How do I write code that reads data at memory location specified by segment and offset?
Use peekb( ) function. This function returns byte(s) read from specific segment and offset locations in memory. The following program illustrates use of this function. In this program from VDU memory we have read characters and its attributes of the first row. The information stored in file is then further read and displayed using peek( ) function.
#include
#include
main( )
{
char far *scr = 0xB8000000 ;
FILE *fp ;
int offset ;
char ch ;
if ( ( fp = fopen ( “scr.dat”, “wb” ) ) == NULL )
{
printf ( “\nUnable to open file” ) ;
exit( ) ;
}
// reads and writes to file
for ( offset = 0 ; offset < 160 ; offset++ ) fprintf ( fp, “%c”, peekb ( scr, offset ) ) ; fclose ( fp ) ; if ( ( fp = fopen ( “scr.dat”, “rb” ) ) == NULL ) { printf ( “\nUnable to open file” ) ; exit( ) ; } // reads and writes to file for ( offset = 0 ; offset < 160 ; offset++ ) { fscanf ( fp, “%c”, &ch ) ; printf ( “%c”, ch ) ; } fclose ( fp ) ; }
How do I write code to find an amount of free disk space available on current drive?
Use getdfree( ) function as shown in follow code.
#include
#include
#include
#include
main( )
{
int dr ; struct dfree disk ;
long freesp ;
dr = getdisk( ) ;
getdfree ( dr + 1 , &disk ) ;
if ( disk.df_sclus == 0xFFFF )
{
printf ( “\ngetdfree( ) function failed\n”);
exit ( 1 ) ;
}
freesp = ( long ) disk.df_avail
* ( long ) disk.df_bsec
* ( long ) disk.df_sclus ;
printf ( “\nThe current drive %c: has %ld bytes
available as free space\n”, ‘A’ + dr, freesp ) ;
}
The functions memcmp( ) and memicmp( )
#include
#include
main( )
{
char str1[] = “This string contains some
characters” ;
char str2[] = “this string contains” ;
int result ;
result = memcmp ( str1, str2, strlen ( str2 ) ) ;
printf ( “\nResult after comapring buffer using
memcmp( )” ) ;
show ( result ) ;
result = memicmp ( str1, str2, strlen ( str2 ) ) ;
printf ( “\nResult after comapring buffer using
memicmp( )” ) ;
show ( result ) ;
}
show ( int r )
{
if ( r == 0 )
printf ( “\nThe buffer str1 and str2 hold
identical data” ) ;
if ( r > 0 )
printf ( “\nThe buffer str1 is bigger than buffer
str2″ ) ;
if ( r < 0 ) printf ( “\nThe buffer str1 is less than buffer str2″ ) ; }
How do I write code to get the current drive as well as set the current drive?
The function getdisk( ) returns the drive number of current drive. The drive number 0 indicates ‘A’ as the current drive, 1 as ‘B’ and so on. The Setdisk( ) function sets the current drive. This function takes one argument which is an integer indicating the drive to be set. Following program demonstrates use of both the functions.
#include
main( )
{
int dno, maxdr ;
dno = getdisk( ) ;
printf ( “\nThe current drive is: %c\n”, 65 + dno
) ;
maxdr = setdisk ( 3 ) ;
dno = getdisk( ) ;
printf ( “\nNow the current drive is: %c\n”, 65 +
dno ) ;
}
How do I write code that would get error number and display error message if any standard error occurs?
Following code demonstrates this.
#include
#include
#include
main( )
{
char *errmsg ;
FILE *fp ;
fp = fopen ( “C:\file.txt”, “r” ) ;
if ( fp == NULL )
{
errmsg = strerror ( errno ) ;
printf ( “\n%s”, errmsg ) ;
}
}
Here, we are trying to open ‘file.txt’ file. However, if the file does not exist, then it would cause an error. As a result, a value (in this case 2) related to the error generated would get set in errno. errno is an external int variable declared in ’stdlib.h’ and also in ‘errno.h’. Next, we have called sterror( ) function which takes an error number and returns a pointer to standard error message related to the given error number.
How do I change the type of cursor and hide a cursor?
We can change the cursor type by using function _setcursortype( ). This function can change the cursor type to solid cursor and can even hide a cursor. Following code shows how to change the cursor type and hide cursor.
#include
main( )
{
/* Hide cursor */
_setcursortype ( _NOCURSOR ) ;
/* Change cursor to a solid cursor */
_setcursortype ( _SOLIDCURSOR ) ;
/* Change back to the normal cursor */
_setcursortype ( _NORMALCURSOR ) ;
}
How do I write code to retrieve current date and time from the system and display it as a string?
Use time( ) function to get current date and time and then ctime( ) function to display it as a string. This is shown in following code snippet.
#include
void main( )
{
time_t curtime ;
char ctm[50] ;
time ( &curtime ) ; //retrieves current time &
stores in curtime
printf ( “\nCurrent Date & Time: %s”, ctime (
&curtime ) ) ;
}
How do I know how many elements an array can hold?
The amount of memory an array can consume depends on the data type of an array. In DOS environment, the amount of memory an array can consume depends on the current memory model (i.e. Tiny, Small, Large, Huge, etc.). In general an array cannot consume more than 64 kb. Consider following program, which shows the maximum number of elements an array of type int, float and char can have in case of Small memory model.
main( )
{
int i[32767] ;
float f[16383] ;
char s[65535] ;
}
Why doesn’t the following statement work?
char str[ ] = “Hello” ;
strcat ( str, ‘!’ ) ;
Ans: The string function strcat( ) concatenates strings and not a character. The basic difference between a string and a character is that a string is a collection of characters, represented by an array of characters whereas a character is a single character. To make the above statement work writes the statement as shown below:
strcat ( str, “!” ) ;
Why doesn’t the following code give the desired result?
int x = 3000, y = 2000 ;
long int z = x * y ;
Ans: Here the multiplication is carried out between two ints x and y, and the result that would overflow would be truncated before being assigned to the variable z of type long int. However, to get the correct output, we should use an explicit cast to force long arithmetic as shown below:
long int z = ( long int ) x * y ;
Note that ( long int )( x * y ) would not give the desired effect
What will be the output of the following code?
void main ()
{ int i = 0 , a[3] ;
a[i] = i++;
printf (“%d”,a[i]) ;
}
Ans: The output for the above code would be a garbage value. In the statement a[i] = i++; the value of the variable i would get assigned first to a[i] i.e. a[0] and then the value of i would get incremented by 1. Since a[i] i.e. a[1] has not been initialized, a[i] will have a garbage value.
What is an adaptor class or Wrapper class?
A class that has no functionality of its own. Its member functions hide the use of a third party software component or an object with the non-compatible interface or a non- object- oriented implementation
What do you mean by Stack unwinding?
It is a process during exception handling when the destructor is called for all local objects between the place where the exception was thrown and where it is caught.
What is a mixin class?
A class that provides some but not all of the implementation for a virtual base class is often called mixin. Derivation done just for the purpose of redefining the virtual functions in the base classes is often called mixin inheritance. Mixin classes typically don’t share common bases.
What is a concrete class?
A concrete class is used to define a useful object that can be instantiated as an automatic variable on the program stack. The implementation of a concrete class is defined. The concrete class is not intended to be a base class and no attempt to minimize dependency on other classes in the implementation or behavior of the class.
What is cloning?
An object can carry out copying in two ways i.e. it can set itself to be a copy of another object, or it can return a copy of itself. The latter process is called cloning.
What is an opaque pointer?
A pointer is said to be opaque if the definition of the type to which it points to is not included in the current translation unit. A translation unit is the result of merging an implementation file with all its headers and header files.
What sorting algos have their best and worst case times equal ?
O(nlogn) for mergesort and heap sort
What is an abstract class?
A class which cannot be Instantiated.
Static binding occurs at Compile Time,Runtime?
Both at compile and runtime.
What will a read() function do?.
A method in Input Stream.It reads a single byte or an array of bytes.Returns no of bytes read or -1 if EOF(End of file)is reached
Differentiate between a deep copy and a shallow copy?
Deep copy involves using the contents of one object to create another instance of the same class. In a deep copy, the two objects may contain ht same information but the target object will have its own buffers and resources. the destruction of either object will not affect the remaining object. The overloaded assignment operator would create a deep copy of objects.
Shallow copy involves copying the contents of one object into another instance of the same class thus creating a mirror image. Owing to straight copying of references and pointers, the two objects will share the same externally contained contents of the other object to be unpredictable.
Using a copy constructor we simply copy the data values member by member. This method of copying is called shallow copy. If the object is a simple class, comprised of built in types and no pointers this would be acceptable. This function would use the values and the objects and its behavior would not be altered with a shallow copy, only the addresses of pointers that are members are copied and not the value the address is pointing to. The data values of the object would then be inadvertently altered by the function. When the function goes out of scope, the copy of the object with all its data is popped off the stack. If the object has any pointers a deep copy needs to be executed. With the deep copy of an object, memory is allocated for the object in free store and the elements pointed to are copied. A deep copy is used for objects that are returned from a function.
What is a smart pointer?
A smart pointer is an object that acts, looks and feels like a normal pointer but offers more functionality. In C++, smart pointers are implemented as template classes that encapsulate a pointer and override standard pointer operators. They have a number of advantages over regular pointers. They are guaranteed to be initialized as either null pointers or pointers to a heap object. Indirection through a null pointer is checked. No delete is ever necessary. Objects are automatically freed when the last pointer to them has gone away. One significant problem with these smart pointers is that unlike regular pointers, they don’t respect inheritance. Smart pointers are unattractive for polymorphic code. Given below is an example for the implementation of smart pointers.
Will the inline function be compiled as the inline function always? Justify.
An inline function is a request and not a command. Hence it won’t be compiled as an inline function always.
Explanation:Inline-expansion could fail if the inline function contains loops, the address of an inline function is used, or an inline function is called in a complex expression. The rules for inlining are compiler dependent.
What is reflexive association?
The ‘is-a’ is called a reflexive association because the reflexive association permits classes to bear the is-a association not only with their super-classes but also with themselves. It differs from a ’specializes-from’ as ’specializes-from’ is usually used to describe the association between a super-class and a sub-class.
For example:Printer is-a printer.
Differentiate between a deep copy and a shallow copy?
Deep copy involves using the contents of one object to create another instance of the same class. In a deep copy, the two objects may contain ht same information but the target object will have its own buffers and resources. the destruction of either object will not affect the remaining object. The overloaded assignment operator would create a deep copy of objects.
Shallow copy involves copying the contents of one object into another instance of the same class thus creating a mirror image. Owing to straight copying of references and pointers, the two objects will share the same externally contained contents of the other object to be unpredictable
What is a parameterized type?
A template is a parameterized construct or type containing generic code that can use or manipulate any type. It is called parameterized because an actual type is a parameter of the code body. Polymorphism may be achieved through parameterized types. This type of polymorphism is called parameteric polymorphism. Parameteric polymorphism is the mechanism by which the same code is used on different types passed as parameters.
Saturday, October 25, 2008
Monday, October 20, 2008
INTERVIEW QUESTIONS & ANSWERS FROM C, C ++ part-IV
What is garbage collection?
Suppose some memory space becomes reusable because a node is released from a linked list. Hence, we want the space to be available for future use. One way to bring this about is to immediately reinsert the space into the free-storage list. However, this method may be too time-consuming for the operating system. The operating system may periodically collect all the deleted space onto the free-storage list. The technique that does this collection is called Garbage Collection. Garbage Collection usually takes place in two steps: First the Garbage Collector runs through all lists, tagging whose cells are currently in use, and then it runs through the memory, collecting all untagged space onto the free-storage list. The Garbage Collection may take place when there is only some minimum amount of space or no space at all left in the free-storage list, or when the CPU is idle and has time to do the collection. Generally speaking, the Garbage Collection is invisible to the programmer..
Can we get the remainder of a floating point division ?
