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Language in C Interview Questions and Answers

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Freshers / Beginner level questions & answers

Ques 1. What will be the output of the following code?

void main ()
{ int i = 0 , a[3] ;
a[i] = i++;
printf ("%d",a[i]) ;
}

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.

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Ques 2. Why doesn't the following code give the desired result?

int x = 3000, y = 2000 ;
long int z = x * y ;

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.

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Ques 3. Why doesn't the following statement work?

char str[ ] = "Hello" ;
strcat ( str, '!' ) ;

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, "!" ) ;

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Ques 4. 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] ;
}

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Ques 5. 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 <stdio.h>
#include <dos.h>

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 ) ;

}

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Ques 6. 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 <mem.h>

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" ) ;
}

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Ques 7. The Spawnl( ) function...

DOS is a single tasking operating system, thus only one program runs at a time. The Spawnl( ) function provides us with the capability of starting the execution of one program from within another program. The first program is called the parent process and the second program that gets called from within the first program is called a child process. Once the second program starts execution, the first is put on hold until the second program completes execution. The first program is then restarted. The following program demonstrates use of spawnl( ) function.

/* Mult.c */

int main ( int argc, char* argv[ ] )
{
int a[3], i, ret ;
if ( argc < 3 || argc > 3 )
{
printf ( "Too many or Too few arguments..." ) ;
exit ( 0 ) ;
}

for ( i = 1 ; i < argc ; i++ )
a[i] = atoi ( argv[i] ) ;
ret = a[1] * a[2] ;
return ret ;
}

/* Spawn.c */
#include <process.h>
#include <stdio.h>

main( )
{
int val ;
val = spawnl ( P_WAIT, "C:\Mult.exe", "3", "10",
"20", NULL ) ;
printf ( "nReturned value is: %d", val ) ;
}

Here, there are two programs. The program 'Mult.exe' works as a child process whereas 'Spawn.exe' works as a parent process. On execution of 'Spawn.exe' it invokes 'Mult.exe' and passes the command-line arguments to it. 'Mult.exe' in turn on execution, calculates the product of 10 and 20 and returns the value to val in 'Spawn.exe'. In our call to spawnl( ) function, we have passed 6 parameters, P_WAIT as the mode of execution, path of '.exe' file to run as child process, total number of arguments to be passed to the child process, list of command line arguments and NULL. P_WAIT will cause our application to freeze execution until the child process has completed its execution. This parameter needs to be passed as the default parameter if you are working under DOS. under other operating systems that support multitasking, this parameter can be P_NOWAIT or P_OVERLAY. P_NOWAIT will cause the parent process to execute along with the child process, P_OVERLAY will load the child process on top of the parent process in the memory.

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Ques 8. Are the following two statements identical?

char str[6] = "Kicit" ;
char *str = "Kicit" ;

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.

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Ques 9. Is the following code fragment correct?

const int x = 10 ;
int arr[x] ;

No! Here, the variable x is first declared as an int so memory is reserved for it. Then it is qualified by a const qualifier. Hence, const qualified object is not a constant fully. It is an object with read only attribute, and in C, an object associated with memory cannot be used in array dimensions.

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Ques 10. 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 <systypes.h>

void main( )
{
time_t curtime ;
char ctm[50] ;

time ( &curtime ) ; //retrieves current time &
stores in curtime
printf ( "nCurrent Date & Time: %s", ctime (
&curtime ) ) ;
}

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Ques 11. 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 <conio.h>
main( )
{
/* Hide cursor */
_setcursortype ( _NOCURSOR ) ;

/* Change cursor to a solid cursor */
_setcursortype ( _SOLIDCURSOR ) ;

/* Change back to the normal cursor */
_setcursortype ( _NORMALCURSOR ) ;
}

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Ques 12. How do I write code that would get error number and display error message if any standard error occurs?

Following code demonstrates this.

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>

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.

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Ques 13. 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 <dir.h>

main( )
{
int dno, maxdr ;

dno = getdisk( ) ;
printf ( "nThe current drive is: %cn", 65 + dno
) ;

maxdr = setdisk ( 3 ) ;
dno = getdisk( ) ;
printf ( "nNow the current drive is: %cn", 65 +
dno ) ;
}

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Ques 14. The functions memcmp( ) and memicmp( )

The functions memcmp( ) and memicmp( ) compares first n bytes of given two blocks of memory or strings. However, memcmp( ) performs comparison as unsigned chars whereas memicmp( ) performs comparison as chars but ignores case (i.e. upper or lower case). Both the functions return an integer value where 0 indicates that two memory buffers compared are identical. If the value returned is greater than 0 then it indicates that the first buffer is bigger than the second one. The value less than 0 indicate that the first buffer is less than the second buffer. The following code snippet demonstrates use of both

#include <stdio.h>
#include <mem.h>

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" ) ;
}

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Ques 15. 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 <stdio.h>
#include <stdlib.h>
#include <dir.h>
#include <dos.h>

main( )
{
int dr ; struct dfree disk ;
long freesp ;

dr = getdisk( ) ;
getdfree ( dr + 1 , &disk ) ;

if ( disk.df_sclus == 0xFFFF )
{
printf ( "ngetdfree( ) function failedn");
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 spacen", 'A' + dr, freesp ) ;
}

17.

