loop each student
  loop each class
    get_class_info(grade,credits)
    if A-F
      gpa_pts += credit_pts(grade,credits)
  gpa = gpa_pts / gpa_credits


get_class_info function: 
  "in" args: none
  "out" args: valid grade (char) and valid credits (int)
  return value will be used to indicate successfully got input or no 
     more class info

credit_pts function:
  "in" args: grade and credits for one class
  "out" args: none
  return value is number of points for the grade and credits combination

gpafunc.cpp