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Testing for end-of-file

In the above the functions open and fail have been attached to a stream name. Another function that can be attached to a stream name is the end-of-file condition, eof. This condition is set true when an an attempt is made to read beyond the end of a file, otherwise it is set to false. Unlike some other languages the end-of-file character is actually entered and the eof function returns true when it is entered. For example to read values from a file and evaluate their average then the following code could be used:

#include <iostream.h>
#include <fstream.h>
#include <iomanip.h>

int main()
{
  int n;
  float x, sum, average;
  ifstream ins;         // input stream
  ofstream outs;        // output stream
  ins.open("indata.dat");

     // open files, exit program if fail
  if (ins.fail())
    {
      cout << "Can't open indata.dat" <<endl;
      return 1; //exit with code 1 for failure
    }
  outs.open("results.dat");
  if (outs.fail())
    {
      cout << "Can't open results.dat" << endl;
      return 1; //exit with code 1 for failure
    }

  // Initialise and let user know something is happening
  sum = 0.0;
  n = 0;
  cout << "Reading input file " << endl;
      // read from file, accumulate sum and output average
      // to output file.
  ins >> x;  // if file was empty then eof would now be true
  while (!ins.eof())
    {
      sum += x;
      n++;
      ins >> x;
    }
  average = sum / n;
  cout << "Writing results to file " << endl;
  outs << "The average of " << n << " numbers is "
       << setprecision(2)
       << average << endl;
  ins.close();     // Close all files - GOOD PRACTICE
  outs.close();
  return 0;        // indicate success
}
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Note that at the end the streams ins and outs were closed. While some operating systems will do this automatically when your program terminates it is good practice to always close files before exiting a program. If this is not done then sometimes not all of the output is written to the file.

If the program was to be run again with data from a different file then the only way this could be done would be to edit the program above and replace each occurrence of indata.dat with the name of the other input file. This is obviously inconvenient and it is much better to allow the user to enter the file name at run-time. However this topic is delayed until Lesson 23 when further consideration has been given to the representation of arbitrary strings within a program.


Next: Summary Up: Streams and External Files Previous: Connecting Streams to External