Pointers

POINTERS

INTRODUCTION

Generally, computer uses memory for storing instruction and values of the variables with in the program, but the pointers has memory address as their values. The memory address is the location where program instructions and data are stored, pointers can be used to access and manipulate data stored in the memory.

The computer's memory is a sequential collection of storage cells as shown below:

In computer's memory each cell is 1 byte, and it has a number called address, this address is numbered consecutively starting from zero to last address (depends upon the memory size, in 64K memory having the last address as 65,535).

While declaring variable, the computer allocates appropriate memory to hold the value of the variable. Since every memory has a unique address, this address location will have its own address number somewhere in the memory area.

Example: int sno=39;

The above statement instructs the system to specify a location for the integer variable ‘sno' and put the value 39 in that location. Assume that the system has chosen the address location 3977 for 'sno' this may be represented as

During the execution of the program, the system always associates the variable 'sno' with the address 3977. We may have access to the value 39 by using either the name of the variable 'sno' or the address 3977.

Example: Program to print address and value of variable.

 /* Program to print address of variable sno and value of variable sno */

#include <stdio.h>

main()

{

int sno=39;

printf ("Address of sno=%u", &sno);

printf ("Value of sno= %d", sno);

} /* main */

OUTPUT

Address of sno=234560

Value of sno=39

EXPLANATION: '&' is 'address of operator. The expression &sno returns the address of the variable sno (i.e.) 39. The '%u' is used for obtaining the address.

Pointers are also allotted storage location in memory, where the addresses of other variable will be stored.

1. Pointer Concept

One of the most important and powerful features in C language is pointer. It has added power and flexibility to the language.

"A pointer is a variable; it may contain the memory address of another variable.” Pointer can have any name that is legal for other variable. It is declared in the same manner like other variables. It is always denoted by '*' operator.

A pointer is a variable whose value is also an address. Each variable has two attributes: address and value. A variable can take any value specified by its data type. A pointer to an integer is a variable that can store the address of that integer.

Assigning values directly to variables, we can indirectly manipulate a variable by creating a variable called a pointer, which contains the memory address of another variable.

The second variable contains the address of the first variable. 39 is the value at the address, 3977 is the address of the first variable.

Simply we can define pointers in two ways.

• First a pointer is a variable and assigns different values to a pointer variable.

• Second the value contained by a pointer must be an address which indicates the location of another variable in the memory.

So, pointer is called as "address variable".

Pointer seems to be complex but it is simple when compared to its advantages. Pointer is more difficult to understand initially, but when we start working once and understood, it will be easy.

2. Features of Pointers

The benefits of using pointers are

a) Pointers are efficient in handling data and associated with array.

b) Pointers are used for saving memory space.

c) Pointers reduce length and complexity of the program.

d) Use of pointer assigns the memory space and also releases it. It helps to make better use of the available memory (dynamic memory allocation).

e) Since the pointer data manipulation is done with address, the execution time is faster.

f) The two‒dimensional and multi‒dimensional array representation is easy in pointers.

g) Pointer allows for references to function, this may facilitate passing of function as arguments to other functions.

3. Advantages of Using Pointers

Pointers provide the following benefits or advantages.

a) Pointers are more compact and efficient code.

b) Pointers can be used to achieve clarity and simplicity.

c) Pointers are used to pass information between function and its reference point.

d) Pointers provide a way to return multiple data items from a function using its function arguments.

e) Pointers also provide an alternate way to access an array element.

f) Pointers enable us to access the memory directly.

4. Pointer Declaration

Pointer is a variable that contains the address of another variable. Like normal variable declared in 'C', pointers can also be declared.

Syntax: data‒type *pointer‒name;

Description:

data‒type Specifies the type of data to which the pointer points.

Pointer‒name Specifies the name of the pointer. (It must preceded with an (*) asterisk.

