You can't do that in C.  Use dynamic allocation method
to implement array.  See Q. 20 and 41 in 
http://www.geocities.com/vijoeyz/faq/

Best,
Vijay Zanvar,
Home Page - http://geocities.com/vijoeyz/

by using calloc() or malloc() functions we create at run 
time 


        i think it will be correct if not forgive me 

              sumankumar..........

This can be done by using malloc...if u have an integer 
array then ...you can ask user the size..and then allocate 
memory for integers...at the run time...using malloc()

that is the main drawback of an array..
u cannot assign memory at run time...

#include <stdlib.h> /* EXIT_FAILURE, EXIT_SUCCESS */
        #include <errno.h>
        #include <stdio.h>

        #define     ROW     5
        #define     COL     5

        int
        main ( void )
        {
            int **cont_arr;
            int **cont_arr;

            int **arr = malloc ( ROW * sizeof ( int * ) );
            if ( !arr )
            {
                perror ( "Error" );
                exit ( EXIT_FAILURE );
            }

            for ( i=0; i < ROW; i++ )
            {
                arr[i] = malloc ( sizeof ( int ) * COL );
                if ( !arr[i] )
                {
                    perror ( "Error" );
                    exit ( EXIT_FAILURE );
                }
            }

            /*
             * Contiguous memory allocated for array.  
Below.
             */

            cont_arr = (int **) malloc ( ROW * sizeof ( int 
* ) );
            if ( !cont_arr )
            {
                perror ( "Error" );
                exit ( EXIT_FAILURE );
            }

            cont_arr[0] = (int *) malloc ( ROW * COL * 
sizeof ( int ) );
            if ( !cont_arr[0] )
            {
                perror ( "Error" );
                exit ( EXIT_FAILURE );
            }

            for ( int i=1; i < ROW; i++ )
                cont_arr[i] = cont_arr[0] + i * COL;

            exit ( EXIT_SUCCESS );
        }

Declare a integer array size at runtime:
1.declare an interget pointer. - int *ptr
2.Get the size of array from user - int n
3.using malloc() ,allocate that many number of location to 
the pointer. - ptr = malloc(n*sizeof(int));
4.Make use of the dynamically allocated array. - ptr[]

main()
{
 int n,i;
 scanf("%d",&n);
 int a[n];
}