Use of array indices...
If we wish to store a character in a char variable ch and the character to be stored depends on the value of another variable say color (of type int), then the code would be as shown below:

switch ( color )
{
case 0 :
ch = 'R' ;
break ;
case 1 :
ch = 'G' ;
break ;
case 2 :
ch = 'B' ;
break ;
}
In place of switch-case we can make use of the value in color as an index for a character array. How to do this is shown in following code snippet.

char *str = "RGB' ;
char ch ;
int color ;
// code
ch = str[ color ] ;

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Ques 16. How do I write a user-defined function, which deletes each character in a string str1, which matches any character in string str2?

The function is as shown below:

Compress ( char str1[], char str2[] )
{
int i, j, k ;

for ( i = k = 0 ; str1[i] != ‚??‚?? ; i++ )
{
for ( j = 0 ; str2[j] != ‚??‚?? && str2[j] !=
str1[i] ; j++ )
;
if ( str2[j] == ‚??‚?? )
str1[k++] = str1[I] ;
}
str1[k] = ‚??‚??
}

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Ques 17. How does free( ) know how many bytes to free?

The malloc( ) / free( ) implementation remembers the size of each block allocated and returned, so it is not necessary to remind it of the size when freeing.

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Ques 18. What is the use of randomize( ) and srand( ) function?

While generating random numbers in a program, sometimes we require to control the series of numbers that random number generator creates. The process of assigning the random number generators starting number is called seeding the generator. The randomize( ) and srand( ) functions are used to seed the random number generators. The randomize( ) function uses PC's clock to produce a random seed, whereas the srand( ) function allows us to specify the random number generator's starting value.

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Ques 19. How do I determine amount of memory currently available for allocating?

We can use function coreleft( ) to get the amount of memory available for allocation. However, this function does not give an exact amount of unused memory. If, we are using a small memory model, coreleft( ) returns the amount of unused memory between the top of the heap and stack. If we are using a larger model, this function returns the amount of memory between the highest allocated memory and the end of conventional memory. The function returns amount of memory in terms of bytes.

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Ques 20. How does a C program come to know about command line arguments?

When we execute our C program, operating system loads the program into memory. In case of DOS, it first loads 256 bytes into memory, called program segment prefix. This contains file table, environment segment, and command line information. When we compile the C program the compiler inserts additional code that parses the command, assigning it to the argv array, making the arguments easily accessible within our C program.

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Ques 21. When we open a file, how does functions like fread( )/fwrite( ), etc. get to know from where to read or to write the data?

When we open a file for read/write operation using function like fopen( ), it returns a pointer to the structure of type FILE. This structure stores the file pointer called position pointer, which keeps track of current location within the file. On opening file for read/write operation, the file pointer is set to the start of the file. Each time we read/write a character, the position pointer advances one character. If we read one line of text at a step from the file, then file pointer advances to the start of the next line. If the file is opened in append mode, the file pointer is placed at the very end of the file. Using fseek( ) function we can set the file pointer to some other place within the file.

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Ques 22. The sizeof( ) function doesn‚??t return the size of the block of memory pointed to by a pointer. Why?

The sizeof( ) operator does not know that malloc( ) has been used to allocate a pointer. sizeof( ) gives us the size of pointer itself. There is no handy way to find out the size of a block allocated by malloc( ).

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Ques 23. Compare FP_SEG And FP_OFF.

Sometimes while working with far pointers we need to break a far address into its segment and offset. In such situations we can use FP_SEG and FP_OFF macros. Following program illustrates the use of these two macros.
#include

main( )
{
unsigned s, o ;
char far *ptr = "Hello!" ;

s = FP_SEG ( ptr ) ;
o = FP_OFF ( ptr ) ;
printf ( "n%u %u", s, o ) ;
}

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Ques 24. How do I write a program to convert a string containing number in a hexadecimal form to its equivalent decimal?

The following program demonstrates this:
main( )
{
char str[] = "0AB" ;
int h, hex, i, n ;
n = 0 ; h = 1 ;
for ( i = 0 ; h == 1 ; i++ )
{
if ( str[i] >= '0' && str[i] <= '9' )
hex = str[i] - '0' ;
else
{
if ( str[i] >= 'a' && str[i] <= 'f' )
hex = str[i] - 'a' + 10 ;
else
if ( str[i] >= 'A' && str[i] <= 'F' )
hex = str[i] - 'A' + 10 ;
else
h = 0 ;
}
if ( h == 1 )
n = 16 * n + hex ;
}
printf ( "nThe decimal equivalent of %s is %d",
str, n ) ;
}
The output of this program would be the decimal equivalent of 0AB is 171.