Yes. Although the % operator fails to work on float numbers we can still get the remainder of floating point division by using a function fmod( ). The fmod( ) function divides the two float numbers passed to it as parameters and returns the remainder as a floating-point value. Following program shows fmod( ) function at work.
#include
main( )
{
printf ( “%f”, fmod ( 5.15, 3.0 ) ) ;
}
The above code snippet would give the output as 2.150000.
What does the error “Null Pointer Assignment” mean and what causes this error?
The Null Pointer Assignment error is generated only in small and medium memory models. This error occurs in programs which attempt to change the bottom of the data segment. In Borland’s C or C++ compilers, Borland places four zero bytes at the bottom of the data segment, followed by the Borland copyright notice “Borland C++ - Copyright 1991 Borland Intl.”. In the small and medium memory models, a null pointer points to DS:0000. Thus assigning a value to the memory referenced by this pointer will overwrite the first zero byte in the data segment. At program termination, the four zeros and the copyright banner are checked. If either has been modified, then the Null Pointer Assignment error is generated. Note that the pointer may not truly be null, but may be a wild pointer that references these key areas in the data segment.
What’s the difference between a null pointer, a NULL macro, the ASCII NUL character and a null string?
A null pointer is a pointer which doesn’t point anywhere. A NULL macro is used to represent the null pointer in source code. It has a value 0 associated with it. The ASCII NUL character has all its bits as 0 but doesn’t have any relationship with the null pointer. The null string is just another name for an empty string “”.
What is the difference between const char *p, char const *p, and char* const p ?
‘const char *p’ and ‘char const *p’ are the same, i.e. p points to a constant character. On the other hand, ‘char* const p’ means p is a constant pointer pointing to a character which means we cannot change the pointer p but we can change the character which p is pointing to.
How to get the memory size ?
Consider the following program
#include
#include
main( )
{
int memsize;
memsize = biosmemory( ) ;
printf ( “RAM size = %dK\n”,memsize ) ;
return 0 ;
}
The function biosmemory uses BIOS interrupt 0×12 to return the size of memory.
How to use function strdup( ) in a program?
The string function strdup( ) copies the given string to a new location. The function uses malloc( ) function to allocate space required for the duplicated string. It takes one argument a pointer to the string to be duplicated. The total number of characters present in the given string plus one bytes get allocated for the new string. As this function uses malloc( ) to allocate memory, it is the programmer’s responsibility to deallocate the memory using free( ).
#include
#include
#include
void main( )
{
char *str1, *str2 = “double”;
str1 = strdup ( str2 ) ;
printf ( “%s\n”, str1 ) ;
free ( str1 ) ;
}
Can we change the system date to some other date?
Yes, We can! The function stime( ) sets the system date to the specified date. It also sets the system time. The time and date is measured in seconds from the 00:00:00 GMT, January 1, 1970. The following program shows how to use this function.
#include
#include
void main( )
{
time_t tm ;
int d ;
tm = time ( NULL ) ;
printf ( “The System Date : %s”, ctime ( &tm ) ) ;
printf ( “\nHow many days ahead you want to set the date : ” ) ;
scanf ( “%d”, &d ) ;
tm += ( 24L * d ) * 60L * 60L ;
stime ( &tm ) ;
printf ( “\nNow the new date is : %s”, ctime ( &tm ) ) ;
}
In this program we have used function ctime( ) in addition to function stime( ). The ctime( ) function converts time value to a 26-character long string that contains date and time
How do I write a function that takes variable number of arguments?
The following program demonstrates this.
#include
#include
void main( )
{
int i = 10 ;
float f = 2.5 ;
char *str = “Hello!” ;
vfpf ( “%d %f %s\n”, i, f, str ) ;
vfpf ( “%s %s”, str, “Hi!” ) ;
}
void vfpf ( char *fmt, … )
{
va_list argptr ;
va_start ( argptr, fmt ) ;
vfprintf ( stdout, fmt, argptr ) ;
va_end ( argptr ) ;
}
Here, the function vfpf( ) has called vfprintf( ) that take variable argument lists. va_list is an array that holds information required for the macros va_start and va_end. The macros va_start and va_end provide a portable way to access the variable argument lists. va_start would set up a pointer argptr to point to the first of the variable arguments being passed to the function. The macro va_end helps the called function to perform a normal return.
Can we get the process identification number of the current program?
Yes! The macro getpid( ) gives us the process identification number of the program currently running. The process id. uniquely identifies a program. Under DOS, the getpid( ) returns the Program Segment Prefix as the process id. Following program illustrates the use of this macro.
#include
#include
void main( )
{
printf ( “The process identification number of this program is %X\n”,
getpid( ) ) ;
}
How to obtain a path of the given file?
The function searchpath( ) searches for the specified file in the subdirectories of the current path. Following program shows how to make use of the searchpath( ) function.
#include “dir.h”
void main ( int argc, char *argv[] )
{
char *path ;
if ( path = searchpath ( argv[ 1 ] ) )
printf ( “Pathname : %s\n”, path ) ;
else
printf ( “File not found\n” ) ;
}
Can we get the x and y coordinate of the current cursor position ?
The function wherex( ) and wherey( ) returns the x-coordinate and y-coordinate of the current cursor position respectively. Both the functions return an integer value. The value returned by wherex( ) is the horizontal position of cursor and the value returned by wherey( ) is the vertical position of the cursor. Following program shows how to use the wherex( ) and wherey( ) functions.
#include
main( )
{
printf ( “Just\n To\n Test\n Where\n the cursor\n goes” ) ;
printf ( “Current location is X: %d Y: %d\n”, wherex( ), wherey( ) ) ;
}
How to obtain the current drive through C ?
We can use the function _getdrive( ) to obtain the current drive. The _getdrive( ) function uses DOS function 0X19 to get the current drive number
#include
main( )
{
int disk ;
disk = _getdrive( ) + ‘A’ - 1 ;
printf ( “The current drive is: %c\n”, disk ) ;
}
How to restrict the program’s output to a specific screen region?
A C function window( ) can be used to restrict the screen output to a specific region. The window( ) function defines a text-mode window. The parameters passed to this function defines the upper-left and lower-right corner of the region within which you want the output. In the following program, the string ‘Hello!’ gets printed within the specified region. To print the string we must use cprintf( ) function which prints directly on the text-mode window.
#include
main( )
{
int i, j ;
window ( 20, 8, 60, 17 ) ;
for ( i = 0 ; i < j =" 0">
How do I print the contents of environment variables?
The following program shows how to achieve this:
main( int argc, char *argv[ ], char *env[ ] )
{
int i = 0 ;
clrscr( ) ;
while ( env[ i ] )
printf ( “\n%s”, env[ i++ ] ) ;
}
main( ) has the third command line argument env, which is an array of pointers to the strings. Each pointer points to an environment variable from the list of environment variables.
What’s the difference between these two declarations?
struct str1 { … } ;
typedef struct { … } str2 ;
Ans : The first form declares a structure tag whereas the second declares a typedef. The main difference is that the second declaration is of a slightly more abstract type — its users don’t necessarily know that it is a structure, and the keyword struct is not used when declaring instances of it.
How do I convert a floating-point number to a string?
Use function gcvt( ) to convert a floating-point number to a string. Following program demonstrates the use of this function.
#include
main( )
{
char str[25] ;
float no ;
int dg = 5 ; /* significant digits */
no = 14.3216 ;
gcvt ( no, dg, str ) ;
printf ( “String: %s\n”, str ) ;
}
How to find the row and column dimension of a given 2-D array?
Whenever we initialize a 2-D array at the same place where it has been declared, it is not necessary to mention the row dimension of an array. The row and column dimensions of such an array can be determined programmatically as shown in following program.
void main( )
{
int a[][3] = { 0, 1, 2,
9,-6, 8,
7, 5, 44,
23, 11,15 } ;
int c = sizeof ( a[0] ) / sizeof ( int ) ;
int r = ( sizeof ( a ) / sizeof ( int ) ) / c ;
int i, j ;
printf ( “\nRow: %d\nCol: %d\n”, r, c ) ;
for ( i = 0 ; i < j =" 0">
How do I write printf( ) so that the width of a field can be specified at runtime?
This is shown in following code snippet.
main( )
{
int w, no ;
printf ( “Enter number and the width for the
number field:” ) ;
scanf ( “%d%d”, &no, &w ) ;
printf ( “%*d”, w, no ) ;
}
Here, an ‘*’ in the format specifier in printf( ) indicates that an int value from the argument list should be used for the field width.
Are the following two statements identical?
char str[6] = “Kicit” ;
char *str = “Kicit” ;
Ans: No! Arrays are not pointers. An array is a single, pre-allocated chunk of contiguous elements (all of the same type), fixed in size and location. A pointer on the other hand, is a reference to any data element (of a particular type) located anywhere. A pointer must be assigned to point to space allocated elsewhere, but it can be reassigned any time. The array declaration char str[6] ; requests that space for 6 characters be set aside, to be known
by name str. In other words there is a location named str at which six characters are stored. The pointer declaration char *str ; on the other hand, requests a place that holds a pointer, to be known by the name str. This pointer can point almost anywhere to any char, to any contiguous array of chars, or nowhere.
How do I compare character data stored at two different memory locations?
Sometimes in a program we require to compare memory ranges containing strings. In such a situation we can use functions like memcmp( ) or memicmp( ). The basic difference between two functions is that memcmp( ) does a case-sensitive comparison whereas memicmp( ) ignores case of characters. Following program illustrates the use of both the functions.
#include
main( )
{
char *arr1 = “Kicit” ;
char *arr2 = “kicitNagpur” ;
int c ;
c = memcmp ( arr1, arr2, sizeof ( arr1 ) ) ;
if ( c == 0 )
printf ( “\nStrings arr1 and arr2 compared using memcmp are identical” ) ;
else
printf ( “\nStrings arr1 and arr2 compared using memcmp are not identical”
) ;
c = memicmp ( arr1, arr2, sizeof ( arr1 ) ) ;
if ( c == 0 )
printf ( “\nStrings arr1 and arr2 compared using memicmp are identical” )
;
else
printf ( “\nStrings arr1 and arr2 compared using memicmp are not
identical” ) ;
}
Suppose some memory space becomes reusable because a node is released from a linked list. Hence, we want the space to be available for future use. One way to bring this about is to immediately reinsert the space into the free-storage list. However, this method may be too time-consuming for the operating system. The operating system may periodically collect all the deleted space onto the free-storage list. The technique that does this collection is called Garbage Collection. Garbage Collection usually takes place in two steps: First the Garbage Collector runs through all lists, tagging whose cells are currently in use, and then it runs through the memory, collecting all untagged space onto the free-storage list. The Garbage Collection may take place when there is only some minimum amount of space or no space at all left in the free-storage list, or when the CPU is idle and has time to do the collection. Generally speaking, the Garbage Collection is invisible to the programmer..
Can we get the remainder of a floating point division ?
Yes. Although the % operator fails to work on float numbers we can still get the remainder of floating point division by using a function fmod( ). The fmod( ) function divides the two float numbers passed to it as parameters and returns the remainder as a floating-point value. Following program shows fmod( ) function at work.
#include
main( )
{
printf ( “%f”, fmod ( 5.15, 3.0 ) ) ;
}
The above code snippet would give the output as 2.150000.
What does the error “Null Pointer Assignment” mean and what causes this error?
The Null Pointer Assignment error is generated only in small and medium memory models. This error occurs in programs which attempt to change the bottom of the data segment. In Borland’s C or C++ compilers, Borland places four zero bytes at the bottom of the data segment, followed by the Borland copyright notice “Borland C++ - Copyright 1991 Borland Intl.”. In the small and medium memory models, a null pointer points to DS:0000. Thus assigning a value to the memory referenced by this pointer will overwrite the first zero byte in the data segment. At program termination, the four zeros and the copyright banner are checked. If either has been modified, then the Null Pointer Assignment error is generated. Note that the pointer may not truly be null, but may be a wild pointer that references these key areas in the data segment.