Example:

int *a;

char *b;

float *c;

Accessing variable through pointers

Once the pointer is declared and assigned to the address of another variable, the variable can be accessed through its pointers. This is done by using another unary operator * (asterisk), usually known as the indirection operator. Another name for the indirection operator is the dereferencing operator.

Example:

int *p;

x = 15;

p = &x;

 Example 1: Program to accessing through variable pointer.

/* Program for accessing variable through pointer */

#include <stdio.h>

main. ( )

{

int a=22, *a; /* *a, "b is a pointer variables */

float b=2.25, *b;

a = &a;  /* & is a address of letter and it represents the address of the variable */

b = &b;

printf ("\n Value of a=%d", *a);

printf("\n Value of b=%d", *b);

} /* main */

OUTPUT

Value of a=22

Value of b=2.25

EXPLANATION: Variable 'a' is assigned values as 22, again 'a' and 'b' declared as pointer variable. The statement a=&a assigns address of a to variable a and the statement b=&b assigns address of b to variable b. Then finally print the value of a and b.

Example 2: Program to print address and value of variable.

 /* Program to accessing variable through pointer */

#include <stdio.h>

main ( )

{

int a=22;  /* Local definition */

int *a; a=&a;

printf ("\n Value of a=%d", *a);

printf ("\n Address of a=%u", &a);

printf ("\n Value at address %u=%d", &a, *(&a));

} /* main */

OUTPUT

Value of a=22

Address of a=4000

value at address 4000=22.

EXPLANATION: Assign a to 22, Address of variable a is assigned to pointer a. Print the value of a using pointer variable and print the address of a variable by using address operator &.

NOTE: (&a)=('a' means the value at address) of the variables.

i. Null Pointer

A pointer is said to be a null pointer when its right value is 0. A null pointer can never point to a valid data. For checking a pointer, if it is assigned to 0, then it is a null pointer and is not valid.

Example:

int *a;

int *b;

b=a=0;

Hence band a become null pointers after the integer value of 0 is assigned to them.

ii. Pointer to Pointer

Pointer is a variable that contains the address of another variable. Similarly another pointer variable can store the address of this pointer variable. So we can say, this is a pointer to pointer variable.

Example: Program to illustrate pointer to pointer concept.

/* Program using pointer to pointer */

#include<stdio.h>

main ()

{

int a=22; /* Local definitions */

int *b; /* *b is a pointer variable */

int **c; /* **c is a pointer to another pointer */

b=&a; /* '&a' is a address variable */

c=&b;

printf ("\n Value of a is %d", a);

printf ("\n Value of a is %d", *(&a));

printf ("\n Value of a is %d", *b);

printf ("\n Value of a is %d", **c);

printf ("\n Value of b and address of a=%u", b);

printf("\n Value of c and address of b = %u", c);

printf ("\n Address of a=%u", &a);

printf ("\n Address of b=%u", &b);

printf ("\n Address of a=%u", *c);

printf ("\n Address of b=%u", &b);

printf ("\n Address of b=%u", c);

printf ("\n Address of c=%u", &c);

} /* main */

OUTPUT

Value of a is 22         Value of a=22

Value of a is 22          Value of a=22

Value of b and address of a=2000

Value of c and address of b=4000

Address of a=2000       Address of b=4000

Address of a=2000       Address of b=4000    

Address of b=4000       Address of c=6000

iii. Pointer Expressions

Pointer variables can be used in expressions. Here, the pointers are preceded by the *(indirection operator) symbol.

Example:

i) c=*a+*b; can be written as c=(*a)+(*b);

ii) s=10*‒*a*b; can be written as s=(10*(‒(a)))/(*b);

iii) *p=*a**b; can be written as*p=(*a)**b;

iv) *a=*a+5; can be written as(*a)+=5;

'C' allows us to add or subtract integers from one pointer to another pointer. In addition to that, the pointer can also be compared using relational operators. But comparisons can be used mainly in handling arrays and strings.