accept this kind of double:
#include <vector>
#include <iostream>
#include <algorithm>
#include <iterator>

#include <cmath>

using namespace std;

void root( double& d )
{
    d = sqrt(d);
}

int main()
{
    vector<double> arr;
    copy( istream_iterator<double>(cin), istream_iterator<double>(), back_inserter(arr) );

    for_each( arr.begin(), arr.end(), root );

    cout.precision(4);
    cout << fixed;
    copy( arr.rbegin(), arr.rend(), ostream_iterator<double>(cout, "\n"));

    return 0;
}

Wrong answer of "float" type:

#include <vector>
#include <iostream>
#include <algorithm>
#include <iterator>

#include <cmath>

using namespace std;

void root( float& d )
{
    d = sqrt(d);
}

int main()
{
    vector<float> arr;
    copy( istream_iterator<float>(cin), istream_iterator<float>(), back_inserter(arr) );

    for_each( arr.begin(), arr.end(), root );

    cout.precision(4);
    cout << fixed;
    copy( arr.rbegin(), arr.rend(), ostream_iterator<float>(cout, "\n"));

    return 0;
}

strange problem ......