What’s the difference between a null pointer, a NULL macro, the ASCII NUL character and a null string?
A null pointer is a pointer which doesn’t point anywhere. A NULL macro is used to represent the null pointer in source code. It has a value 0 associated with it. The ASCII NUL character has all its bits as 0 but doesn’t have any relationship with the null pointer. The null string is just another name for an empty string “”.
What is the difference between const char *p, char const *p, and char* const p ?
‘const char *p’ and ‘char const *p’ are the same, i.e. p points to a constant character. On the other hand, ‘char* const p’ means p is a constant pointer pointing to a character which means we cannot change the pointer p but we can change the character which p is pointing to.
How to get the memory size ?
Consider the following program
#include
#include
main( )
{
int memsize;
memsize = biosmemory( ) ;
printf ( “RAM size = %dK\n”,memsize ) ;
return 0 ;
}
The function biosmemory uses BIOS interrupt 0×12 to return the size of memory.
How to use function strdup( ) in a program?
The string function strdup( ) copies the given string to a new location. The function uses malloc( ) function to allocate space required for the duplicated string. It takes one argument a pointer to the string to be duplicated. The total number of characters present in the given string plus one bytes get allocated for the new string. As this function uses malloc( ) to allocate memory, it is the programmer’s responsibility to deallocate the memory using free( ).
#include
#include
#include
void main( )
{
char *str1, *str2 = “double”;
str1 = strdup ( str2 ) ;
printf ( “%s\n”, str1 ) ;
free ( str1 ) ;
}
Can we change the system date to some other date?
Yes, We can! The function stime( ) sets the system date to the specified date. It also sets the system time. The time and date is measured in seconds from the 00:00:00 GMT, January 1, 1970. The following program shows how to use this function.
#include
#include
void main( )
{
time_t tm ;
int d ;
tm = time ( NULL ) ;
printf ( “The System Date : %s”, ctime ( &tm ) ) ;
printf ( “\nHow many days ahead you want to set the date : ” ) ;
scanf ( “%d”, &d ) ;
tm += ( 24L * d ) * 60L * 60L ;
stime ( &tm ) ;
printf ( “\nNow the new date is : %s”, ctime ( &tm ) ) ;
}
In this program we have used function ctime( ) in addition to function stime( ). The ctime( ) function converts time value to a 26-character long string that contains date and time
How do I write a function that takes variable number of arguments?
The following program demonstrates this.
#include
#include
void main( )
{
int i = 10 ;
float f = 2.5 ;
char *str = “Hello!” ;
vfpf ( “%d %f %s\n”, i, f, str ) ;
vfpf ( “%s %s”, str, “Hi!” ) ;
}
void vfpf ( char *fmt, … )
{
va_list argptr ;
va_start ( argptr, fmt ) ;
vfprintf ( stdout, fmt, argptr ) ;
va_end ( argptr ) ;
}
Here, the function vfpf( ) has called vfprintf( ) that take variable argument lists. va_list is an array that holds information required for the macros va_start and va_end. The macros va_start and va_end provide a portable way to access the variable argument lists. va_start would set up a pointer argptr to point to the first of the variable arguments being passed to the function. The macro va_end helps the called function to perform a normal return.
Can we get the process identification number of the current program?
Yes! The macro getpid( ) gives us the process identification number of the program currently running. The process id. uniquely identifies a program. Under DOS, the getpid( ) returns the Program Segment Prefix as the process id. Following program illustrates the use of this macro.
#include
#include
void main( )
{
printf ( “The process identification number of this program is %X\n”,
getpid( ) ) ;
}
How to obtain a path of the given file?
The function searchpath( ) searches for the specified file in the subdirectories of the current path. Following program shows how to make use of the searchpath( ) function.
#include “dir.h”
void main ( int argc, char *argv[] )
{
char *path ;
if ( path = searchpath ( argv[ 1 ] ) )
printf ( “Pathname : %s\n”, path ) ;
else
printf ( “File not found\n” ) ;
}
Can we get the x and y coordinate of the current cursor position ?
The function wherex( ) and wherey( ) returns the x-coordinate and y-coordinate of the current cursor position respectively. Both the functions return an integer value. The value returned by wherex( ) is the horizontal position of cursor and the value returned by wherey( ) is the vertical position of the cursor. Following program shows how to use the wherex( ) and wherey( ) functions.
#include
main( )
{
printf ( “Just\n To\n Test\n Where\n the cursor\n goes” ) ;
printf ( “Current location is X: %d Y: %d\n”, wherex( ), wherey( ) ) ;
}
How to obtain the current drive through C ?
We can use the function _getdrive( ) to obtain the current drive. The _getdrive( ) function uses DOS function 0X19 to get the current drive number
#include
main( )
{
int disk ;
disk = _getdrive( ) + ‘A’ - 1 ;
printf ( “The current drive is: %c\n”, disk ) ;
}
How to restrict the program’s output to a specific screen region?
A C function window( ) can be used to restrict the screen output to a specific region. The window( ) function defines a text-mode window. The parameters passed to this function defines the upper-left and lower-right corner of the region within which you want the output. In the following program, the string ‘Hello!’ gets printed within the specified region. To print the string we must use cprintf( ) function which prints directly on the text-mode window.
#include
main( )
{
int i, j ;
window ( 20, 8, 60, 17 ) ;
for ( i = 0 ; i < j =" 0">
How do I print the contents of environment variables?
The following program shows how to achieve this:
main( int argc, char *argv[ ], char *env[ ] )
{
int i = 0 ;
clrscr( ) ;
while ( env[ i ] )
printf ( “\n%s”, env[ i++ ] ) ;
}
main( ) has the third command line argument env, which is an array of pointers to the strings. Each pointer points to an environment variable from the list of environment variables.
What’s the difference between these two declarations?
struct str1 { … } ;
typedef struct { … } str2 ;
Ans : The first form declares a structure tag whereas the second declares a typedef. The main difference is that the second declaration is of a slightly more abstract type — its users don’t necessarily know that it is a structure, and the keyword struct is not used when declaring instances of it.
How do I convert a floating-point number to a string?
Use function gcvt( ) to convert a floating-point number to a string. Following program demonstrates the use of this function.
#include
main( )
{
char str[25] ;
float no ;
int dg = 5 ; /* significant digits */
no = 14.3216 ;
gcvt ( no, dg, str ) ;
printf ( “String: %s\n”, str ) ;
}
How to find the row and column dimension of a given 2-D array?
Whenever we initialize a 2-D array at the same place where it has been declared, it is not necessary to mention the row dimension of an array. The row and column dimensions of such an array can be determined programmatically as shown in following program.
void main( )
{
int a[][3] = { 0, 1, 2,
9,-6, 8,
7, 5, 44,
23, 11,15 } ;
int c = sizeof ( a[0] ) / sizeof ( int ) ;
int r = ( sizeof ( a ) / sizeof ( int ) ) / c ;
int i, j ;
printf ( “\nRow: %d\nCol: %d\n”, r, c ) ;
for ( i = 0 ; i < j =" 0">
How do I write printf( ) so that the width of a field can be specified at runtime?
This is shown in following code snippet.
main( )
{
int w, no ;
printf ( “Enter number and the width for the
number field:” ) ;
scanf ( “%d%d”, &no, &w ) ;
printf ( “%*d”, w, no ) ;
}
Here, an ‘*’ in the format specifier in printf( ) indicates that an int value from the argument list should be used for the field width.
Are the following two statements identical?
char str[6] = “Kicit” ;
char *str = “Kicit” ;
Ans: No! Arrays are not pointers. An array is a single, pre-allocated chunk of contiguous elements (all of the same type), fixed in size and location. A pointer on the other hand, is a reference to any data element (of a particular type) located anywhere. A pointer must be assigned to point to space allocated elsewhere, but it can be reassigned any time. The array declaration char str[6] ; requests that space for 6 characters be set aside, to be known
by name str. In other words there is a location named str at which six characters are stored. The pointer declaration char *str ; on the other hand, requests a place that holds a pointer, to be known by the name str. This pointer can point almost anywhere to any char, to any contiguous array of chars, or nowhere.
How do I compare character data stored at two different memory locations?
Sometimes in a program we require to compare memory ranges containing strings. In such a situation we can use functions like memcmp( ) or memicmp( ). The basic difference between two functions is that memcmp( ) does a case-sensitive comparison whereas memicmp( ) ignores case of characters. Following program illustrates the use of both the functions.
#include
main( )
{
char *arr1 = “Kicit” ;
char *arr2 = “kicitNagpur” ;
int c ;
c = memcmp ( arr1, arr2, sizeof ( arr1 ) ) ;
if ( c == 0 )
printf ( “\nStrings arr1 and arr2 compared using memcmp are identical” ) ;
else
printf ( “\nStrings arr1 and arr2 compared using memcmp are not identical”
) ;
c = memicmp ( arr1, arr2, sizeof ( arr1 ) ) ;
if ( c == 0 )
printf ( “\nStrings arr1 and arr2 compared using memicmp are identical” )
;
else
printf ( “\nStrings arr1 and arr2 compared using memicmp are not
identical” ) ;
}
Sunday, October 12, 2008
INTERVIEW QUESTIONS & ANSWERS FROM C, C ++ part-III
Can math operations be performed on a void pointer?
No. Pointer addition and subtraction are based on advancing the pointer by a number of elements. By definition, if you have a void pointer, you don’t know what it’s pointing to, so you don’t know the size of what it’s pointing to. If you want pointer arithmetic to work on raw addresses, use character pointers.
What is the difference between NULL and NUL?
NULL is a macro defined in for the null pointer.
NUL is the name of the first character in the ASCII character set. It corresponds to a zero value. There’s no standard macro NUL in C, but some people like to define it.
The digit 0 corresponds to a value of 80, decimal. Don’t confuse the digit 0 with the value of (NUL)!
NULL can be defined as ((void*)0), NUL as ‘’.
What are the advantages of the functions?
Debugging is easier
It is easier to understand the logic involved in the program
Testing is easier
Recursive call is possible
Irrelevant details in the user point of view are hidden in functions
Functions are helpful in generalizing the program
Difference between arrays and pointers?
Pointers are used to manipulate data using the address. Pointers use * operator to access the data pointed to by them
Arrays use subscripted variables to access and manipulate data.Array variables can be equivalently written using pointer expression.
When should a far pointer be used?
Sometimes you can get away with using a small memory model in most of a given program. There might be just a few things that don’t fit in your small data and code segments. When that happens, you can use explicit far pointers and function declarations to get at the rest of memory. A far function can be outside the 64KB segment most functions are shoehorned into for a small-code model. (Often, libraries are declared explicitly far, so they’ll work no matter what code model the program uses.)
A far pointer can refer to information outside the 64KB data segment. Typically, such pointers are used with farmalloc() and such, to manage a heap separate from where all the rest of the data lives. If you use a small-data, large-code model, you should explicitly make your function pointers far.
What is static memory allocation and dynamic memory allocation?
Static memory allocation: The compiler allocates the required memory space for a declared variable.By using the address of operator,the reserved address is obtained and this address may be assigned to a pointer variable.Since most of the declared variable have static memory,this way of assigning pointer value to a pointer variable is known as static memory allocation. memory is assigned during compilation time.
Dynamic memory allocation: It uses functions such as malloc( ) or calloc( ) to get memory dynamically.If these functions are used to get memory dynamically and the values returned by these functions are assingned to pointer variables, such assignments are known as dynamic memory allocation.memory is assined during run time.
What is a const pointer?
The access modifier keyword const is a promise the programmer makes to the compiler that the value of a variable will not be changed after it is initialized. The compiler will enforce that promise as best it can by not enabling the programmer to write code which modifies a variable that has been declared const.
A “const pointer,” or more correctly, a “pointer to const,” is a pointer which points to data that is const (constant, or unchanging). A pointer to const is declared by putting the word const at the beginning of the pointer declaration. This declares a pointer which points to data that can’t be modified. The pointer itself can be modified. The following example illustrates some legal and illegal uses of a const pointer:
const char *str = “hello”;
char c = *str /* legal */
str++; /* legal */
*str = ‘a’; /* illegal */
str[1] = ‘b’; /* illegal */
What is the purpose of realloc( )?
