Pointer

Pointer

Pointer is variable which hold the address of a variable

Pointer declaration :

It is similar to normal variable declaration only difference is  variable contain * symbol before variable name

Syntax   data type    *pointer variable name

Where * : asterisk

        Data type : Basic data type like int,  float, char, double

        Pointer variable name is valid identifier

Example

 int     *i;    /* pointer to an integer */

double  *d;    /* pointer to a double */

float   *f;    /* pointer to a float */

char    *c     /* pointer to a character */

Initialization of Pointer variable

Pointer Initialization is the process of assigning address of a variable to a pointer variable. Pointer variable can only contain address of a variable of the same data type. In C language address operator & is used to determine the address of a variable. The & (immediately preceding a variable name) returns the address of the variable associated with it.

#include<stdio.h>

   

void main()

{

    int a = 10;

    int *ptr;       //pointer declaration

    ptr = &a;       //pointer initialization

}

Pointer variablea always point to variables of same datatype. Let's have an example to showcase this:
Example 2

Advantage of pointer

1) Pointer reduces the code and improves the performance, it is used to retrieving strings, trees etc. and used with arrays, structures and functions.

2) We can return multiple values from function using pointer.

3) It makes you able to access any memory location in the computer's mem

/*C program to create, initialize, assign and access a pointer variable.*/

#include <stdio.h>

int main()

{

    int num;    /*declaration of integer variable*/

    int *pNum;  /*declaration of integer pointer*/

    pNum=& num; /*assigning address of num*/

    num=100;    /*assigning 100 to variable num*/

    //access value and address using variable num

    printf("Using variable num:\n");

    printf("value of num: %d\naddress of num: %u\n",num,&num);

    //access value and address using pointer variable num

    printf("Using pointer variable:\n");

    printf("value of num: %d\naddress of num: %u\n",*pNum,pNum);

   return 0;

}

Output

    Using variable num:

    value of num: 100

    address of num: 2764564284

    Using pointer variable:

    value of num: 100

    address of num: 2764564284

Pointer & Array

 Array is a collection of similar data element

Ex  int arr[5] = { 11, 22, 33, 44, 55 };

Assuming the base address of arr is 1000 and each integer requires two bytes, the five elements will be stored as follows:

Element arr[0]           arr[1]                  arr[2]                     arr[3]           arr[4]

11

22

33

44

55

Address     1000             1002           1004                     1006           1008

 pointer   p

1000

Pointer variable p holds the address of array ie arr[0].

We can write it as

int *p; //pointer variable

int arr[5]={11,22,33,44,55};

p=&arr;  //or

p=&arr[0];

Example write a c program that print  array element using pointer

#include <stdio.h>

    main () {

   int  arr[3] = {5, 10, 15};

   int i, *ptr[3];

   for ( i = 0; i < 3; i++) {

      ptr[i] = &arr[i]; /* assign the address of integer. */

   }

  

   for ( i = 0; i < arr; i++) {

      printf("Value of arr[%d] = %d\n", i, *ptr[i] );

   }

}

Out put

Value of arr[0] = 5

Value of arr[1] = 10

Value of arr[2] = 15

function in pointer
 Definition a small code is written separately whenever code is required called and executed is called function.
syntax of function
 data type function_name( parameter list)
{
 body of function
}

in these function we are passing address of variable instead of value through pointer . Any changes made in function will effect 
 example call by pointer value
#include<stdio.h>
 void add(int ,int);// function prototype
void main()
{
  int a=5,b=6;
  int *p,*q;  // pointer variable
  p=&a; // assign the address of variable a to pointer variable p
  q=&b;//assign the address of variable b to pointer variable q
   
  add(*p,*q);
}
 void add(int *p,int *q)
 {
       int sum=0;
    sum=*p+*q;
    printf("sum of two number =%d",sum);
 }
  
example 2 swapping of two number using intermediate variable 
  
#include<stdio.h>
 void add(int ,int);// function prototype
void main()
{
  int a=10,b=20;
  int *p,*q;  // pointer variable
  p=&a; // assign the address of variable a to pointer variable p
  q=&b;//assign the address of variable b to pointer variable q
 printf(" Before swapping a=%d & b=%d ",a,b); 
  swap(*p,*q);
}
 void swap(int *p,int *q)
 {
       int *temp;
     temp=p;
     p=q;
      q=temp;
    printf("After  swapping a=%d & b=%d ",p,q); 
 }
  output 
  Before swapping a=10 & b=20
  After  swapping a=20 & b=10

example 2 swapping of two number without using intermediate variable 
  
#include<stdio.h>
 void add(int ,int);// function prototype
void main()
{
  int a=10,b=20;
 printf(" Before swapping a=%d & b=%d ",a,b); 
  swap(&a,&b); //call by Address 
}
 void swap(int *p,int *q)
 {
      *p=*p+*q;
      *q=*P-*q;
       *p=*P-*q;
    printf("After  swapping a=%d & b=%d ",p,q); 
 }
  output 
  Before swapping a=10 & b=20
  After  swapping a=20 & b=10