Pointer

Pointer in c

Pointers are used to access the variables directly by using their address Locations. The value has been assigned to the variable gets stored in that particular memory location.

• Null pointer

A pointer that is assigned NULL is called a "null pointer".The value of null pointer is 0.

• Void pointer

A void pointer is also called as generic pointer. It doesn't have any standard data type. In this pointer void keyword is used. It is used to store the address of any variable.

Syntax:

void *p = NULL;    //Void pointer  

Pointer operators

Two types of pointer operators:

1. address operator[&]
2. indirection/derefrencing operator[*]

1.address operator[&]

& is used to determine the address of the variable.

Example:

int a=1;
            int *aptr;
            aptr= &a;   //aptr gets address of a   

2.indirection/derefrencing operator[*]

* can be used to assign a value to a location in memory.

Example:

*aptr=2;    //changes a to 2 

Declaration of pointer

It contain data type and pointer name with astric sign.

Syntax:

data type *pointer_name;

here, * is pointer variable.

Example :

int *ptr;

This declares that the pointer variable points to an integer data type.

Initialization of pointer

Initialization of pointer variable is done by assigning to it the address of a variable

Syntax:

pointer_name= &variable;

Example:

 int a=2081;
           int *ptr;   //Declaration of pointer
           ptr = &a;  //Initialization of pointer 

Advantages of pointer

1. Dynamic memory Allocation
2. Pointer provide direct access to memory.
3. Reduce the execution time of the program.
4. It provide an alternate way to access array elements.
5. Using pointers, arrays and structures can be handled in more efficient way.
6. Searchin of large size data is very efficient.

Disadvantages of pointrer

1. Pointers are slower than normal variables.
2. Poiner lead to memory leaks.
3. Pointer are a litle complex to understand.
4. If pointers are updated with incorrect values, it might lead to memory corruption.

Methods of parameter passing

• Call by value

[It is completed in function tutorial.]

• Call by reference

It is a method of passing arguments to a function copies the address of an actual parameter into the formal parameter.
To pass value by reference, pointer is passed as argument to function call like any other value.

Example:

             
             #include<stdio.h> 
             
             int main () 
             {
                 /* local variable definition */
                 int a = 100;
                 int b = 200;
               
                 printf("Before swap, value of a : %d\n", a );
                 printf("Before swap, value of b : %d\n", b );
               
                 /* calling a function to swap the values */
                 swap(&a, &b);
               
                 printf("After swap, value of a : %d\n", a );
                 printf("After swap, value of b : %d\n", b );
               
                 return 0;
              }
              void swap(int *a, int *b) {
              
                 int temp;
              
                 temp = a; / save the value of a */
                 a = *b;    / put y into a */
                 b = temp; / put temp into b */
                
                 return;
              }
         

Output

             Before swap, value of a : 100
             Before swap, value of b : 200
             After swap, value of a : 200
             After swap, value of b : 100
         

Sample programs:

1.C program to addition of two number using pointer

         
         #include<stdio.h> 
         
         int main()
             {
                int first, second, *p, *q, sum;
             
                printf("Enter any two integers\n");
                scanf("%d%d", &first, &second);
             
                p = &first;
                q = &second;
             
                sum = *p + *q;
             
                printf("Sum of the two numbers = %d\n", sum);
             
                return 0;
             }
     

         Enter any two integers
         2
         3
         Sum of the two numbers = 5
     

2.C Program to reverse a number using pointer

        
        #include<stdio.h> 
         
        int  main( )
        {
             int  num, rem, rev=0 ;
             int  *pn, *pr ;
             printf(" Enter the number \n") ;
             scanf("%f ",& num) ;
             pn = & num ;
             pr = & rev ;
             do
              {
                rem = ( *pn ) % 10 ;
                *pr = ( *pr * 10 ) + rem ;
                *pn = ( *pn ) / 10 ;
              }
              while(  *pn > 0) ;
                 printf("\n Reverse of Number is : %d ",*pr) ;
                 return ( 0 );
        }
    

        Enter the number
        123 
        Reverse of Number is : 321
    

3.C program to find the largest number using pointer

        
        #include<stdio.h> 
        #include<conio.h> 
         
        void main()
        {
           int x,y, *large, *xptr,*yptr;
           clrscr();
           printf("Enter the value of x and y \n");
           scanf("%d%d",&x,&y);
           xptr=&x;
           yptr=&y;
           if(*xptr>*yptr)
           {
            large=xptr;
           } 
           else
           {
            large=yptr;
           } 
           printf("The largest number is : %d",*large);
           getch();
        }  
    

        Enter the value of x and y 
        34
        45
        The largest number is : 45
    

Examples For Practice

1. Find reverse of the string using pointer.
2. Write a program to print the elements of a 1D integer array in reverse order along with memory address of each element using pointer.
   (use decrement operator on pointer to print the address)
   e.g for array containing 10,20,30…output will be
   30 at 65528……..
   20 at 65526
   10 at 65524
3. 5) Write a program to print characters in string along with memory addresses of each character
   e.g for sting : “hello” output will be
   h at 65524
   e at 65525……..