The function realloc(ptr,n) uses two arguments.the first argument ptr is a pointer to a block of memory for which the size is to be altered.The second argument n specifies the new size.The size may be increased or decreased.If n is greater than the old size and if sufficient space is not available subsequent to the old region, the function realloc( )
may create a new region and all the old data are moved to the new region.
How can you restore a redirected standard stream?
The preceding example showed how you can redirect a standard stream from within your program. But what if later in your program you wanted to restore the standard stream to its original state? By using the standard C library functions named dup() and fdopen(), you can restore a standard stream such as stdout to its original state.
The dup() function duplicates a file handle. You can use the dup() function to save the file handle corresponding to the stdout standard stream. The fdopen() function opens a stream that has been duplicated with the dup() function.
What is Preprocessor?
The preprocessor is used to modify your program according to the preprocessor directives in your source code. Preprocessor directives (such as #define) give the preprocessor specific instructions on how to modify your source code. The preprocessor reads in all of your include files and the source code you are compiling and creates a preprocessed version of your source code. This preprocessed version has all of its macros and constant symbols replaced by their corresponding code and value assignments. If your source code contains any conditional preprocessor directives (such as #if), the preprocessor evaluates the condition and modifies your source code accordingly.
The preprocessor contains many features that are powerful to use, such as creating macros, performing conditional compilation, inserting predefined environment variables into your code, and turning compiler features on and off. For the professional programmer, in-depth knowledge of the features of the preprocessor can be one of the keys to creating fast, efficient programs.
How do you print an address?
The safest way is to use printf() (or fprintf() or sprintf()) with the %P specification. That prints a void pointer (void*). Different compilers might print a pointer with different formats. Your compiler will pick a format that’s right for your environment.
If you have some other kind of pointer (not a void*) and you want to be very safe, cast the pointer to a void*:
printf( “%Pn”, (void*) buffer );
Is it possible to execute code even after the program exits the main() function?
The standard C library provides a function named atexit() that can be used to perform “cleanup” operations when your program terminates. You can set up a set of functions you want to perform automatically when your program exits by passing function pointers to the at exit() function.
How can I convert a number to a string?
The standard C library provides several functions for converting numbers of all formats (integers, longs, floats, and so on) to strings and vice versa The following functions can be used to convert integers to strings:
Function Name Purpose
itoa() Converts an integer value to a string.
ltoa() Converts a long integer value to a string.
ultoa() Converts an unsigned long integer value to a string.
The following functions can be used to convert floating-point values to strings:
Function Name Purpose
ecvt() Converts a double-precision floating-point value to a string without an embedded decimal point.
fcvt() Same as ecvt(), but forces the precision to a specified number of digits.
gcvt() Converts a double-precision floating-point value to a string with an embedded decimal point.
How can I convert a string to a number?
The standard C library provides several functions for converting strings to numbers of all formats (integers, longs, floats, and so on) and vice versa.
The following functions can be used to convert strings to numbers:
Function Name Purpose
atof() Converts a string to a double-precision floating-point value.
atoi() Converts a string to an integer.
atol() Converts a string to a long integer.
strtod() Converts a string to a double-precision floating-point value and reports any “leftover” numbers that could not be converted.
strtol() Converts a string to a long integer and reports any “leftover” numbers that could not be converted.
strtoul() Converts a string to an unsigned long integer and reports any “leftover” numbers that could not be converted.
What is a null pointer?
There are times when it’s necessary to have a pointer that doesn’t point to anything. The macro NULL, defined in , has a value that’s guaranteed to be different from any valid pointer. NULL is a literal zero, possibly cast to void* or char*. Some people, notably C++ programmers, prefer to use 0 rather than NULL.
The null pointer is used in three ways:
1) To stop indirection in a recursive data structure
2) As an error value
3) As a sentinel value
When should a type cast be used?
There are two situations in which to use a type cast. The first use is to change the type of an operand to an arithmetic operation so that the operation will be performed properly.
The second case is to cast pointer types to and from void * in order to interface with functions that expect or return void pointers. For example, the following line type casts the return value of the call to malloc() to be a pointer to a foo structure.
struct foo *p = (struct foo *) malloc(sizeof(struct foo));
Can include files be nested?
Yes. Include files can be nested any number of times. As long as you use precautionary measures , you can avoid including the same file twice. In the past, nesting header files was seen as bad programming practice, because it complicates the dependency tracking function of the MAKE program and thus slows down compilation. Many of today’s popular compilers make up for this difficulty by implementing a concept called precompiled headers, in which all headers and associated dependencies are stored in
a precompiled state.
Many programmers like to create a custom header file that has #include statements for every header needed for each module. This is perfectly acceptable and can help avoid potential problems relating to #include files, such as accidentally omitting an #include file in a module.
How can you check to see whether a symbol is defined?
You can use the #ifdef and #ifndef preprocessor directives to check whether a symbol has been defined (#ifdef) or whether it has not been defined (#ifndef).
How do you override a defined macro?
You can use the #undef preprocessor directive to undefine (override) a previously defined macro.
What is hashing?
To hash means to grind up, and that’s essentially what hashing is all about. The heart of a hashing algorithm is a hash function that takes your nice, neat data and grinds it into some random-looking integer.
The idea behind hashing is that some data either has no inherent ordering (such as images) or is expensive to compare (such as images). If the data has no inherent ordering, you can’t perform comparison searches.
If the data is expensive to compare, the number of comparisons used even by a binary search might be too many. So instead of looking at the data themselves, you’ll condense (hash) the data to an integer (its hash value) and keep all the data with the same hash value in the same place. This task is carried out by using the hash value as an index into an array.
To search for an item, you simply hash it and look at all the data whose hash values match that of the data you’re looking for. This technique greatly lessens the number of items you have to look at. If the parameters are set up with care and enough storage is available for the hash table, the number of comparisons needed to find an item can be made arbitrarily close to one.
One aspect that affects the efficiency of a hashing implementation is the hash function itself. It should ideally distribute data randomly throughout the entire hash table, to reduce the likelihood of collisions. Collisions occur when two different keys have the same hash value. There are two ways to resolve this problem. In “open addressing,” the collision is resolved by the choosing of another position in the hash table for the element inserted later. When the hash table is searched, if the entry is not found at its
hashed position in the table, the search continues checking until either the element is found or an empty position in the table is found
The second method of resolving a hash collision is called “chaining.” In this method, a “bucket” or linked list holds all the elements whose keys hash to the same value.
When the hash table is searched, the list must be searched linearly.
What is the benefit of using an enum rather than a #define constant?
The use of an enumeration constant (enum) has many advantages over using the traditional symbolic constant style of #define. These advantages include a lower maintenance requirement, improved program readability, and better debugging capability.
1) The first advantage is that enumerated constants are generated automatically by the compiler. Conversely, symbolic constants must be manually assigned values by the Programmer..
For instance, if you had an enumerated constant type for error codes that could occur in your program, your enum definition could look something like this:
enum Error_Code
{
OUT_OF_MEMORY,
INSUFFICIENT_DISK_SPACE,
LOGIC_ERROR,
FILE_NOT_FOUND
};
In the preceding example, OUT_OF_MEMORY is automatically assigned the value of 0 (zero) by the compiler because it appears first in the definition. The compiler then continues to automatically assign numbers to the enumerated constants, making INSUFFICIENT_DISK_SPACE equal to 1, LOGIC_ERROR equal to 2, and FILE_NOT_FOUND equal to 3, so on.
If you were to approach the same example by using symbolic constants, your code would look something like this:
#define OUT_OF_MEMORY 0
#define INSUFFICIENT_DISK_SPACE 1
#define LOGIC_ERROR 2
#define FILE_NOT_FOUND 3
values by the programmer. Each of the two methods arrives at the same result: four constants assigned numeric values to represent error codes. Consider the maintenance required, however, if you were to add two constants to represent the error codes DRIVE_NOT_READY and CORRUPT_FILE. Using the enumeration constant method, you simply would put these two constants anywhere in the enum definition. The compiler would generate two unique values for these constants. Using the symbolic constant method, you would have to manually assign two new numbers to these constants. Additionally, you would want to ensure that the numbers you assign to these constants are unique.
2) Another advantage of using the enumeration constant method is that your programs are more readable and thus can be understood better by others who might have to update your program later.
3) A third advantage to using enumeration constants is that some symbolic debuggers can print the value of an enumeration constant. Conversely, most symbolic debuggers cannot print the value of a symbolic constant. This can be an enormous help in debugging your program, because if your program is stopped at a line that uses an enum, you can simply inspect that constant and instantly know its value. On the other hand, because most debuggers cannot print #define values, you would most likely have to search for that value by manually looking it up in a header file.
How’s method overriding different from overloading?
When overriding, you change the method behavior for a derived class. Overloading simply involves having a method with the same name within the class.
No. Pointer addition and subtraction are based on advancing the pointer by a number of elements. By definition, if you have a void pointer, you don’t know what it’s pointing to, so you don’t know the size of what it’s pointing to. If you want pointer arithmetic to work on raw addresses, use character pointers.
What is the difference between NULL and NUL?
NULL is a macro defined in for the null pointer.
NUL is the name of the first character in the ASCII character set. It corresponds to a zero value. There’s no standard macro NUL in C, but some people like to define it.
The digit 0 corresponds to a value of 80, decimal. Don’t confuse the digit 0 with the value of (NUL)!
NULL can be defined as ((void*)0), NUL as ‘’.
What are the advantages of the functions?
Debugging is easier
It is easier to understand the logic involved in the program
Testing is easier
Recursive call is possible
Irrelevant details in the user point of view are hidden in functions
Functions are helpful in generalizing the program
Difference between arrays and pointers?
Pointers are used to manipulate data using the address. Pointers use * operator to access the data pointed to by them
Arrays use subscripted variables to access and manipulate data.Array variables can be equivalently written using pointer expression.
When should a far pointer be used?
Sometimes you can get away with using a small memory model in most of a given program. There might be just a few things that don’t fit in your small data and code segments. When that happens, you can use explicit far pointers and function declarations to get at the rest of memory. A far function can be outside the 64KB segment most functions are shoehorned into for a small-code model. (Often, libraries are declared explicitly far, so they’ll work no matter what code model the program uses.)
A far pointer can refer to information outside the 64KB data segment. Typically, such pointers are used with farmalloc() and such, to manage a heap separate from where all the rest of the data lives. If you use a small-data, large-code model, you should explicitly make your function pointers far.
What is static memory allocation and dynamic memory allocation?
Static memory allocation: The compiler allocates the required memory space for a declared variable.By using the address of operator,the reserved address is obtained and this address may be assigned to a pointer variable.Since most of the declared variable have static memory,this way of assigning pointer value to a pointer variable is known as static memory allocation. memory is assigned during compilation time.
Dynamic memory allocation: It uses functions such as malloc( ) or calloc( ) to get memory dynamically.If these functions are used to get memory dynamically and the values returned by these functions are assingned to pointer variables, such assignments are known as dynamic memory allocation.memory is assined during run time.
What is a const pointer?
The access modifier keyword const is a promise the programmer makes to the compiler that the value of a variable will not be changed after it is initialized. The compiler will enforce that promise as best it can by not enabling the programmer to write code which modifies a variable that has been declared const.
A “const pointer,” or more correctly, a “pointer to const,” is a pointer which points to data that is const (constant, or unchanging). A pointer to const is declared by putting the word const at the beginning of the pointer declaration. This declares a pointer which points to data that can’t be modified. The pointer itself can be modified. The following example illustrates some legal and illegal uses of a const pointer:
const char *str = “hello”;
char c = *str /* legal */
str++; /* legal */
*str = ‘a’; /* illegal */
str[1] = ‘b’; /* illegal */
What is the purpose of realloc( )?
