#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;

int main()
{
        long double array[128*1024];
    long double x = 0;
    int k = 0;
    while ( cin >> x )
    {
        array[k] = sqrt(x);
        ++k;
    }
    for ( int i = k-1; i >= 0; --i )
        cout << fixed << setprecision(4) << array[i] << endl;
    return 0;
}

try to define array in global namespace (not in main())

Thank you very much !