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Ques 25. How do I write code that reads the segment register settings?

We can use segread( ) function to read segment register settings. There are four segment registers‚??code segment, data segment, stack segment and extra segment. Sometimes when we use DOS and BIOS services in a program we need to know the segment register's value. In such a situation we can use segread( ) function. The following program illustrates the use of this function.
#include <dos.h>
main( )
{
struct SREGS s ;
segread ( &s ) ;
printf ( "nCS: %X DS: %X SS: %X ES: %X",s.cs,
s.ds, s.ss, s.es ) ;
}

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Ques 26. What is environment and how do I get environment for a specific entry?

While working in DOS, it stores information in a memory region called environment. In this region we can place configuration settings such as command path, system prompt, etc. Sometimes in a program we need to access the information contained in environment. The function getenv( ) can be used when we want to access environment for a specific entry. Following program demonstrates the use of this function.
#include <stdio.h>
#include <stdlib.h>

main( )
{
char *path = NULL ;

path = getenv ( "PATH" ) ;
if ( *path != NULL )
printf ( "nPath: %s", path ) ;
else
printf ( "nPath is not set" ) ;
}

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Ques 27. How do I display current date in the format given below?
Saturday October 12, 2002

Following program illustrates how we can display date in above given format.

#include
#include

main( )
{
struct tm *curtime ;
time_t dtime ;

char str[30] ;

time ( &dtime ) ;
curtime = localtime ( &dtime ) ;
strftime ( str, 30, "%A %B %d, %Y", curtime ) ;

printf ( "n%s", str ) ;
}
Here we have called time( ) function which returns current time. This time is returned in terms of seconds, elapsed since 00:00:00 GMT, January 1, 1970. To extract the week day, day of month, etc. from this value we need to break down the value to a tm structure. This is done by the function localtime( ). Then we have called strftime( ) function to format the time and store it in a string str.

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Ques 28. If we have declared an array as global in one file and we are using it in another file then why doesn't the sizeof operator works on an extern array?

An extern array is of incomplete type as it does not contain the size. Hence we cannot use sizeof operator, as it cannot get the size of the array declared in another file. To resolve this use any of one the following two solutions:
1. In the same file declare one more variable that holds the size of array. For example,

array.c

int arr[5] ;
int arrsz = sizeof ( arr ) ;

myprog.c

extern int arr[] ;
extern int arrsz ;
2. Define a macro which can be used in an array
declaration. For example,

myheader.h

#define SZ 5

array.c

#include "myheader.h"
int arr[SZ] ;

myprog.c

#include "myheader.h"
extern int arr[SZ] ;

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Ques 29. 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.

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Ques 30. 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: %dnCol: %dn", r, c ) ;
for ( i = 0 ; i < r ; i++ )
{
for ( j = 0 ; j < c ; j++ )
printf ( "%d ", a[i][j] ) ;
printf ( "n" ) ;
}
}

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Ques 31. The access( ) function...

The access( ) function checks for the existence of a file and also determines whether it can be read, written to or executed. This function takes two arguments the filename and an integer indicating the access mode. The values 6, 4, 2, and 1 checks for read/write, read, write and execute permission of a given file, whereas value 0 checks whether the file exists or not. Following program demonstrates how we can use access( ) function to check if a given file exists.

#include <io.h>

main( )
{
char fname[67] ;

printf ( "nEnter name of file to open" ) ;
gets ( fname ) ;

if ( access ( fname, 0 ) != 0 )
{
printf ( "nFile does not exist." ) ;
return ;
}
}

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Ques 32. 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 <stdlib.h>

main( )
{
char str[25] ;
float no ;
int dg = 5 ; /* significant digits */

no = 14.3216 ;
gcvt ( no, dg, str ) ;
printf ( "String: %sn", str ) ;
}

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Ques 33. What is a stack ?

The stack is a region of memory within which our programs temporarily store data as they execute. For example, when a program passes parameters to functions, C places the parameters on the stack. When the function completes, C removes the items from the stack. Similarly, when a function declares local variables, C stores the variable's values on the stack during the function's execution. Depending on the program's use of functions and parameters, the amount of stack space that a program requires will differ.

