Chapter 4

CHAPTER 4

Pointers

1. What is a pointer in C?

A pointer is a special variable in C language meant just to store address of any other variable or function. Pointer variables unlike ordinary variables cannot be operated with all the arithmetic operations such as '*','%' operators. It follows a special arithmetic called as pointer arithmetic.

A pointer is declared as:

int *ap;

int a = 5;

In the above two statements an integer a was declared and initialized to 5. A pointer to an integer with name ap was declared. Next before ap is used

ap=&a;

This operation would initialize the declared pointer to int. The pointer ap is now said to point to a.

Operations on a pointer:

· Dereferencing operator ' * ': This operator gives the value at the address pointed by the pointer . For

example after the above C statements if we give

printf("%d",*ap);

Actual value of a that is 5 would be printed. That is because ap points to a.

· Addition operator ' + ': Pointer arithmetic is different from ordinary arithmetic.

ap=ap+1;

Above expression would not increment the value of ap by one, but would increment it by the number of bytes of the data type it is pointing to. Here ap is pointing to an integer variable hence ap is incremented by 2 or 4 bytes depending upon the compiler.

2. What are the advantages of using pointers?

Pointers are special variables which store address of some other variables.

Syntax: datatype *ptr;

Here * indicates that ptr is a pointer variable which represents value stored at a particular address.

Example: int *p;

'p' is a pointer variable pointing to address location where an integer type is stored.

Advantages:

1. Pointers allow us to pass values to functions using call by reference. This is useful when large sized

arrays are passed as arguments to functions. A function can return more than one value by using call by

reference.

2. Dynamic allocation of memory is possible with the help of pointers.

3. We can resize data structures. For instance, if an array's memory is fixed, it cannot be resized. But in

case of an array whose memory is created out of malloc can be resized.

4. Pointers point to physical memory and allow quicker access to data.

3. What are the differences between malloc() and calloc()?

Allocation of memory at the time of execution is called dynamic memory allocation. It is done using the standard library functions malloc() and calloc(). It is defined in "stdlib.h".

malloc(): used to allocate required number of bytes in memory at runtime. It takes one argument, viz. size in bytes to be allocated.

Syntax:

void * malloc(size_t size);

Example:

a = (int*) malloc(4);

4 is the size (in bytes) of memory to be allocated.

calloc(): used to allocate required number of bytes in memory at runtime. It needs two arguments viz.,

1. total number of data and

2. size of each data.

Syntax:

void * calloc(size_t nmemb, size_t size);

Example:

a = (int*) calloc(8, sizeof(int));

Here sizeof indicates the size of the data type and 8 indicates that we want to reserve space for storing 8

integers.

Differences between malloc() and calloc() are:

1. Number of arguments differ.

2. By default, memory allocated by malloc() contains garbage values. Whereas memory allocated by calloc() contains all zeros.

4. How to use realloc() to dynamically increase size of an already allocated array?

realloc(): This function is used to increase or decrease the size of any dynamic memory which is allocated using malloc() or calloc() functions.

Syntax: void *realloc(void *ptr, size_t newsize);

The first argument 'ptr' is a pointer to the memory previously allocated by the malloc or calloc functions. The second argument 'newsize' is the size in bytes, of a new memory region to be allocated by realloc. This value can be larger or smaller than the previously allocated memory. The realloc function adjusts the old memory region if newsize is smaller than the size of old memory. If the newsize is larger than the existing memory size, it increases the size by copying the contents of old memory region to new memory region. The function then deallocates the old memory region. realloc function is helpful in managing a dynamic array whose size may change during execution.

Example: a program that reads input from standard input may not know the size of data in advance. In this case, dynamically allocated array can be used so that it is possible allocate the exact amount of memory using realloc function.

5. What is the equivalent pointer expression for referring an element a[i][j][k][l], in a four

dimensional array?

Consider a multidimensional array a[w][x][y][z].

In this array, a[i] gives address of a[i][0][0][0] and a[i]+j gives the address of a[i][j][0][0]

Similarly, a[i][j] gives address of a[i][j][0][0] and a[i][j]+k gives the address of a[i][j][k][0]

a[i][j][k] gives address of a[i][j][k][0] and a[i][j][k]+l gives address of a[i][j][k][l]

Hence a[i][j][k][l] can be accessed using pointers as *(a[i][j][k]+l)

where * stands for value at address and a[i][j][k]+l gives the address location of a[i][j][k][l].

