'C' language faqs

19.

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

Ans: The function is as shown below:

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

for ( i = k = 0 ; str1[i] != ‘\0’ ; i++ )
{
for ( j = 0 ; str2[j] != ‘\0’ && str2[j] !=
str1[i] ; j++ )
;
if ( str2[j] == ‘\0’ )
str1[k++] = str1[I] ;
}
str1[k] = ‘\0’
}
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20.

How does free( ) know how many bytes to free?

Ans: 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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21.

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

Ans: 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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22.

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

Ans: 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.

-------------------------------------------------------------------------------------23.

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

Ans: 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.

24.
When we open a file, how does functions like fread( )/fwrite( ), etc. get to know from where to read or to write the data?

Ans: 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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25.

The sizeof( ) function doesn’t return the size of the block of memory pointed to by a pointer. Why?

Ans: 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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26.
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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27.

How do I write a program to convert a string containing number in a hexadecimal form to its equivalent decimal?
Ans: 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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28.

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

Ans: 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
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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29.

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

Ans: 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
#include

main( )
{
char *path = NULL ;

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

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30.

How do I display current date in the format given below?

Saturday October 12, 2002
Ans: 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.

31.
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?

Ans: 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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32.

How do I write printf( ) so that the width of a field can be specified at runtime?

Ans: 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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33.

How to find the row and column dimension of a given 2-D array?

Ans: 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 < r ; i++ )
{
for ( j = 0 ; j < c ; j++ )
printf ( "%d ", a[i][j] ) ;
printf ( "\n" ) ;
}
}
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34.

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

main( )
{
char fname[67] ;

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

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

-------------------------------------------------------------------------------------35.

How do I convert a floating-point number to a string?

Ans: 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 ) ;
}

36.
What is a stack ?

Ans: 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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37.
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 ( "\n\n" ) ;
}
}
Data Structures
How to distinguish between a binary tree and a tree?

Ans: 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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38.
How do I use the function ldexp( ) in a program?

Ans: 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
#include

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

ans = ldexp ( n, 2 ) ;
printf ( "\nThe ldexp value is : %lf\n", 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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39.
Can we get the mantissa and exponent form of a given number?

Ans: 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
#include

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 %d\n", exponent ) ;

return 0 ;
}
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40.

How do I write code that executes certain function only at program termination?
Ans: Use atexit( ) function as shown in following program.

#include
main( )
{
int ch ;
void fun ( void ) ;
atexit ( fun ) ;
// code
}
void fun( void )
{
printf ( "\nTerminate program......" ) ;
getch( ) ;
}
-------------------------------------------------------------------------------------41.
What are memory models?
Ans: 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.

42.
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] ) ;
}
Ans: 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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43.

How do I use scanf( ) to read the date in the form 'dd-mm-yy' ?
Ans: 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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44.

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

Ans: 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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45.
Are the expressions *ptr++ and ++*ptr same?

Ans: 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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46.
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
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