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Ques 34. Allocating memory for a 3-D array

#include "alloc.h"
#define MAXX 3
#define MAXY 4
#define MAXZ 5
main( )
{
int ***p, i, j, k ;
p = ( int *** ) malloc ( MAXX * sizeof ( int ** ) ) ;
for ( i = 0 ; i < MAXX ; i++ )
{
p[i] = ( int ** ) malloc ( MAXY * sizeof ( int * ) ) ;
for ( j = 0 ; j < MAXY ; j++ )
p[i][j] = ( int * ) malloc ( MAXZ * sizeof ( int ) ) ;
}
for ( k = 0 ; k < MAXZ ; k++ )
{
for ( i = 0 ; i < MAXX ; i++ )
{
for ( j = 0 ; j < MAXY ; j++ )
{
p[i][j][k] = i + j + k ;
printf ( "%d ", p[i][j][k] ) ;
}
printf ( "n" ) ;
}
printf ( "nn" ) ;
}
}
Data Structures
How to distinguish between a binary tree and a tree?

A node in a tree can have any number of branches. While a binary tree is a tree structure in which any node can have at most two branches. For binary trees we distinguish between the subtree on the left and subtree on the right, whereas for trees the order of the subtrees is irrelevant.
Consider the following figure...

This above figure shows two binary trees, but these binary trees are different. The first has an empty right subtree while the second has an empty left subtree. If the above are regarded as trees (not the binary trees), then they are same despite the fact that they are drawn differently. Also, an empty binary tree can exist, but there is no tree having zero nodes.

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Ques 35. How do I use the function ldexp( ) in a program?

The math function ldexp( ) is used while solving the complex mathematical equations. This function takes two arguments, a double value and an int respectively. The order in which ldexp( ) function performs calculations is ( n * pow ( 2, exp ) ) where n is the double value and exp is the integer. The following program demonstrates the use of this function.

#include <stdio.h>
#include <math.h>

void main( )
{
double ans ;
double n = 4 ;

ans = ldexp ( n, 2 ) ;
printf ( "nThe ldexp value is : %lfn", ans ) ;
}
Here, ldexp( ) function would get expanded as ( 4 * 2 * 2 ), and the output would be the ldexp value is : 16.000000

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Ques 36. Can we get the mantissa and exponent form of a given number?

The function frexp( ) splits the given number into a mantissa and exponent form. The function takes two arguments, the number to be converted as a double value and an int to store the exponent form. The function returns the mantissa part as a double value. Following example demonstrates the use of this function.

#include <math.h>
#include <stdio.h>

void main( )
{
double mantissa, number ;
int exponent ;

number = 8.0 ;
mantissa = frexp ( number, &exponent ) ;

printf ( "The number %lf is ", number ) ;
printf ( "%lf times two to the ", mantissa ) ;
printf ( "power of %dn", exponent ) ;

return 0 ;
}

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Ques 37. How do I write code that executes certain function only at program termination?

Use atexit( ) function as shown in following program.

#include <stdlib.h>
main( )
{
int ch ;
void fun ( void ) ;
atexit ( fun ) ;
// code
}
void fun( void )
{
printf ( "nTerminate program......" ) ;
getch( ) ;
}

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Ques 38. What are memory models?

The compiler uses a memory model to determine how much memory is allocated to the program. The PC divides memory into blocks called segments of size 64 KB. Usually, program uses one segment for code and a second segment for data. A memory model defines the number of segments the compiler can use for each. It is important to know which memory model can be used for a program. If we use wrong memory model, the program might not have enough memory to execute. The problem can be solved using larger memory model. However, larger the memory model, slower is your program execution. So we must choose the smallest memory model that satisfies our program needs. Most of the compilers support memory models like tiny, small, medium, compact, large and huge.

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Ques 39. How does C compiler store elements in a multi-dimensional array?

The compiler maps multi-dimensional arrays in two ways‚??Row major order and Column order. When the compiler places elements in columns of an array first then it is called column-major order. When the compiler places elements in rows of an array first then it is called row-major order. C compilers store multidimensional arrays in row-major order. For example, if there is a multi-dimensional array a[2][3], then according row-major order, the elements would get stored in memory following order:
a[0][0], a[0][1], a[0][2], a[1][0], a[1][1], a[1][2]

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Ques 40. If the result of an _expression has to be stored to one of two variables, depending on a condition, can we use conditional operators as shown below?

( ( i < 10 ) ? j : k ) = l * 2 + p ;

No! The above statement is invalid. We cannot use the conditional operators in this fashion. The conditional operators like most operators, yields a value, and we cannot assign the value of an _expression to a value. However, we can use conditional operators as shown in following code snippet.

main( )
{
int i, j, k, l ;
i = 5 ; j = 10 ; k = 12, l = 1 ;
* ( ( i < 10 ) ? &j : &k ) = l * 2 + 14 ;
printf ( "i = %d j = %d k = %d l = %d", i, j, k, l ) ;
}

The output of the above program would be as given below:
i = 5 j = 16 k = 12 l = 1

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Ques 41. How can I find the day of the week of a given date?