Program: Example program to illustrate pointer denotation of multi-dimensional arrays.

#include<stdio.h>

#include<string.h>

int main() {

int a[3][3][3][3];

//it gives address of a[0][0][0][0] .

printf(" \n address of array a is %u", a);

printf("\n address of a[2][0][0][0] is %u ,given by a[2], %u given by a+2",

a[2], a + 2);

printf("\n address of a[2][2][0][0] is %u ,given by a[2][2], %u given by a[2]+2",

a[2][2], a[2] + 2);

printf("\n address of a[2][2][1][0] is %u ,given by a[2][2][1] , %u given by a[2][2]+1",

a[2][2][1], a[2][2] + 1);

return 0;

}

Output:

address of array a is 65340

address of a[2][0][0][0] is 65448, given by a[2] , 65448 given by a+2

address of a[2][2][0][0] is 65484, given by a[2][2] ,65484 given by a[2]+2

address of a[2][2][1][0] is 65490, given by a[2][2][1] , 65490 given by a[2][2]+1

Explanation:

This output may differ from computer to computer as the address locations are not same for every computer.

6. Declare an array of three function pointers where each function receives two integers and returns float.

Declaration:

float (*fn[3])(int, int);

Program: Illustrates the usage of above declaration

#include<stdio.h>

float (*fn[3])(int, int);

float add(int, int);

int main() {

int x, y, z, j;

for (j = 0; j < 3; j++){

fn[j] = &add;

}

x = fn[0](10, 20);

y = fn[1](100, 200);

z = fn[2](1000, 2000);

printf("sum1 is: %d \n", x);

printf("sum2 is: %d \n", y);

printf("sum3 is: %d \n", z);

return 0;

}f

loat add(int x, int y) {

float f = x + y;

return f;

}

Output:

sum1 is: 30

sum2 is: 300

sum3 is: 3000

Explanation:

Here 'fn[3]' is an array of function pointers. Each element of the array can store the address of function 'float

add(int, int)'.

fn[0]=fn[1]=fn[2]=&add

Wherever this address is encountered add(int, int) function is called.

7. Explain the variable assignment in the declaration

int *(*p[10])(char *, char *);

It is an array of function pointers that returns an integer pointer. Each function has two arguments which in turn are pointers to character type variable. p[0], p[1],....., p[9] are function pointers.

return type : integer pointer.

p[10] : array of function pointers

char * : arguments passed to the function

Program: Example program to explain function pointers.

#include<stdio.h>

#include<stdlib.h>

int *(*p[10])(char *, char *);

//average function which returns pointer to integer whose value is average of ascii value of characters passed by

pointers

int *average(char *, char *);

//function which returns pointer to integer whose value is sum of ascii value of characters passed by pointers

int *sum(char *, char *);

int retrn;

int main(void) {

int i;

for (i = 0; i < 5; i++) {

//p[0] to p[4] are pointers to average function.

p[i] = &(average);

}

for (i = 5; i < 10; i++) {

//p[5] to p[9] are pointers to sum function

p[i] = &(sum);

}

char str[10] = "nodalo.com";

int *intstr[10];

for (i = 0; i < 9; i++) {

//upto p[4] average function is called, from p[5] sum is called.

intstr[i] = p[i](&str[i], &str[i + 1]);

if (i < 5) {

//prints the average of ascii of both characters

printf(" \n average of %c and %c is %d",

str[i], str[i + 1],*intstr[i]);

}

else {

//prints the sum of ascii of both characters.

printf(" \n sum of %c and %c is %d",

str[i], str[i + 1], *intstr[i]);

}

}

return 0;

}/

/function average is defined here

int *average(char *arg1, char *arg2) {

retrn = (*arg1 + *arg2) / 2;

return (&retrn);

}/

/function sum is defined here

int *sum(char *arg1, char *arg2) {

retrn = (*arg1 + *arg2);

return (&retrn);

}

Output:

average of n and o is 110

average of o and d is 105

average of d and a is 98 average of d and a is 98

average of a and l is 102

average of l and o is 109

sum of o and . is 157

sum of . and c is 145

sum of c and o is 210

sum of o and m is 220

Explanation:

In this program p[10] is an array of function pointers. First five elements of p[10] point to the function: int

*average(char *arg1,char *arg2). Next five elements point to the function int *sum(char *arg1,char *arg2). They return pointer to an integer and accept pointer to char as arguments.

