#include <iostream>
//#include <conio.h>   Non-standard. Please avoid this header.
#include <limits>
#include <cctype>   // For toupper(), used below.
                    // Don't forget to include all the proper headers!

using namespace std;

float GetTestScore();
float ComputeAverage(int, float);
bool  MoreScores();

int main()
  {
  float Sum        = 0.0f;
  int   ScoreCount = 0;

  do
    {
    Sum += GetTestScore();
    ScoreCount++;
    }
  while (MoreScores());

  float Average = ComputeAverage(ScoreCount, Sum);

  cout << "\n\nAverage score: " << Average << endl;

  //getch();
  //  This is a non-standard function.
  //  The better way is as follows:
  cout << "\nPress ENTER to continue..." << flush;
  cin.sync();
  cin.ignore( numeric_limits <streamsize> ::max(), '\n' );

  return 0;
  }

float GetTestScore()
  {
  float score;
  cout << "Enter next score: ";
  cin >> score;
  while (score < 0 || score > 100)
    {
    cout << "***Invalid test score. ";
    cout << "\nRe-enter test score: ";
    cin >> score;
    }

  return score;
  //  This is an unconditional return, meaning anything that follows
  //  is meaningless. Your function should have ended here.

  //MoreScores();
  //  The function has accomplished its purpose: it asked for and validated a
  //  number from the user (a "score"), which it then returned. (See line 20,
  //  where the return value is added to 'Sum' in the main() function.)
  //  Hence, there is no need to call the MoreScores() function from here.
  }

float ComputeAverage(int ScoreCount, float Sum)
  {
  float Average = Sum / ScoreCount;
  return Average;

  //main();
  //  Please, never, ever do this. It is wrong, evil, and immoral.
  //  (Don't take that the wrong way -- just some strong language to imprint it
  //  in your brain.)
  //
  //  Again, the function accomplished its purpose and unconditionally returned
  //  a value to the main function (line 25). There is no need to execute another
  //  instance of main() -- which would be equivalent to starting the program
  //  again, except that it is illegal and/or undefined by the language standards.
  }

bool MoreScores()
  {
  char UserResponse;
  cout << "Are there more scores to be processed? ";
  cout << "\nEnter Y or N: ";
  cin >> UserResponse;

#if 1
  if(toupper(UserResponse) == 'Y')
    {
    return true;
    //GetTestScore();
    }
  if(toupper(UserResponse) == 'N')
    {
    return false;
    //main();
    }
  int count = 0;
  //while(toupper(UserResponse) != 'Y' || toupper(UserResponse) != 'N' && count <= 3)
  //  This line is what ak555 was complaining about.
  //  The && operator has a higher precedence than does the || operator.
  //  As a rule of thumb, I always add those extra parentheses to enforce
  //  proper evaluation order. In your case, you wanted:
  while ((toupper(UserResponse) != 'Y' || toupper(UserResponse) != 'N') && count <= 3)
    {
    cout << "***Invalid Response. ";
    cout << "\nMust be Y or N ---> ";
    cin >> UserResponse;
    count++;
    }
  if(count == 3)
    {
    cout << "****WARNING: Three invalid entry attempts. ";
    cout << "\nNo more scores assumed. ";
    return false;
    //main();
    }

  // Here, the function terminates without returning anything.
  // You must return something if your function is declared with a
  // non-void type.

#else
  // I think this function needs some rethinking. I suggest you use a switch
  // statement in a loop. Here is an improved version. To use it, either change the
  //   #if 1
  // line to read
  //   #if 0
  // or just delete everything above and remove the #if #else #endif lines.

  int count = 0;
  do
    {
    switch (toupper(UserResponse))
      {
      case 'Y': return true;
      case 'N': return false;
      default:
        count++;
        if (count < 3)
          {
          cout << "***Invalid Response. ";
          cout << "\nMust be Y or N ---> ";
          cin >> UserResponse;
          }
      }
    }
  while (count < 3);
        
  cout << "***WARNING: Three invalid entry attempts. "
          "\nI assume that there aren't any more scores. ";
  return false;
#endif
  }