The following code snippet shows how to get the day of week from the given date.

dayofweek ( int yy, int mm, int dd )
{
/*Monday = 1 and Sunday = 0 */
/* month number >= 1 and <= 12, yy > 1752 or so */
static int arr[ ] = { 0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4 } ;
yy = yy - mm < 3 ;
return ( yy + yy / 4 - yy / 100 + yy / 400 + arr[ mm - 1] + dd ) % 7 ;
}

void main( )
{
printf ( "nnnDay of week : %d ", dayofweek ( 2002, 5, 18 ) ) ;
}

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Ques 42. What's the difference between these two declarations?

struct str1 { ... } ;
typedef struct { ... } str2 ;

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.

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Ques 43. 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.

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Ques 44. What would the second and the third printf( ) output the following program?

main( )
{
char *str[ ] = {
"Good Morning"
"Good Evening"
"Good Afternoon"
} ;
printf ( "nFirst string = %s", str[0] ) ;
printf ( "nSecond string = %s", str[1] ) ;
printf ( "nThird string = %s", str[2] ) ;
}

For the above given program, we expect the output as Good Evening and Good Afternoon, for the second and third printf( ). However, the output would be as shown below.

First string = Good MorningGood EveningGood Afternoon
Second string = ( null )
Third string =

What is missing in the above given code snippet is a comma separator which should separate the strings Good Morning, Good Evening and Good Afternoon. On adding comma, we would get the output as shown below.

First string = Good Morning
Second string = Good Evening
Third string = Good Afternoon

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Ques 45. How do I use scanf( ) to read the date in the form 'dd-mm-yy' ?

There are two ways to read the date in the form of 'dd-mm-yy' one possible way is...

int dd, mm, yy ;
char ch ; /* for char '-' */
printf ( "nEnter the date in the form of dd-mm-yy : " ) ;
scanf( "%d%c%d%c%d", &dd, &ch, &mm, &ch, &yy ) ;

And another best way is to use suppression character * as...

int dd, mm, yy ;
scanf( "%d%*c%d%*c%d", &dd, &mm, &yy ) ;

The suppression character * suppresses the input read from the standard input buffer for the assigned control character.

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Ques 46. How do I print a floating-point number with higher precision say 23.34568734 with only precision up to two decimal places?

This can be achieved through the use of suppression char '*' in the format string of printf( ) as shown in the following program.
main( )
{
int i = 2 ;
float f = 23.34568734 ;
printf ( "%.*f", i, f ) ;
}
The output of the above program would be 23.35.

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Ques 47. Are the expressions *ptr++ and ++*ptr same?

No. *ptr++ increments the pointer and not the value pointed by it, whereas ++*ptr increments the value being pointed to by ptr.

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Ques 48. strpbrk( )

The function strpbrk( ) takes two strings as parameters. It scans the first string, to find, the first occurrence of any character appearing in the second string. The function returns a pointer to the first occurrence of the character it found in the first string. The following program demonstrates the use of string function strpbrk( ).

#include <string.h>
main( )
{
char *str1 = "Hello!" ;
char *str2 = "Better" ;
char *p ;
p = strpbrk ( str1, str2 ) ;

if ( p )
printf ( "The first character found in str1 is %c", *p ) ;
else
printf ( "The character not found" ) ;
}
The output of the above program would be the first character found in str1 is e


div( )...

The function div( ) divides two integers and returns the quotient and remainder. This function takes two integer values as arguments; divides first integer with the second one and returns the answer of division of type div_t. The data type div_t is a structure that contains two long ints, namely quot and rem, which store quotient and remainder of division respectively. The following example shows the use of div( ) function.

#include <stdlib.h>
void main( )
{
div_t res ;

res = div ( 32, 5 ) ;
printf ( "nThe quotient = %d and remainder = %d ", res.quot, res.rem ) ;

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Ques 49. Can we convert an unsigned long integer value to a string?

The function ultoa( ) can be used to convert an unsigned long integer value to a string. This function takes three arguments, first the value that is to be converted, second the base address of the buffer in which the converted number has to be stored (with a string terminating null character '') and the last argument specifies the base to be used in converting the value. Following example demonstrates the use of this function.

#include <stdlib.h>
void main( )
{
unsigned long ul = 3234567231L ;
char str[25] ;

ultoa ( ul, str, 10 ) ;
printf ( "str = %s unsigned long = %lun", str, ul ) ;
}

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Ques 50. ceil( ) and floor( )

The math function ceil( ) takes a double value as an argument. This function finds the smallest possible integer to which the given number can be rounded up. Similarly, floor( ) being a math function, takes a double value as an argument and returns the largest possible integer to which the given double value can be rounded down. The following program demonstrates the use of both the functions.

#include <math.h>
void main( )
{
double no = 1437.23167 ;
double down, up ;

down = floor ( no ) ;
up = ceil ( no ) ;

printf ( "The original number %7.5lfn", no ) ;
printf ( "The number rounded down %7.5lfn", down ) ;
printf ( "The number rounded up %7.5lfn", up ) ;
}

The output of this program would be,
The original number 1437.23167
The number rounded down 1437.00000
The number rounded up 1438.00000

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Ques 51. How do I use function ecvt( ) in a program?

