// Principle: how to get a computed number to a file name 
// a cleaner way: see basic/string_to_filename.cpp
#include <iostream>
#include <sstream>
#include <fstream>
#include <math.h>
using namespace std;
int main()
{
  stringstream s;
  const string str = "res";
  for (unsigned i=1;i<5;i++){
    s<<str<<"_"<<cos(i);
    ofstream out(s.str().c_str()); // dig out C-string         
    s.str(""); // clears s
    out.close();
  }
}
/* opened files 
res_0.540302
res_-0.416147
res_-0.989992
res_-0.653644
*/