Function average:

int *average(char *arg1,char *arg2) This function finds the average of the two values of the addresses passed to it as arguments and returns address of the average value as an integer pointer.

Function sum:

int *sum(char *arg1,char *arg2) This function finds the sum of the two values of the addresses passed to it as arguments and returns address of the sum value as an integer pointer.

8. What is the value of sizeof(a) /sizeof(char *) in a code snippet:

char *a[4]={"sridhar","raghava","shashi","srikanth"};

Explanation:

Here a[4] is an array which holds the address of strings. Strings are character arrays themselves.

Memory required to store an address is 4 bits. So memory required to store 4 addresses is equal to 4*4=16 bits.

char *; is a pointer variable which stores the address of a char variable.

So sizeof(char *) is 4 bits. Therefore sizeof(a) /sizeof(char *) = 16/4 = 4 bytes.

9. (i) What are the differences between the C statements below:

char *str = "Hello";

char arr[] = "Hello";

(ii) Whether following statements get complied or not? Explain each statement.

arr++;

*(arr + 1) = 's';

printf("%s",arr);

(i) char *str="Hello";

"Hello" is an anonymous string present in the memory. 'str' is a pointer variable that holds the address of this string.

char arr[]="Hello";

This statement assigns space for six characters: 'H' 'e' 'l' 'l' 'o' '\0' . 'arr' is the variable name assigned to this array of characters.

str[4] and arr[4] also have different meanings.

str[4]: adds 4 to the value of 'str' and points to the address same as value of str + 4.

arr[4]: points to the fourth element in array named 'arr'.

(ii) 'arr' is variable name of an array. A variable name can not be incremented or decremented. Hence arr++ is an invalid statement and would result in a compilation error.

*(arr+1)='s';

'arr' is the name of a character array that holds string "Hello". Usually, name of an array points to its base address. Hence value of arr is same as &arr[0].

arr+1 is address of the next element: &arr[1] Character 's' is assigned to the second element in array 'arr', thereby string changes from "Hello" to "Hsllo".

printf("%s",arr ); This statement prints the string stored in character array 'arr'.

Chapter 3

CHAPTER 3

Functions

1. What is the purpose of main() function?

In C, program execution starts from the main() function. Every C program must contain a main() function. The main function may contain any number of statements. These statements are executed sequentially in the order which they are written. The main function can in-turn call other functions. When main calls a function, it passes the execution control to that function. The function returns control to main when a return statement is executed or when end of function is reached. In C, the function prototype of the 'main' is one of the following:

int main(); //main with no arguments

int main(int argc, char *argv[]); //main with arguments

The parameters argc and argv respectively give the number and value of the program's command-line arguments.

Example:

#include <stdio.h>

/* program section begins here */

int main() {

// opening brace - program execution starts here

printf("Welcome to the world of C");

return 0;

}/

/ closing brace - program terminates here

Output:

Welcome to the world of C

2. Explain command line arguments of main function?

In C, we can supply arguments to 'main' function. The arguments that we pass to main ( ) at command prompt are called command line arguments. These arguments are supplied at the time of invoking the program. The main ( ) function can take arguments as: main(int argc, char *argv[]) { } The first argument argc is known as 'argument counter'. It represents the number of arguments in the command line. The second argument argv is known as 'argument vector'. It is an array of char type pointers that points to the command line arguments. Size of this array will be equal to the value of argc. Example: at the command prompt if we give:

C:\> fruit.exe apple mango

then

argc would contain value 3

argv [0] would contain base address of string " fruit.exe" which is the command name that invokes the program.

argv [1] would contain base address of string "apple"

argv [2] would contain base address of string "mango"

here apple and mango are the arguments passed to the program fruit.exe

Program:

#include <stdio.h>

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

int n;

printf("Following are the arguments entered in the command line");

for (n = 0; n < argc; n++) {

printf("\n %s", argv[n]);

}

printf("\n Number of arguments entered are\n %d\n", argc);

return 0;

}

Output:

Following are the arguments entered in the command line

C:\testproject.exe

apple

mango

Number of arguments entered are

3

3. What are header files? Are functions declared or defined in header files ?

Functions and macros are declared in header files. Header files would be included in source files by the compiler at the time of compilation. Header files are included in source code using #include directive.#include<some.h> includes all the declarations present in the header file 'some.h'. A header file may contain declarations of sub-routines, functions, macros and also variables which we may want to use in our program. Header files help in reduction of repetitive code.