The function ecvt( ) converts a floating-point value to a null terminated string. This function takes four arguments, such as, the value to be converted to string, the number of digits to be converted to string, and two integer pointers. The two-integer pointer stores the position of the decimal point (relative to the string) and the sign of the number, respectively. If the value in a variable, used to store sign is 0, then the number is positive and, if it is non-zero, then the number is negative. The function returns a pointer to the string containing digits. Following program demonstrates the use of this function.

#include <stdlib.h>
main( )
{
char *str ;
double val ;
int dec, sign ;
int ndig = 4 ;

val = 22 ;
str = ecvt ( val, ndig, &dec, &sign ) ;
printf ( "string = %s dec = %d sign = %dn", str, dec, sign ) ;

val = -345.67 ;
ndig = 8 ;
str = ecvt ( val, ndig, &dec, &sign ) ;
printf ( "string = %s dec = %d sign = %dn", str, dec, sign ) ;

// number with a scientific notation
val = 3.546712e5 ;
ndig = 5 ;
str = ecvt ( val, ndig, &dec, &sign ) ;
printf ( "string = %s dec = %d sign = %dn", str, dec, sign ) ;
}

The output of this program would be

string = 2200 dec = 2 sign = 0
string = 34567000 dec = 3 sign = 1
string = 35467 dec = 6 sign = 0

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Ques 52. How to run DIR command programmatically?

We can use the system( ) function to execute the DIR command along with its options. Following program shows how this can be achieved:

// mydir.c

main ( int argc, char *argv[ ] )
{
char str[30] ;

if ( argc < 2 )
exit ( 0 ) ;

sprintf ( str, "dir %s %s", argv[1], argv[2] ) ;
system ( str ) ;
}

If we run the executable file of this program at command prompt passing the command line arguments as follows:

> mydir abc.c /s

This will search the file 'abc.c' in the current directory.

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Ques 53. Suppose I have a structure having fields name, age, salary and have passed address of age to a function fun( ). How I can access the other member of the structure using the address of age?

struct emp
{
char name[20] ;
int age ;
float salary ;
} ;
main( )
{
struct emp e ;
printf ( "nEnter name: " ) ;
scanf ( "%s", e.name ) ;
printf ( "nEnter age: " ) ;
scanf ( "%d", &e.age ) ;
printf ( "nEnter salary: " ) ;
scanf ( "%f", &e.salary ) ;
fun ( &e.age ) ;
}
fun ( int *p )
{
struct emp *q ;
int offset ;
offset = ( char * ) ( & ( ( struct emp * ) 0 ) -> age ) - ( char * ) ( (
struct emp* ) 0 ) ;
q = ( struct emp * ) ( ( char * ) p - offset ) ;
printf ( "nname: %s", q -> name ) ;
printf ( "nage: %d", q -> age ) ;
printf ( "nsalary: %f", q -> salary ) ;
}

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Ques 54. 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 <conio.h>
main( )
{
int i, j ;

window ( 20, 8, 60, 17 ) ;
for ( i = 0 ; i < 8 ; i++ )
for ( j = 0 ; j < 10 ; j++ )
cprintf ( "Hello!" ) ;
}

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Ques 55. Sometimes you need to prompt the user for a password. When the user types in the password, the characters the user enters should not appear on the screen. A standard library function getpass( ) can be used to perform such function. Maximum number of characters that can be entered as password is 8.

main( )
{
char *pwd ;

pwd = getpass ( "Enter Password" ) ;

if ( strcmp ( pwd, "orgcity" ) )
printf ( "nPassword %s is incorrect", pwd ) ;
else
printf ( "nCorrect Password" ) ;
}

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Ques 56. 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 <direct.h>
main( )
{
int disk ;
disk = _getdrive( ) + 'A' - 1 ;
printf ( "The current drive is: %cn", disk ) ;
}

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Ques 57. How come the output for both the programs is different when the logic is same?

main( )
{
int i, j ;

for ( i = 1, j = 1 ; i <= 5, j <= 100 ; i++, j++ )
{
gotoxy ( 1, 1, ) ;
printf ( "%d %d", i, j ) ;
}
}

main( )
{
int i, j ;

for ( i =1, j = 1; j <= 100, i <= 5; i++, j++ )
{
gotoxy ( 1, 1 ) ;
printf ( "%d %d", i, j ) ;
}
}

Output -> 5 5

Even if logic of both the programs is same the output of the first program comes out to be 100, 100, but of the second program it is 5, 5. The comma operator plays a vital role inside the for loop. It always considers the value of the latest variable. So, at the time of testing the condition in for loop, the value of j will be considered in the first program and value of i in the second.