The function realloc(ptr,n) uses two arguments.the first argument ptr is a pointer to a block of memory for which the size is to be altered.The second argument n specifies the new size.The size may be increased or decreased.If n is greater than the old size and if sufficient space is not available subsequent to the old region, the function realloc( )
may create a new region and all the old data are moved to the new region.
How can you restore a redirected standard stream?
The preceding example showed how you can redirect a standard stream from within your program. But what if later in your program you wanted to restore the standard stream to its original state? By using the standard C library functions named dup() and fdopen(), you can restore a standard stream such as stdout to its original state.
The dup() function duplicates a file handle. You can use the dup() function to save the file handle corresponding to the stdout standard stream. The fdopen() function opens a stream that has been duplicated with the dup() function.
What is Preprocessor?
The preprocessor is used to modify your program according to the preprocessor directives in your source code. Preprocessor directives (such as #define) give the preprocessor specific instructions on how to modify your source code. The preprocessor reads in all of your include files and the source code you are compiling and creates a preprocessed version of your source code. This preprocessed version has all of its macros and constant symbols replaced by their corresponding code and value assignments. If your source code contains any conditional preprocessor directives (such as #if), the preprocessor evaluates the condition and modifies your source code accordingly.
The preprocessor contains many features that are powerful to use, such as creating macros, performing conditional compilation, inserting predefined environment variables into your code, and turning compiler features on and off. For the professional programmer, in-depth knowledge of the features of the preprocessor can be one of the keys to creating fast, efficient programs.
How do you print an address?
The safest way is to use printf() (or fprintf() or sprintf()) with the %P specification. That prints a void pointer (void*). Different compilers might print a pointer with different formats. Your compiler will pick a format that’s right for your environment.
If you have some other kind of pointer (not a void*) and you want to be very safe, cast the pointer to a void*:
printf( “%Pn”, (void*) buffer );
Is it possible to execute code even after the program exits the main() function?
The standard C library provides a function named atexit() that can be used to perform “cleanup” operations when your program terminates. You can set up a set of functions you want to perform automatically when your program exits by passing function pointers to the at exit() function.
How can I convert a number to a string?
The standard C library provides several functions for converting numbers of all formats (integers, longs, floats, and so on) to strings and vice versa The following functions can be used to convert integers to strings:
Function Name Purpose
itoa() Converts an integer value to a string.
ltoa() Converts a long integer value to a string.
ultoa() Converts an unsigned long integer value to a string.
The following functions can be used to convert floating-point values to strings:
Function Name Purpose
ecvt() Converts a double-precision floating-point value to a string without an embedded decimal point.
fcvt() Same as ecvt(), but forces the precision to a specified number of digits.
gcvt() Converts a double-precision floating-point value to a string with an embedded decimal point.
How can I convert a string to a number?
The standard C library provides several functions for converting strings to numbers of all formats (integers, longs, floats, and so on) and vice versa.
The following functions can be used to convert strings to numbers:
Function Name Purpose
atof() Converts a string to a double-precision floating-point value.
atoi() Converts a string to an integer.
atol() Converts a string to a long integer.
strtod() Converts a string to a double-precision floating-point value and reports any “leftover” numbers that could not be converted.
strtol() Converts a string to a long integer and reports any “leftover” numbers that could not be converted.
strtoul() Converts a string to an unsigned long integer and reports any “leftover” numbers that could not be converted.
What is a null pointer?
There are times when it’s necessary to have a pointer that doesn’t point to anything. The macro NULL, defined in , has a value that’s guaranteed to be different from any valid pointer. NULL is a literal zero, possibly cast to void* or char*. Some people, notably C++ programmers, prefer to use 0 rather than NULL.
The null pointer is used in three ways:
1) To stop indirection in a recursive data structure
2) As an error value
3) As a sentinel value
When should a type cast be used?
There are two situations in which to use a type cast. The first use is to change the type of an operand to an arithmetic operation so that the operation will be performed properly.
The second case is to cast pointer types to and from void * in order to interface with functions that expect or return void pointers. For example, the following line type casts the return value of the call to malloc() to be a pointer to a foo structure.
struct foo *p = (struct foo *) malloc(sizeof(struct foo));
Can include files be nested?
Yes. Include files can be nested any number of times. As long as you use precautionary measures , you can avoid including the same file twice. In the past, nesting header files was seen as bad programming practice, because it complicates the dependency tracking function of the MAKE program and thus slows down compilation. Many of today’s popular compilers make up for this difficulty by implementing a concept called precompiled headers, in which all headers and associated dependencies are stored in
a precompiled state.
Many programmers like to create a custom header file that has #include statements for every header needed for each module. This is perfectly acceptable and can help avoid potential problems relating to #include files, such as accidentally omitting an #include file in a module.
How can you check to see whether a symbol is defined?
You can use the #ifdef and #ifndef preprocessor directives to check whether a symbol has been defined (#ifdef) or whether it has not been defined (#ifndef).
How do you override a defined macro?
You can use the #undef preprocessor directive to undefine (override) a previously defined macro.
What is hashing?
To hash means to grind up, and that’s essentially what hashing is all about. The heart of a hashing algorithm is a hash function that takes your nice, neat data and grinds it into some random-looking integer.
The idea behind hashing is that some data either has no inherent ordering (such as images) or is expensive to compare (such as images). If the data has no inherent ordering, you can’t perform comparison searches.
If the data is expensive to compare, the number of comparisons used even by a binary search might be too many. So instead of looking at the data themselves, you’ll condense (hash) the data to an integer (its hash value) and keep all the data with the same hash value in the same place. This task is carried out by using the hash value as an index into an array.
To search for an item, you simply hash it and look at all the data whose hash values match that of the data you’re looking for. This technique greatly lessens the number of items you have to look at. If the parameters are set up with care and enough storage is available for the hash table, the number of comparisons needed to find an item can be made arbitrarily close to one.
One aspect that affects the efficiency of a hashing implementation is the hash function itself. It should ideally distribute data randomly throughout the entire hash table, to reduce the likelihood of collisions. Collisions occur when two different keys have the same hash value. There are two ways to resolve this problem. In “open addressing,” the collision is resolved by the choosing of another position in the hash table for the element inserted later. When the hash table is searched, if the entry is not found at its
hashed position in the table, the search continues checking until either the element is found or an empty position in the table is found
The second method of resolving a hash collision is called “chaining.” In this method, a “bucket” or linked list holds all the elements whose keys hash to the same value.
When the hash table is searched, the list must be searched linearly.
What is the benefit of using an enum rather than a #define constant?
The use of an enumeration constant (enum) has many advantages over using the traditional symbolic constant style of #define. These advantages include a lower maintenance requirement, improved program readability, and better debugging capability.
1) The first advantage is that enumerated constants are generated automatically by the compiler. Conversely, symbolic constants must be manually assigned values by the Programmer..
For instance, if you had an enumerated constant type for error codes that could occur in your program, your enum definition could look something like this:
enum Error_Code
{
OUT_OF_MEMORY,
INSUFFICIENT_DISK_SPACE,
LOGIC_ERROR,
FILE_NOT_FOUND
};
In the preceding example, OUT_OF_MEMORY is automatically assigned the value of 0 (zero) by the compiler because it appears first in the definition. The compiler then continues to automatically assign numbers to the enumerated constants, making INSUFFICIENT_DISK_SPACE equal to 1, LOGIC_ERROR equal to 2, and FILE_NOT_FOUND equal to 3, so on.
If you were to approach the same example by using symbolic constants, your code would look something like this:
#define OUT_OF_MEMORY 0
#define INSUFFICIENT_DISK_SPACE 1
#define LOGIC_ERROR 2
#define FILE_NOT_FOUND 3
values by the programmer. Each of the two methods arrives at the same result: four constants assigned numeric values to represent error codes. Consider the maintenance required, however, if you were to add two constants to represent the error codes DRIVE_NOT_READY and CORRUPT_FILE. Using the enumeration constant method, you simply would put these two constants anywhere in the enum definition. The compiler would generate two unique values for these constants. Using the symbolic constant method, you would have to manually assign two new numbers to these constants. Additionally, you would want to ensure that the numbers you assign to these constants are unique.
2) Another advantage of using the enumeration constant method is that your programs are more readable and thus can be understood better by others who might have to update your program later.
3) A third advantage to using enumeration constants is that some symbolic debuggers can print the value of an enumeration constant. Conversely, most symbolic debuggers cannot print the value of a symbolic constant. This can be an enormous help in debugging your program, because if your program is stopped at a line that uses an enum, you can simply inspect that constant and instantly know its value. On the other hand, because most debuggers cannot print #define values, you would most likely have to search for that value by manually looking it up in a header file.
How’s method overriding different from overloading?
When overriding, you change the method behavior for a derived class. Overloading simply involves having a method with the same name within the class.
Wednesday, October 8, 2008
INTERVIEW QUESTIONS & ANSWERS FROM C, C ++ part II
Difference between C structure and C++ structure?
C++ places greater emphasis on type checking, compiler can diagnose every diff between C and C++
1. structures are a way of storing many different values in variables of potentially diff types under under the same name
2. classes and structures make program modular, easier to modify make things compact
3. useful when a lot of data needs to be grouped together
4. struct Tag {…}struct example {Int x;}example ex; ex.x = 33; //accessing variable of structure
5. members of a struct in C are by default public, in C++ private
6. unions like structs except they share memory – allocates largest data type in memory - like a giant storage: store one small OR one large but never both at the same time
7. pointers can point to struct:
8. C++ can use class instead of struct (almost the same thing) - difference: C++ classes can include functions as members
9. members can be declared as: private: members of a class are accessible only from other members of their same class; protected: members are accessible from members of their same class and friend classes and also members of their derived classes; public: members are accessible from anywhere the class is visible
10. structs usually used for data only structures, classes for classes that have procedures and member functions
11. use private because in large projects important that values not be modified in an unexpected way from the POV of the object
12. advantage of class declare several diff objects from it, each object of Rect has its own variable x, y AND its own functions
13. concept of OO programming: data and functions are properties of the object instead of the usual view of objects as function parameters in structured programming
Difference between calloc and malloc?
malloc: allocate n bytes
calloc: allocate m times n bytes initialized to 0
Difference between printf and sprintf?
sprintf: a function that puts together a string, output goes to an array of char instead of stdout
printf: prints to stdout
What is the difference between = and == in C?
“=” is an assignment operator while, “==” is comparative operator
a=b; //b’s value is stored in a
if( a==b) // a’s value is compared with b
What is the difference between superclass and subclass?
A super class is a class that is inherited whereas sub class is a class that does the inheriting.
What is difference between overloading and overriding?
a) In overloading, there is a relationship between methods available in the same class whereas in overriding, there is relationship between a superclass method and subclass method.
b) Overloading does not block inheritance from the superclass whereas overriding blocks inheritance from the superclass.
c) In overloading, separate methods share the same name whereas in overriding, subclass method replaces the superclass.
d) Overloading must have different method signatures whereas overriding must have same signature.
How many ways can an argument be passed to a subroutine?
An argument can be passed in two ways. They are Pass by Value and Passing by Reference.
Passing by value: This method copies the value of an argument into the formal parameter of the subroutine.
Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to the parameter.
What do you mean by multiple inheritance in C++ ?
Multiple inheritance is a feature in C++ by which one class can be of different types. Say class teachingAssistant is inherited from two classes say teacher and Student.
What do you mean by virtual methods?
Virtual Methods are used to use the polymorphisms feature in C++. Say class A is inherited from class B. If we declare say function f() as virtual in class B and override the same function in class A then at runtime appropriate method of the class will be called depending upon the type of the object.
How many ways are there to initialize an int with a constant?
Two.
There are two formats for initializers in C++ as shown in the example that follows. The first format uses the traditional C notation. The second format uses constructor notation.
int foo = 123;
int bar (123);
Explain the scope resolution operator.
It permits a program to reference an identifier in the global scope that has been hidden by another identifier with the same name in the local scope.
