#include <string>
#include <iostream>
#include <fstream>
using namespace std;

/**
   Reads numbers from a file and finds the maximum value
   @param in the input stream to read from
   @return the maximum value or 0 if the file has no numbers
*/            
double read_data(fstream& in)
{  double highest;
   double next;
   if (in >> next) highest = next;
   else return 0;

   while (in >> next)
      if (next > highest) highest = next;
   
   return highest;
}

int main()
{  double max;

   string input;
   cout << "Do you want to read from a file? (y/n) ";
   cin >> input;

   if (input == "y")
   {  string filename;
      cout << "Please enter the data file name: ";
      cin >> filename;

      ifstream infile;
      infile.open(filename.c_str());

      if (infile.fail()) 
      {  cout << "Error opening " << filename << "\n";
         return 1;
      }

      max = read_data(infile);
      infile.close();
   }
   else max = read_data(cin);

   cout << "The maximum value is " << max << "\n";
   return 0;
}