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Ques 58. 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 <conio.h>
main( )
{
printf ( "Justn Ton Testn Wheren the cursorn goes" ) ;

printf ( "Current location is X: %d Y: %dn", wherex( ), wherey( ) ) ;
}

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Ques 59. How do I programmatically delete lines in the text window?

While writing programs that perform screen-based I/O, you may want to-delete the current line's contents, moving one line up, all of the output that follows. In such cases a function called delline( ) can be used. Following code snippet illustrates the use of function delline( ).

#include <conio.h>
main( )
{
int i ;
clrscr( ) ;

for ( i = 0; i <= 23; i++ )
printf ( "Line %drn", i ) ;

printf ( "Press a key to continue : " ) ;
getch( ) ;

gotoxy ( 2, 6 ) ;

for ( i = 6; i <= 12; i++ )
delline( ) ;

getch( ) ;
}

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Ques 60. How do I get the time elapsed between two function calls ?

The function difftime( ) finds the difference between two times. It calculates the elapsed time in seconds and returns the difference between two times as a double value.

#include <time.h>
#include <stdio.h>
#include <dos.h>

main( )
{
int a[] = { 2, -34, 56, 78, 112, 33, -7, 11, 45, 29, 6 } ;
int s ;
time_t t1, t2 ; // time_t defines the value used for time function

s = sizeof ( a ) / 2 ;
t1 = time ( NULL ) ;
sel_sort ( a, s ) ; // sort array by selection sort
bub_sort ( a, s ) ; // sort array by bubble sort method
t2 = time ( NULL ) ;
printf ( "nThe difference between two function calls is %f", difftime (
t2, t1 ) ) ;
}

In the above program we have called difftime( ) function that returns the time elapsed from t1 to t2.

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Ques 61. How do I use swab( ) in my program ?

The function swab( ) swaps the adjacent bytes of memory. It copies the bytes from source string to the target string, provided that the number of characters in the source string is even. While copying, it swaps the bytes which are then assigned to the target string.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

main ( )
{
char *str1 = "hS eesll snsiasl not eh es as oher " ;
char *str2 ;
clrscr( ) ;
swab ( str1, str2, strlen ( str1 ) ) ;
printf ( "The target string is : %sn", str2 ) ; // output -- She sells
snails on the sea shore
getch( ) ;
}

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Ques 62. 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.

Data Structures

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Ques 63. How to build an expression trees ?

An expression tree is a binary tree which is built from simple operands and operators of an (arithmetic or logical ) expression by placing simple operands as the leaves of a binary tree and the operators as the interior nodes. If an operator is binary , then it has two nonempty subtrees, that are its left and right operands (either simple operands or sub expressions). If an operator is unary, then only one of its subtrees is nonempty, the one on the left or right according as the operator is written on the right or left of its operand. We traditionally write some unary operators to the left of their operands, such as "-" ( unary negation) or the standard functions like log( ), sin( ) etc. Others are written on the right, such as the factorial function ()!. If the operator is written on the left, then in the expression tree we take its left subtree as empty. If it appears on the right, then its right subtree will be empty. An example of an expression tree is shown below for the expression ( -a < b ) or ( c + d ) .

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Ques 64. 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 <math.h>

main( )
{
printf ( "%f", fmod ( 5.15, 3.0 ) ) ;
}

The above code snippet would give the output as 2.150000.

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Ques 65. How to extract the integer part and a fractional part of a floating point number?

C function modf( ) can be used to get the integer and fractional part of a floating point.

#include "math.h"

main( )
{
double val, i, f ;
val = 5.15 ;
f = modf ( val, &i ) ;
printf ( "nFor the value %f integer part = %f and fractional part = %f",
val, i, f ) ;
}

The output of the above program will be:

For the value 5.150000 integer part = 5.000000 and fractional part =
0.150000

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Ques 66. How do I define a pointer to a function which returns a char pointer?

char * ( *p )( ) ;
or
typedef char * ( * ptrtofun )( ) ;
ptrtofun p ;
Here is a sample program which uses this definition.
main( )
{
typedef char * ( * ptrtofun ) ( ) ;
char * fun( ) ;
ptrtofun fptr ;
char *cptr ;
fptr = fun ;
cptr = (*fptr) ( ) ;
printf ( "nReturned string is "%s"", cptr ) ;
}
char * fun( )
{
static char s[ ] = "Hello!" ;
printf ( "n%s", s ) ;
return s ;
}

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Ques 67. What's wrong with the following declaration: char* ptr1, ptr2 ; get errors when I try to use ptr2 as a pointer.

char * applies only to ptr1 and not to ptr2. Hence ptr1 is getting declared as a char pointer, whereas, ptr2 is being declared merely as a char. This can be rectified in two ways :
char *ptr1, *ptr2 ;
typedef char* CHARPTR ; CHARPTR ptr1, ptr2 ;

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Ques 68. How to use scanf( ) to read the date in the form of dd-mm-yy?