Syntax of include directive:

#include<stdio.h> //includes the header file stdio.h, standard input output header into the source code Functions can be declared as well as defined in header files. But it is recommended only to declare functions and not to define in the header files. When we include a header file in our program we actually are including all the functions, macros and variables declared in it.

In case of pre-defined C standard library header files ex(stdio.h), the functions calls are replaced by equivalent binary code present in the pre-compiled libraries. Code for C standard functions are linked and then the program is executed. Header files with custom names can also be created.

Program: Custom header files example

/****************

Index: restaurant.h

****************/

int billAll(int food_cost, int tax, int tip);

/****************

Index: restaurant.c

****************/

#include<stdio.h>

int billAll(int food_cost, int tax, int tip) {

int result;

result = food_cost + tax + tip;

printf("Total bill is %d\n",result);

return result;

}

/****************

Index: main.c

****************/

#include<stdio.h>

#include"restaurant.h"

int main() {

int food_cost, tax, tip;

food_cost = 50;

tax = 10;

tip = 5;

billAll(food_cost,tax,tip);

return 0;

}

4. What are the differences between formal arguments and actual arguments of a function?

Argument: An argument is an expression which is passed to a function by its caller (or macro by its invoker) in order for the function(or macro) to perform its task. It is an expression in the comma-separated list bound by the parentheses in a function call expression.

Actual arguments:

The arguments that are passed in a function call are called actual arguments. These arguments are defined in the calling function.

Formal arguments:

The formal arguments are the parameters/arguments in a function declaration. The scope of formal arguments is local to the function definition in which they are used. Formal arguments belong to the called function. Formal arguments are a copy of the actual arguments. A change in formal arguments would not be reflected in the actual arguments.

Example:

#include <stdio.h>

void sum(int i, int j, int k);

/* calling function */

int main() {

int a = 5;

// actual arguments

sum(3, 2 * a, a);

return 0;

}

/* called function */

/* formal arguments*/

void sum(int i, int j, int k) {

int s;

s = i + j + k;

printf("sum is %d", s);

}

Here 3,2*a,a are actual arguments and i,j,k are formal arguments.

5. What is pass by value in functions?

Pass by Value: In this method, the value of each of the actual arguments in the calling function is copied into corresponding formal arguments of the called function. In pass by value, the changes made to formal arguments in the called function have no effect on the values of actual arguments in the calling function.

Example:

#include <stdio.h>

void swap(int x, int y) {

int t;

t = x;

x = y;

y = t;

} int main() {

int m = 10, n = 20;

printf("Before executing swap m=%d n=%d\n", m, n);

swap(m, n);

printf("After executing swap m=%d n=%d\n", m, n);

return 0;

Output:

Before executing swap m=10 n=20

After executing swap m=10 n=20

Explanation:

In the main function, value of variables m, n are not changed though they are passed to function 'swap'. Swap function has a copy of m, n and hence it can not manipulate the actual value of arguments passed to it.

6. What is pass by reference in functions?

Pass by Reference: In this method, the addresses of actual arguments in the calling function are copied into formal arguments of the called function. This means that using these addresses, we would have an access to the actual arguments and hence we would be able to manipulate them. C does not support Call by reference. But it can be simulated using pointers.

Example:

#include <stdio.h>

/* function definition */

void swap(int *x, int *y) {

int t;

t = *x; /* assign the value at address x to t */

*x = *y; /* put the value at y into x */

*y = t; /* put the value at to y */

} int main() {

int m = 10, n = 20;

printf("Before executing swap m=%d n=%d\n", m, n);

swap(&m, &n);

printf("After executing swap m=%d n=%d\n", m, n);

return 0;

}

Output:

Before executing swap m=10 n=20

After executing swap m=20 n=10

Explanation:

In the main function, address of variables m, n are sent as arguments to the function 'swap'. As swap function has the access to address of the arguments, manipulation of passed arguments inside swap function would be directly reflected in the values of m, n.

