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

int main()
{
    double d;
    int n = 0;
    double vect[1024*128];

    while (scanf("%lf", &d) != EOF) vect[n++] = sqrt(d);

    while (--n >= 0) printf("%.5lf\n", vect[n]);

    return 0;
}

"double vect[1024*128];"
You can use "malloc" or "new" operator.

Or you can allocate memory as a global variable. (Before line: int main(void))