In C, why is the void pointer useful? When would you use it?
The void pointer is useful becuase it is a generic pointer that any pointer can be cast into and back again without loss of information.
In C, what is the difference between a static variable and global variable?
A static variable declared outside of any function is accessible only to all the functions defined in the same file (as the static variable). However, a global variable can be accessed by any function (including the ones from different files).
What methods can be overridden in Java?
In C++ terminology, all public methods in Java are virtual. Therefore, all Java methods can be overwritten in subclasses except those that are declared final, static, and private.
In C++, what is the difference between method overloading and method overriding?
Overloading a method (or function) in C++ is the ability for functions of the same name to be defined as long as these methods have different signatures (different set of parameters). Method overriding is the ability of the inherited class rewriting the virtual method of the base class.
What is pure virtual function?
A class is made abstract by declaring one or more of its virtual functions to be pure. A pure virtual function is one with an initializer of = 0 in its declaration
How can you tell what shell you are running on UNIX system?
You can do the Echo $RANDOM. It will return a undefined variable if you are from the C-Shell, just a return prompt if you are from the Bourne shell, and a 5 digit random numbers if you are from the Korn shell. You could also do a ps -l and look for the shell with the highest PID.
How do you find out if a linked-list has an end? (i.e. the list is not a cycle)
You can find out by using 2 pointers. One of them goes 2 nodes each time. The second one goes at 1 nodes each time. If there is a cycle, the one that goes 2 nodes each time will eventually meet the one that goes slower. If that is the case, then you will know the linked-list is a cycle.
What is polymorphism?
Polymorphism is the idea that a base class can be inherited by several classes. A base class pointer can point to its child class and a base class array can store different child class objects.
How do you write a function that can reverse a linked-list?
void reverselist(void)
{
if(head==0)
return;
if(head->next==0)
return;
if(head->next==tail)
{
head->next = 0;
tail->next = head;
}
else
{
node* pre = head;
node* cur = head->next;
node* curnext = cur->next;
head->next = 0;
cur->next = head;
for(; curnext!=0; )
{
cur->next = pre;
pre = cur;
cur = curnext;
curnext = curnext->next;
}
curnext->next = cur;
}
}
Write a fucntion that will reverse a string.
char *strrev(char *s)
{
int i = 0, len = strlen(s);
char *str;
if ((str = (char *)malloc(len+1)) == NULL) /*cannot allocate memory */
err_num = 2;
return (str);
}
while(len)
str[i++]=s[?len];
str[i] = NULL;
return (str);
}
What is the difference between Stack and Queue?
Stack is a Last In First Out (LIFO) data structure.
Queue is a First In First Out (FIFO) data structure
Suppose a 3-bit sequence number is used in the selective-reject ARQ, what is the maximum number of frames that could be transmitted at a time?
If a 3-bit sequence number is used, then it could distinguish 8 different frames. Since the number of frames that could be transmitted at a time is no greater half the numner of frames that could be distinguished by the sequence number, so at most 4 frames can be transmitted at a time.
Describe Stacks and name a couple of places where stacks are useful.
A Stack is a linear structure in which insertions and deletions are always made at one end, called the top. This updating policy is called last in, first out (LIFO). It is useful when we need to check some syntex errors, such as missing parentheses.
Describe one simple rehashing policy.
The simplest rehashing policy is linear probing. Suppose a key K hashes to location i. Suppose other key occupies H[i]. The following function is used to generate alternative locations:
rehash(j) = (j + 1) mod h
where j is the location most recently probed. Initially j = i, the hash code for K. Notice that this version of rehash does not depend on K.
Write the psuedo code for the Depth first Search.
dfs(G, v) //OUTLINE
Mark v as “discovered”
For each vertex w such that edge vw is in G:
If w is undiscovered:
dfs(G, w); that is, explore vw, visit w, explore from there
as much as possible, and backtrack from w to v.
Otherwise:
“Check” vw without visiting w.
Mark v as “finished”.
What are the advantages and disadvantages of B-star trees over Binary trees?
B-star trees have better data structure and are faster in search than Binary trees, but it’s harder to write codes for B-start trees.
What is an HTML tag?
An HTML tag is a syntactical construct in the HTML language that abbreviates specific instructions to be executed when the HTML script is loaded into a Web browser. It is like a method in Java, a function in C++, a procedure in Pascal, or a subroutine in FORTRAN.
Why are arrays usually processed with for loop?
The real power of arrays comes from their facility of using an index variable to traverse the array, accessing each element with the same expression a[i]. All that is needed to make this work is a iterated statement in which the variable i serves as a counter, incrementing from 0 to a.length -1. That is exactly what a loop does.
What is the difference between an external iterator and an internal iterator? Describe an advantage of an external iterator.
An internal iterator is implemented with member functions of the class that has items to step through. .An external iterator is implemented as a separate class that can be “attach” to the object that has items to step through. .An external iterator has the advantage that many difference iterators can be active simultaneously on the same object
Which recursive sorting technique always makes recursive calls to sort subarrays that are about half size of the original array?
Mergesort always makes recursive calls to sort subarrays that are about half size of the original array, resulting in O(n log n) time.
Suppose that data is an array of 1000 integers. Write a single function call that will sort the 100 elements data [222] through data [321].
quicksort ((data + 222), 100);
Tell how to check whether a linked list is circular.
Create two pointers, each set to the start of the list. Update each as follows:
while (pointer1)
{
pointer1 = pointer1->next;
pointer2 = pointer2->next; if (pointer2) pointer2=pointer2->next;
if (pointer1 == pointer2)
{
print (\”circular\n\”);
}
}
Differentiate between functions read() and write().
Functions read() and write() are used for reading and writing blocks of binary data to a file. Their prototypes are
istream &read(unsigned char *buf, int num);
ostream &write(const unsigned char *buf, int num);
The read() function reads num bytes from the associated stream and puts them in the buffer pointed to by buf. The write() function writes num bytes to the associated stream from the buffer pointed to by buf.
What do you understand by visibility modes in class derivations? What are these modes?
The visibility mode determines the availability of the members of a base class to the deriving class (and other classes, functions and the main function) from the derived class. There are three visibility modes:
(i) Public (ii) Private (iii) Protected
What are proxy objects?
Objects that stand for other objects are called proxy objects or surrogates.
template
class Array2D
{
public:
class Array1D
{
public:
T& operator[] (int index);
const T& operator[] (int index)const;
};
Array1D operator[] (int index);
const Array1D operator[] (int index) const;
};
The following then becomes legal:
Array2Ddata(10,20);
cout<<>
Here data[3] yields an Array1D object and the operator [] invocation on that object yields the float in position(3,6) of the original two dimensional array. Clients of the Array2D class need not be aware of the presence of the Array1D class. Objects of this latter class stand for one-dimensional array objects that, conceptually, do not exist for clients of Array2D. Such clients program as if they were using real, live, two-dimensional arrays. Each Array1D object stands for a one-dimensional array that is absent from a conceptual model used by the clients of Array2D. In the above example, Array1D is a proxy class. Its instances stand for one-dimensional arrays that, conceptually, do not exist.
What are the conditions that have to be met for a condition to be an invariant of the class?
The condition should hold at the end of every constructor.
The condition should hold at the end of every mutator (non-const) operation.
What is a Null object?
It is an object of some class whose purpose is to indicate that a real object of that class does not exist. One common use for a null object is a return value from a member function that is supposed to return an object with some specified properties but cannot find such an object.
When does a name clash occur?
A name clash occurs when a name is defined in more than one place. For example., two different class libraries could give two different classes the same name. If you try to use many class libraries at the same time, there is a fair chance that you will be unable to compile or link the program because of name clashes.
Why does the function arguments are called as “signatures”?
The arguments distinguish functions with the same name (functional polymorphism). The name alone does not necessarily identify a unique function. However, the name and its arguments (signatures) will uniquely identify a function.
In real life we see suppose, in class there are two guys with same name, but they can be easily identified by their signatures. The same concept is applied here.
ex:
class person
{
public:
char getsex();
void setsex(char);
void setsex(int);
};
In the above example we see that there is a function setsex() with same name but with different signature.
Suppose a class acts an Actor in the problem domain, how to represent it in the static model?
In this scenario you can use “stereotype”. Since stereotype is just a string that gives extra semantic to the particular entity/model element. It is given with in the << >>.
class A
<>
attributes
methods.
What is guard condition?
Guard condition is one, which acts as a firewall. The access from a particular object can be made only when the particular condition is met.
For Example:- customer check customer number ATM.
Here the object on the customer accesses the ATM facility only when the guard condition is met.
What is the difference between an argument and a parameter?
While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.
What is an Object and how do you allocate memory to it?
Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data. When an object is created using new operator, memory is allocated to it.
what is the difference between Procedural and OOPs?
a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOPs program, unit of program is object, which is nothing but combination of data and code.
b) In procedural program,data is exposed to the whole program whereas in OOPs program,it is accessible within the object and which in turn assures the security of the code.
What are wrapper classes?
Wrapper classes are classes that allow primitive types to be accessed as objects.
Can the size of an array be declared at runtime?
No. In an array declaration, the size must be known at compile time. You can’t specify a size that’s known only at runtime. For example, if i is a variable, you can’t write code like this:
char array[i]; /* not valid C */
Some languages provide this latitude. C doesn’t. If it did, the stack would be more complicated, function calls would be more expensive, and programs would run a lot slower. If you know that you have an array but you won’t know until runtime how big it will be, declare a pointer to it and use malloc() or calloc() to allocate the array from the heap.
C++ places greater emphasis on type checking, compiler can diagnose every diff between C and C++
1. structures are a way of storing many different values in variables of potentially diff types under under the same name
2. classes and structures make program modular, easier to modify make things compact
3. useful when a lot of data needs to be grouped together
4. struct Tag {…}struct example {Int x;}example ex; ex.x = 33; //accessing variable of structure
5. members of a struct in C are by default public, in C++ private
6. unions like structs except they share memory – allocates largest data type in memory - like a giant storage: store one small OR one large but never both at the same time
7. pointers can point to struct:
8. C++ can use class instead of struct (almost the same thing) - difference: C++ classes can include functions as members
9. members can be declared as: private: members of a class are accessible only from other members of their same class; protected: members are accessible from members of their same class and friend classes and also members of their derived classes; public: members are accessible from anywhere the class is visible
10. structs usually used for data only structures, classes for classes that have procedures and member functions
11. use private because in large projects important that values not be modified in an unexpected way from the POV of the object
12. advantage of class declare several diff objects from it, each object of Rect has its own variable x, y AND its own functions
13. concept of OO programming: data and functions are properties of the object instead of the usual view of objects as function parameters in structured programming
Difference between calloc and malloc?
malloc: allocate n bytes
calloc: allocate m times n bytes initialized to 0
Difference between printf and sprintf?
sprintf: a function that puts together a string, output goes to an array of char instead of stdout
printf: prints to stdout
What is the difference between = and == in C?
“=” is an assignment operator while, “==” is comparative operator
a=b; //b’s value is stored in a
if( a==b) // a’s value is compared with b
What is the difference between superclass and subclass?
A super class is a class that is inherited whereas sub class is a class that does the inheriting.
What is difference between overloading and overriding?
a) In overloading, there is a relationship between methods available in the same class whereas in overriding, there is relationship between a superclass method and subclass method.
b) Overloading does not block inheritance from the superclass whereas overriding blocks inheritance from the superclass.
c) In overloading, separate methods share the same name whereas in overriding, subclass method replaces the superclass.
d) Overloading must have different method signatures whereas overriding must have same signature.
How many ways can an argument be passed to a subroutine?
An argument can be passed in two ways. They are Pass by Value and Passing by Reference.
Passing by value: This method copies the value of an argument into the formal parameter of the subroutine.
Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to the parameter.
What do you mean by multiple inheritance in C++ ?
Multiple inheritance is a feature in C++ by which one class can be of different types. Say class teachingAssistant is inherited from two classes say teacher and Student.
