drew_grades.cpp

#include <iostream>
using namespace std;

const int ENDGRADE = 999;

int main() {
  cout << "This program accepts user-entered grades and calculates the average." << endl;

  double grade_entered = 0;
  double grade_total;
  int num_grades = 0;

  do {
    cout << "Please enter a grade: ";
    cin >> grade_entered;

    if (grade_entered >= 0 && grade_entered <= 100) {
      grade_total += grade_entered;
      num_grades++;
    } else if (grade_entered != ENDGRADE) {
      cerr << grade_entered << " is an invalid grade." << endl;
    }
  } while (grade_entered != ENDGRADE);

  if (num_grades > 0) {
    double average = grade_total / num_grades;
    cout << "The average of all grades is " << average << " percent" << endl;
  } else {
    cout << "No grades were entered." << endl;
  }

  return 0;
}