To read the date in the form of dd-mm-yy one possible way is,
int dd, mm, yy ;
char ch ; /* for char '-' */
printf ( "nEnter the date in the form of dd-mm-yy : " ) ;
scanf( "%d%c%d%c%d", &dd, &ch, &mm, &ch, &yy ) ;
Another way is to use suppression character * is as follows:
int dd, mm, yy ;
scanf( "%d%*c%d%*c%d", &dd, &mm, &yy ) ;
The suppression character '*' suppresses the input read from the standard input buffer for the assigned control character.

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Ques 69. Why the output of sizeof ( 'a' ) is 2 and not 1 ?

Character constants in C are of type int, hence sizeof ( 'a' ) is equivalent to sizeof ( int ), i.e. 2. Hence the output comes out to be 2 bytes.

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Ques 70. Can we use scanf( ) function to scan a multiple words string through keyboard?

Yes. Although we usually use scanf( ) function to receive a single word string and gets( ) to receive a multi-word string from keyboard we can also use scanf( ) function for scanning a multi-word string from keyboard. Following program shows how to achieve this.

main( )
{
char buff[15] ;
scanf ( "%[^n]s", buff ) ;
puts ( buff ) ;
}

In the scanf( ) function we can specify the delimiter in brackets after the ^ character. We have specified 'n' as the delimiter. Hence scanf( ) terminates only when the user hits Enter key.

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Ques 71. How to set the system date through a C program ?

We can set the system date using the setdate( ) function as shown in the following program. The function assigns the current time to a
structure date.

#include "stdio.h"
#include "dos.h"

main( )
{
struct date new_date ;

new_date.da_mon = 10 ;
new_date.da_day = 14 ;
new_date.da_year = 1993 ;

setdate ( &new_date ) ;
}

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Ques 72. How can I write a general-purpose swap without using templates?

Given below is the program which uses the stringizing preprocessor directive ## for building a general purpose swap macro which can swap two integers, two floats, two chars, etc.
#define swap( a, b, t ) ( g ## t = ( a ), ( a ) = ( b ), ( b ) = g ## t )
int gint;
char gchar;
float gfloat ;
main( )
{
int a = 10, b = 20 ;
char ch1 = 'a' , ch2 = 'b' ;
float f1 = 1.12, f2 = 3.14 ;
swap ( a, b, int ) ;
printf ( "na = %d b = %d", a, b ) ;
swap ( ch1, ch2, char ) ;
printf ( "nch1 = %c ch2 = %c", ch1, ch2 ) ;
swap ( f1, f2, float ) ;
printf ( "nf1 = %4.2f f2 = %4.2f", f1, f2 ) ;
}
swap ( a, b, int ) would expand to,
( gint = ( a ), ( a ) = ( b ), ( b ) = gint )

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Ques 73. What is a heap ?

Heap is a chunk of memory. When in a program memory is allocated dynamically, the C run-time library gets the memory from a collection of unused memory called the heap. The heap resides in a program's data segment. Therefore, the amount of heap space available to the program is fixed, and can vary from one program to another.

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Ques 74. 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 : %sn", path ) ;
else
printf ( "File not foundn" ) ;
}

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Ques 75. 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 <stdio.h>
#include <process.h>

void main( )
{
printf ( "The process identification number of this program is %Xn",
getpid( ) ) ;
}

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Ques 76. How do I write a function that takes variable number of arguments?

The following program demonstrates this.

#include <stdio.h>
#include <stdarg.h>

void main( )
{
int i = 10 ;
float f = 2.5 ;
char *str = "Hello!" ;
vfpf ( "%d %f %sn", 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.

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Ques 77. 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 <stdio.h>
#include <time.h>

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.

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Ques 78. 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 <stdio.h>
#include <string.h>
#include <alloc.h>

void main( )
{
char *str1, *str2 = "double";

str1 = strdup ( str2 ) ;
printf ( "%sn", str1 ) ;
free ( str1 ) ;
}

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Ques 79. On including a file twice I get errors reporting redefinition of function.
How can I avoid duplicate inclusion?

Redefinition errors can be avoided by using the following macro definition. Include this definition in the header file.
#if !defined filename_h
#define filename_h
/* function definitions */
#endif
Replace filename_h with the actual header file name. For example, if name of file to be included is 'goto.h' then replace filename_h with 'goto_h'.

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Ques 80. How to write a swap( ) function which swaps the values of the variables using bitwise operators.

Here is the swap( ) function.
swap ( int *x, int *y )
{
*x ^= *y ;
*y ^= *x ;
*x ^= *y ;
}
The swap( ) function uses the bitwise XOR operator and does not require any temporary variable for swapping.

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