What do you mean by virtual methods?
Virtual Methods are used to use the polymorphisms feature in C++. Say class A is inherited from class B. If we declare say function f() as virtual in class B and override the same function in class A then at runtime appropriate method of the class will be called depending upon the type of the object.
How many ways are there to initialize an int with a constant?
Two.
There are two formats for initializers in C++ as shown in the example that follows. The first format uses the traditional C notation. The second format uses constructor notation.
int foo = 123;
int bar (123);
Explain the scope resolution operator.
It permits a program to reference an identifier in the global scope that has been hidden by another identifier with the same name in the local scope.
In C, why is the void pointer useful? When would you use it?
The void pointer is useful becuase it is a generic pointer that any pointer can be cast into and back again without loss of information.
In C, what is the difference between a static variable and global variable?
A static variable declared outside of any function is accessible only to all the functions defined in the same file (as the static variable). However, a global variable can be accessed by any function (including the ones from different files).
What methods can be overridden in Java?
In C++ terminology, all public methods in Java are virtual. Therefore, all Java methods can be overwritten in subclasses except those that are declared final, static, and private.
In C++, what is the difference between method overloading and method overriding?
Overloading a method (or function) in C++ is the ability for functions of the same name to be defined as long as these methods have different signatures (different set of parameters). Method overriding is the ability of the inherited class rewriting the virtual method of the base class.
What is pure virtual function?
A class is made abstract by declaring one or more of its virtual functions to be pure. A pure virtual function is one with an initializer of = 0 in its declaration
How can you tell what shell you are running on UNIX system?
You can do the Echo $RANDOM. It will return a undefined variable if you are from the C-Shell, just a return prompt if you are from the Bourne shell, and a 5 digit random numbers if you are from the Korn shell. You could also do a ps -l and look for the shell with the highest PID.
How do you find out if a linked-list has an end? (i.e. the list is not a cycle)
You can find out by using 2 pointers. One of them goes 2 nodes each time. The second one goes at 1 nodes each time. If there is a cycle, the one that goes 2 nodes each time will eventually meet the one that goes slower. If that is the case, then you will know the linked-list is a cycle.
What is polymorphism?
Polymorphism is the idea that a base class can be inherited by several classes. A base class pointer can point to its child class and a base class array can store different child class objects.
How do you write a function that can reverse a linked-list?
void reverselist(void)
{
if(head==0)
return;
if(head->next==0)
return;
if(head->next==tail)
{
head->next = 0;
tail->next = head;
}
else
{
node* pre = head;
node* cur = head->next;
node* curnext = cur->next;
head->next = 0;
cur->next = head;
for(; curnext!=0; )
{
cur->next = pre;
pre = cur;
cur = curnext;
curnext = curnext->next;
}
curnext->next = cur;
}
}
Write a fucntion that will reverse a string.
char *strrev(char *s)
{
int i = 0, len = strlen(s);
char *str;
if ((str = (char *)malloc(len+1)) == NULL) /*cannot allocate memory */
err_num = 2;
return (str);
}
while(len)
str[i++]=s[?len];
str[i] = NULL;
return (str);
}
What is the difference between Stack and Queue?
Stack is a Last In First Out (LIFO) data structure.
Queue is a First In First Out (FIFO) data structure
Suppose a 3-bit sequence number is used in the selective-reject ARQ, what is the maximum number of frames that could be transmitted at a time?
If a 3-bit sequence number is used, then it could distinguish 8 different frames. Since the number of frames that could be transmitted at a time is no greater half the numner of frames that could be distinguished by the sequence number, so at most 4 frames can be transmitted at a time.
Describe Stacks and name a couple of places where stacks are useful.
A Stack is a linear structure in which insertions and deletions are always made at one end, called the top. This updating policy is called last in, first out (LIFO). It is useful when we need to check some syntex errors, such as missing parentheses.
Describe one simple rehashing policy.
The simplest rehashing policy is linear probing. Suppose a key K hashes to location i. Suppose other key occupies H[i]. The following function is used to generate alternative locations:
rehash(j) = (j + 1) mod h
where j is the location most recently probed. Initially j = i, the hash code for K. Notice that this version of rehash does not depend on K.
Write the psuedo code for the Depth first Search.
dfs(G, v) //OUTLINE
Mark v as “discovered”
For each vertex w such that edge vw is in G:
If w is undiscovered:
dfs(G, w); that is, explore vw, visit w, explore from there
as much as possible, and backtrack from w to v.
Otherwise:
“Check” vw without visiting w.
Mark v as “finished”.
What are the advantages and disadvantages of B-star trees over Binary trees?
B-star trees have better data structure and are faster in search than Binary trees, but it’s harder to write codes for B-start trees.
What is an HTML tag?
An HTML tag is a syntactical construct in the HTML language that abbreviates specific instructions to be executed when the HTML script is loaded into a Web browser. It is like a method in Java, a function in C++, a procedure in Pascal, or a subroutine in FORTRAN.
Why are arrays usually processed with for loop?
The real power of arrays comes from their facility of using an index variable to traverse the array, accessing each element with the same expression a[i]. All that is needed to make this work is a iterated statement in which the variable i serves as a counter, incrementing from 0 to a.length -1. That is exactly what a loop does.
What is the difference between an external iterator and an internal iterator? Describe an advantage of an external iterator.
An internal iterator is implemented with member functions of the class that has items to step through. .An external iterator is implemented as a separate class that can be “attach” to the object that has items to step through. .An external iterator has the advantage that many difference iterators can be active simultaneously on the same object
Which recursive sorting technique always makes recursive calls to sort subarrays that are about half size of the original array?
Mergesort always makes recursive calls to sort subarrays that are about half size of the original array, resulting in O(n log n) time.
Suppose that data is an array of 1000 integers. Write a single function call that will sort the 100 elements data [222] through data [321].
quicksort ((data + 222), 100);
Tell how to check whether a linked list is circular.
Create two pointers, each set to the start of the list. Update each as follows:
while (pointer1)
{
pointer1 = pointer1->next;
pointer2 = pointer2->next; if (pointer2) pointer2=pointer2->next;
if (pointer1 == pointer2)
{
print (\”circular\n\”);
}
}
Differentiate between functions read() and write().
Functions read() and write() are used for reading and writing blocks of binary data to a file. Their prototypes are
istream &read(unsigned char *buf, int num);
ostream &write(const unsigned char *buf, int num);
The read() function reads num bytes from the associated stream and puts them in the buffer pointed to by buf. The write() function writes num bytes to the associated stream from the buffer pointed to by buf.
What do you understand by visibility modes in class derivations? What are these modes?
The visibility mode determines the availability of the members of a base class to the deriving class (and other classes, functions and the main function) from the derived class. There are three visibility modes:
(i) Public (ii) Private (iii) Protected
What are proxy objects?
Objects that stand for other objects are called proxy objects or surrogates.
template
class Array2D
{
public:
class Array1D
{
public:
T& operator[] (int index);
const T& operator[] (int index)const;
};
Array1D operator[] (int index);
const Array1D operator[] (int index) const;
};
The following then becomes legal:
Array2Ddata(10,20);
cout<<>
Here data[3] yields an Array1D object and the operator [] invocation on that object yields the float in position(3,6) of the original two dimensional array. Clients of the Array2D class need not be aware of the presence of the Array1D class. Objects of this latter class stand for one-dimensional array objects that, conceptually, do not exist for clients of Array2D. Such clients program as if they were using real, live, two-dimensional arrays. Each Array1D object stands for a one-dimensional array that is absent from a conceptual model used by the clients of Array2D. In the above example, Array1D is a proxy class. Its instances stand for one-dimensional arrays that, conceptually, do not exist.
What are the conditions that have to be met for a condition to be an invariant of the class?
The condition should hold at the end of every constructor.
The condition should hold at the end of every mutator (non-const) operation.
What is a Null object?
It is an object of some class whose purpose is to indicate that a real object of that class does not exist. One common use for a null object is a return value from a member function that is supposed to return an object with some specified properties but cannot find such an object.
When does a name clash occur?
A name clash occurs when a name is defined in more than one place. For example., two different class libraries could give two different classes the same name. If you try to use many class libraries at the same time, there is a fair chance that you will be unable to compile or link the program because of name clashes.
Why does the function arguments are called as “signatures”?
The arguments distinguish functions with the same name (functional polymorphism). The name alone does not necessarily identify a unique function. However, the name and its arguments (signatures) will uniquely identify a function.
In real life we see suppose, in class there are two guys with same name, but they can be easily identified by their signatures. The same concept is applied here.
ex:
class person
{
public:
char getsex();
void setsex(char);
void setsex(int);
};
In the above example we see that there is a function setsex() with same name but with different signature.
Suppose a class acts an Actor in the problem domain, how to represent it in the static model?
In this scenario you can use “stereotype”. Since stereotype is just a string that gives extra semantic to the particular entity/model element. It is given with in the << >>.
class A
<>
attributes
methods.
What is guard condition?
Guard condition is one, which acts as a firewall. The access from a particular object can be made only when the particular condition is met.
For Example:- customer check customer number ATM.
Here the object on the customer accesses the ATM facility only when the guard condition is met.
What is the difference between an argument and a parameter?
While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.
What is an Object and how do you allocate memory to it?
Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data. When an object is created using new operator, memory is allocated to it.
what is the difference between Procedural and OOPs?
a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOPs program, unit of program is object, which is nothing but combination of data and code.
b) In procedural program,data is exposed to the whole program whereas in OOPs program,it is accessible within the object and which in turn assures the security of the code.
What are wrapper classes?
Wrapper classes are classes that allow primitive types to be accessed as objects.
Can the size of an array be declared at runtime?
No. In an array declaration, the size must be known at compile time. You can’t specify a size that’s known only at runtime. For example, if i is a variable, you can’t write code like this:
char array[i]; /* not valid C */
Some languages provide this latitude. C doesn’t. If it did, the stack would be more complicated, function calls would be more expensive, and programs would run a lot slower. If you know that you have an array but you won’t know until runtime how big it will be, declare a pointer to it and use malloc() or calloc() to allocate the array from the heap.
Sunday, October 5, 2008
INTERVIEW QUESTIONS & ANSWERS FROM C, C ++ Part-1
Here are some useful materials for the software aspirants that I would like to share.These are some of the most likely asked interview questions in a software company.
What is the use of ‘using’ declaration?
A using declaration makes it possible to use a name from a namespace without the scope operator.
What problem does the namespace feature solve?
Multiple providers of libraries might use common global identifiers causing a name collision when an application tries to link with two or more such libraries. The namespace feature surrounds a library’s external declarations with a unique namespace that eliminates the potential for those collisions.
namespace [identifier] { namespace-body }
A namespace declaration identifies and assigns a name to a declarative region. The identifier in a namespace declaration must be unique in the declarative region in which it is used. The identifier is the name of the namespace and is used to reference its members.
What is Memory alignment?
The term alignment primarily means the tendency of an address pointer value to be a multiple of some power of two. So a pointer with two byte alignment has a zero in the least significant bit. And a pointer with four byte alignment has a zero in both the two least significant bits. And so on. More alignment means a longer sequence of zero bits in the lowest bits of a pointer.
What is virtual function?
When derived class overrides the base class method by redefining the same function, then if client wants to access redefined the method from derived class through a pointer from base class object, then you must define this function in base class as virtual function.
class parent
{
void Show()
{
cout << "i'm parent" << parent_object_ptr =" new">
parent_object_ptr->show() // calls parent->show() i
now we goto virtual world…
class parent
{
virtual void Show()
{
cout << "i'm parent" << parent_object_ptr =" new">show() // calls child->show()
When do use “const” reference arguments in function?
a) Using const protects you against programming errors that inadvertently alter data.
b) Using const allows function to process both const and non-const actual arguments, while a function without const in the prototype can only accept non constant arguments.
c) Using a const reference allows the function to generate and use a temporary variable appropriately.
What is passing by reference?
