double Student::getGpa() const
{
	// for every course, add (course units * grade) to the total,
	double total = 0.0;
	for (const auto& course : coursesCompleted)
	{
		switch (course.second)
		{
			case 'A': total += course.first.getCourseUnits() * 4.0; break;
			case 'B': total += course.first.getCourseUnits() * 3.0; break;
			case 'C': total += course.first.getCourseUnits() * 2.0; break;
			case 'D': total += course.first.getCourseUnits() * 1.0; break;
			case 'F': break;
			default: "Invalid Course Grade"; break;
		}
	}

	// return the total / all units
	return static_cast<double>(total / getUnitsCompleted());
}