#include <fstream.h>
#include <stdlib.h>

const int MAXLENGTH = 21;  // maximum file name length

void openInput(ifstream&);   // pass a reference to an ifstream
void openOutput(ofstream&);  // pass a reference to an ofstream
float pollenUpdate(ifstream&, ofstream&);  // pass two references


int main()
{
  ifstream inFile;   // in_file is an istream object
  ofstream outFile;  // out_file is an ofstream object
  float average;

    // display a user message
  cout << "\n\nThis program reads the old pollen count file, "
       << "creates a current pollen"
       << "\n    count file and calculates and displays "
       << "the latest ten week average.";

  openInput(inFile);
  openOutput(outFile);

  average = pollenUpdate(inFile, outFile);

  cout << "\nThe new ten week average is: " << average << endl;

  return 0;
}

// this function gets an external file name and opens the file for input
void openInput(ifstream& fname)
{
  char filename[MAXLENGTH];

  cout << "\n\nEnter the input pollen count file name: ";
  cin  >> filename;

  fname.open(filename, ios::nocreate);

  if (fname.fail())    // check for a successful open
  {
    cout << "\nFailed to open the file named " << fname << "for input"
	 << "\n Please check that this file exits"
	 << endl;
    exit(1);
  }

  return;
}

// this function gets an external file name and opens the file for output
void openOutput(ofstream& fname)
{
  char filename[MAXLENGTH];

  cout << "Enter the output pollen count file name: ";
  cin  >> filename;

  fname.open(filename);

  if (fname.fail())    // check for a successful open
  {
    cout << "\nFailed to open the file named " << fname << "for output"
	 << endl;
    exit(1);
  }

  return;
}

// the following function reads the pollen file
// writes a new file
// and returns the new weekly average
float pollenUpdate(ifstream& infile, ofstream& outfile)
{
  const int POLNUMS = 10; // maximum number of pollen counts

  int i, polreading;
  int oldreading, newcount;
  float sum = 0;
  float average;

    // get the latest pollen count
  cout << "Enter the latest pollen count reading: ";
  cin  >> newcount;


    // read the oldest pollen count
  infile >> oldreading;

  // read, sum and write out the rest of the pollen counts
  for(i = 0; i < POLNUMS - 1; i++)
  {
    infile >> polreading;
    sum += polreading;
    outfile << polreading << endl;
  }
    // write out the latest reading
  outfile << newcount << endl;

    // compute the new average
  average = (sum + newcount) / (float) POLNUMS;

  infile.close();
  outfile.close();

  cout << "\nThe output file has been written.\n";

  return average;
}