Method of passing arguments to a function which takes parameter of type reference.
for example:
void swap( int & x, int & y )
{
int temp = x;
x = y;
y = x;
}
int a=2, b=3;
swap( a, b );
Basically, inside the function there won’t be any copy of the arguments “x” and “y” instead they refer to original variables a and b. so no extra memory needed to pass arguments and it is more efficient.
What is reference ?
Reference is a name that acts as an alias, or alternative name, for a previously defined variable or an object. prepending variable with “&” symbol makes it as reference.
for example:
int a;
int &b = a;
What are C++ storage classes?
C++ Storage Classes:
auto: the default. Variables are automatically created and initialized when they are defined
and are destroyed at the end of the block containing their definition. They are not visible
outside that block
register: a type of auto variable. a suggestion to the compiler to use a CPU register for
performance
static: a variable that is known only in the function that contains its definition but is
never destroyed and retains its value between calls to that function. It exists from the
time the program begins execution
extern: a static variable whose definition and placement is determined when all object and
library modules are combined (linked) to form the executable code file. It can be visible
outside the file where it is defined.
What is conversion operator?
Class can have a public method for specific data type conversions.
for example:
class Boo
{
double value;
public:
Boo(int i )
operator double()
{
return value;
}
};
Boo BooObject;
double i = BooObject; // assigning object to variable i of type double. now conversion
How can a ‘::’ operator be used as unary operator?
The scope operator can be operator gets called to assign the value. used to refer to members of the global namespace. Because the global namespace doesn’t have a name, the notation :: member-name refers to a member of the global namespace. This can be useful for referring to members of global namespace whose names have been hidden by names declared in nested local scope. Unless we specify to the compiler in which namespace to search for a declaration, the compiler simple searches the current scope, and any scopes in which the current scope is nested, to find the declaration for the name.
Describe the main characteristics of static functions.
The main characteristics of static functions include,
It is without the a this pointer,
It can’t directly access the non-static members of its class
It can’t be declared const, volatile or virtual.
It doesn’t need to be invoked through an object of its class, although for convenience, it may.
What is name mangling?
Name mangling is the process through which your c++ compilers give each function in your program a unique name. In C++, all programs have at-least a few functions with the same name. Name mangling is a concession to the fact that linker always insists on all function names being unique.
Example:
In general, member names are made unique by concatenating the name of the member with that of the class e.g. given the declaration:
class Bar
{
public:
int ival;
…
};
ival becomes something like:
// a possible member name mangling
ival__3Bar
Consider this derivation:
class Foo : public Bar
{
public:
int ival;
…
}
The internal representation of a Foo object is the concatenation of its base and derived class members.
// Pseudo C++ code
// Internal representation of Foo
class Foo
{
public:
int ival__3Bar;
int ival__3Foo;
…
};
Unambiguous access of either ival members is achieved through name mangling. Member functions, because they can be overloaded, require an extensive mangling to provide each with a unique name. Here the compiler generates the same name for the two overloaded instances(Their argument lists make their instances unique).
What is slicing?
Slicing means that the data added by a subclass are discarded when an object of the subclass is passed or returned by value or from a function expecting a base class object.
Explanation:
Consider the following class declaration:
class base
{
…
base& operator =(const base&);
base (const base&);
}
void fun( )
{
base e=m;
e=m;
}
As base copy functions don’t know anything about the derived only the base part of the derived is copied. This is commonly referred to as slicing. One reason to pass objects of classes in a hierarchy is to avoid slicing. Other reasons are to preserve polymorphic behavior and to gain efficiency.
Why doesn’t C have nested functions?
C Compilers do not allow class types so they do not support internal function reference table like a C++ compiler does. Instead, you can use a structure having members function pointers. Older C++ compilers use to transform the C++ sources to C using this isomorphic transformation.
Difference between overloading and overriding?
1. Overload - two functions that appear in the same scope are overloaded if they have the same name but have different parameter list
2. main() cannot be overloaded
3. notational convenience - compiler invokes the functions that is the best match on the args – found by finding the best match between the type of arg expr and parameter
4. if declare a function locally, that function hides rather than overload the same function declared in an outer scope
5. Overriding - the ability of the inherited class rewriting the virtual method of a base class - a method which completely replaces base class FUNCTIONALITY in subclass
6. the overriding method in the subclass must have exactly the same signature as the function of the base class it is replacing - replacement of a method in a child class
7. writing a different body in a derived class for a function defined in a base class, ONLY if the function in the base class is virtual and ONLY if the function in the derived class has the same signature
8. all functions in the derived class hide the base class functions with the same name except in the case of a virtual functions which override the base class functions with the same signature
What is Constructor ? How it is called ?
Constructor is a member function of the class, with the name of the function being the same as the class name. It also specifies how the object should be initialized.Ways of calling constructor:
1) Implicitly: automatically by complier when an object is created.
2) Calling the constructors explicitly is possible, but it makes the code unverifiable.
What are inline functions?
1. Treated like macro definitions by C++ compiler.
2. Meant to be used if there’s a need to repetitively execute a small block if code which is smaller.
3. Always evaluates every argument once.
4. Defined in header file.
5. Avoids function call overload because calling a function is slower than evaluating the equivalent expression.
6. It’s a request to the compiler, the compiler can ignore the request.
What is the use of ‘using’ declaration?
A using declaration makes it possible to use a name from a namespace without the scope operator.
What problem does the namespace feature solve?
Multiple providers of libraries might use common global identifiers causing a name collision when an application tries to link with two or more such libraries. The namespace feature surrounds a library’s external declarations with a unique namespace that eliminates the potential for those collisions.
namespace [identifier] { namespace-body }
A namespace declaration identifies and assigns a name to a declarative region. The identifier in a namespace declaration must be unique in the declarative region in which it is used. The identifier is the name of the namespace and is used to reference its members.
What is Memory alignment?
The term alignment primarily means the tendency of an address pointer value to be a multiple of some power of two. So a pointer with two byte alignment has a zero in the least significant bit. And a pointer with four byte alignment has a zero in both the two least significant bits. And so on. More alignment means a longer sequence of zero bits in the lowest bits of a pointer.
What is virtual function?
When derived class overrides the base class method by redefining the same function, then if client wants to access redefined the method from derived class through a pointer from base class object, then you must define this function in base class as virtual function.
class parent
{
void Show()
{
cout << "i'm parent" << parent_object_ptr =" new">
parent_object_ptr->show() // calls parent->show() i
now we goto virtual world…
class parent
{
virtual void Show()
{
cout << "i'm parent" << parent_object_ptr =" new">show() // calls child->show()
When do use “const” reference arguments in function?
a) Using const protects you against programming errors that inadvertently alter data.
b) Using const allows function to process both const and non-const actual arguments, while a function without const in the prototype can only accept non constant arguments.
c) Using a const reference allows the function to generate and use a temporary variable appropriately.
What is passing by reference?
Method of passing arguments to a function which takes parameter of type reference.
for example:
void swap( int & x, int & y )
{
int temp = x;
x = y;
y = x;
}
int a=2, b=3;
swap( a, b );
Basically, inside the function there won’t be any copy of the arguments “x” and “y” instead they refer to original variables a and b. so no extra memory needed to pass arguments and it is more efficient.
What is reference ?
Reference is a name that acts as an alias, or alternative name, for a previously defined variable or an object. prepending variable with “&” symbol makes it as reference.
for example:
int a;
int &b = a;
What are C++ storage classes?
C++ Storage Classes:
auto: the default. Variables are automatically created and initialized when they are defined
and are destroyed at the end of the block containing their definition. They are not visible
outside that block
register: a type of auto variable. a suggestion to the compiler to use a CPU register for
performance
static: a variable that is known only in the function that contains its definition but is
never destroyed and retains its value between calls to that function. It exists from the
time the program begins execution
extern: a static variable whose definition and placement is determined when all object and
library modules are combined (linked) to form the executable code file. It can be visible
outside the file where it is defined.
What is conversion operator?
Class can have a public method for specific data type conversions.
for example:
class Boo
{
double value;
public:
Boo(int i )
operator double()
{
return value;
}
};
Boo BooObject;
double i = BooObject; // assigning object to variable i of type double. now conversion
How can a ‘::’ operator be used as unary operator?
The scope operator can be operator gets called to assign the value. used to refer to members of the global namespace. Because the global namespace doesn’t have a name, the notation :: member-name refers to a member of the global namespace. This can be useful for referring to members of global namespace whose names have been hidden by names declared in nested local scope. Unless we specify to the compiler in which namespace to search for a declaration, the compiler simple searches the current scope, and any scopes in which the current scope is nested, to find the declaration for the name.
Describe the main characteristics of static functions.
The main characteristics of static functions include,
It is without the a this pointer,
It can’t directly access the non-static members of its class
It can’t be declared const, volatile or virtual.
It doesn’t need to be invoked through an object of its class, although for convenience, it may.
What is name mangling?
Name mangling is the process through which your c++ compilers give each function in your program a unique name. In C++, all programs have at-least a few functions with the same name. Name mangling is a concession to the fact that linker always insists on all function names being unique.
Example:
In general, member names are made unique by concatenating the name of the member with that of the class e.g. given the declaration:
class Bar
{
public:
int ival;
…
};
ival becomes something like:
// a possible member name mangling
ival__3Bar
Consider this derivation:
class Foo : public Bar
{
public:
int ival;
…
}
The internal representation of a Foo object is the concatenation of its base and derived class members.
// Pseudo C++ code
// Internal representation of Foo
class Foo
{
public:
int ival__3Bar;
int ival__3Foo;
…
};
Unambiguous access of either ival members is achieved through name mangling. Member functions, because they can be overloaded, require an extensive mangling to provide each with a unique name. Here the compiler generates the same name for the two overloaded instances(Their argument lists make their instances unique).
What is slicing?
Slicing means that the data added by a subclass are discarded when an object of the subclass is passed or returned by value or from a function expecting a base class object.
Explanation:
Consider the following class declaration:
class base
{
…
base& operator =(const base&);
base (const base&);
}
void fun( )
{
base e=m;
e=m;
}
As base copy functions don’t know anything about the derived only the base part of the derived is copied. This is commonly referred to as slicing. One reason to pass objects of classes in a hierarchy is to avoid slicing. Other reasons are to preserve polymorphic behavior and to gain efficiency.
Why doesn’t C have nested functions?
C Compilers do not allow class types so they do not support internal function reference table like a C++ compiler does. Instead, you can use a structure having members function pointers. Older C++ compilers use to transform the C++ sources to C using this isomorphic transformation.
Difference between overloading and overriding?
1. Overload - two functions that appear in the same scope are overloaded if they have the same name but have different parameter list
2. main() cannot be overloaded
3. notational convenience - compiler invokes the functions that is the best match on the args – found by finding the best match between the type of arg expr and parameter
4. if declare a function locally, that function hides rather than overload the same function declared in an outer scope
5. Overriding - the ability of the inherited class rewriting the virtual method of a base class - a method which completely replaces base class FUNCTIONALITY in subclass
6. the overriding method in the subclass must have exactly the same signature as the function of the base class it is replacing - replacement of a method in a child class
7. writing a different body in a derived class for a function defined in a base class, ONLY if the function in the base class is virtual and ONLY if the function in the derived class has the same signature
8. all functions in the derived class hide the base class functions with the same name except in the case of a virtual functions which override the base class functions with the same signature
What is Constructor ? How it is called ?
Constructor is a member function of the class, with the name of the function being the same as the class name. It also specifies how the object should be initialized.Ways of calling constructor:
1) Implicitly: automatically by complier when an object is created.
2) Calling the constructors explicitly is possible, but it makes the code unverifiable.
What are inline functions?
1. Treated like macro definitions by C++ compiler.
2. Meant to be used if there’s a need to repetitively execute a small block if code which is smaller.
3. Always evaluates every argument once.
4. Defined in header file.
5. Avoids function call overload because calling a function is slower than evaluating the equivalent expression.
6. It’s a request to the compiler, the compiler can ignore the request.
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