7. What are the differences between getchar() and scanf() functions for reading strings?

Differences between getchar and scanf functions for reading strings:

scanf getchar

1. Entering of each character should be followed by return key. 1. Need not type return key.

2. Continuous stream of characters cannot be directly supplied using scanf function.

2. Continuous stream of characters can be directly supplied using getchar function

3. Scanf function can be used to provide data at execution time irrespective of its type(int, char,float).

Example:

#include<stdio.h>

int main() {

char a[10];

printf("Enter a: \n");

scanf("%s",a);

return 0;

}

3. getchar() function is used only with character type.

Example:

#include<stdio.h>

int main() {

char a;

printf("Enter any character: \n");

a = getchar();

printf("Character entered:%c \n",a);

return 0;

}

4. scanf() returns the number of items read successfully. A return value 0 indicates that no fields were read. EOF(end of file) is returned in case of an error or if end-of-file/end-of-string character is encountered.

4. getchar() returns the character entered as the value of the function. It returns EOF in case of an error. It is recommended to use getchar instead of scanf.

8. Out of the functions fgets() and gets(), which one is safer to use and why?

Out of functions fgets( ) and gets( ), fgets( ) is safer to use. gets( ) receives a string from the keyboard and it is terminated only when the enter key is hit. There is no limit for the input string. The string can be too long and may lead to buffer overflow.

Example:

gets(s) /* s is the input string */

Whereas fgets( ) reads string with a specified limit, from a file and displays it on screen. The function fgets( ) takes three arguments.

First argument : address where the string is stored.

Second argument : maximum length of the string.

Third argument : pointer to a FILE.

Example:

fgets(s,20,fp); /* s: address of the string, 20: maximum length of string, fp: pointer to a file */

The second argument limits the length of string to be read. Thereby it avoids overflow of input buffer. Thus fgets( ) is preferable to gets( ).

9. What is the difference between the functions strdup() and strcpy()?

strcpy function: copies a source string to a destination defined by user. In strcpy function both source and destination strings are passed as arguments. User should make sure that destination has enough space to accommodate the string to be copied. 'strcpy' sounds like short form of "string copy".

Syntax:

strcpy(char *destination, const char *source);

Source string is the string to be copied and destination string is string into which source string is copied. If successful, strcpy subroutine returns the address of the copied string. Otherwise, a null pointer is returned.

Example Program:

#include<stdio.h>

#include<string.h>

int main() {

char myname[10];

//copy contents to myname

strcpy(myname, "interviewmantra.net");

//print the string

puts(myname);

return 0;

}

Output:

interviewmantra.net

Explanation:

If the string to be copied has more than 10 letters, strcpy cannot copy this string into the string 'myname'. This is because string 'myname' is declared to be of size 10 characters only. In the above program, string "nodalo" is copied in myname and is printed on output screen. strdup function: duplicates a string to a location that will be decided by the function itself. Function will copy the contents of string to certain memory location and returns the address to that location. 'strdup' sounds like short form of "string duplicate"

Syntax:

strdup (const char *s);

strdup returns a pointer to a character or base address of an array. Function returns address of the memory location where the string has been copied. In case free space could not be created then it returns a null pointer. Both strcpy and strdup functions are present in header file <string.h>

Program: Program to illustrate strdup().

#include<stdio.h>

#include<string.h>

#include<stdlib.h>

int main() {

char myname[] = "interviewmantra.net";

//name is pointer variable which can store the address of memory location of string char* name;

//contents of myname are copied in a memory address and are assigned to name name = strdup(myname);

//prints the contents of 'name'

puts(name);

//prints the contents of 'myname'

puts(myname);

//memory allocated to 'name' is now freed

free(name);

return 0;

}

Output:

interviewmantra.net

interviewmantra.net

Explanation:

String myname consists of "interviewmantra.net" stored in it. Contents of myname are copied in a memory address and memory is assigned to name. At the end of the program, memory can be freed using free(name);