#include<stdio.h>
#include<math.h>

int main(void)
{
    double a[256*1024/sizeof(double)];

    int i=0;

    while(i < 256*1024/sizeof(double))
    {
        scanf("%lf",&a[i]);

        i++;
    }



    for(--i;i >=0 ; i--)
        printf("%.4lf\n",sqrt(a[i]));




    return 0;



}

The number of input numbers can be less than 256*1024. Use the fact that when scanf cannot read next double it returns 0 (it returns the number of successfully read parameters), and break your while then.

just use <stack> from stl