#!/usr/bin/env python
#
# Author  : Ahmed Obied (amaobied@ucalgary.ca)
# Date    : Jan. 17th 2008
# Purpose : Calculates the percentage and letter grade of a given total
#           and mark to demonstrate the use of functions
#

def read_total_marks():
    """
    Purpose    : prompts the user to enter in the total marks for the test. This
                 function also does some basic error checking on the number entered.
    Returns    : the number entered by the user 
    """
    end_loop = False
    while not end_loop:

        # get the total number of marks
        marks = float(raw_input('Enter the total number of marks: '))

        # if it's less than zero then print an error
        if marks < 0:
            print 'Error: The number must be greater than zero.'
        else:

            # end he loop if we get a proper number
            end_loop = True
    return marks

def read_users_mark(total_marks):
    """
    Purpose    : prompts the user to enter the mark they got for the test. This
                 function also does some basic error checking on the number entered.
    Parameters : total_marks - the total number of possible marks on the test. This
                               value is used for error checking
    Returns    : the number entered by the user          
    """
    end_loop = False
    while not end_loop:

        # get the mark
        mark = float(raw_input('Enter your mark: '))

        # if it's not in the range then print an error
        if mark < 0 or mark > total_marks:
            print 'Error: incorrect input'
        else:
            end_loop = True
    return mark

def compute_percentage(total_marks, mark):
    """
    Purpose    : computes the percentage the user got
    Parameters : total_marks - the total number of possible marks on the test
                 mark        - the mark the user got on the test
    Returns    : the percentage 
    """
    return mark * 100.0 / total_marks

def find_letter_grade(percentage):
    """
    Purpose    : find the letter grade based upon a given percentage value
    Parameters : percentage - the percentage used to find the letter grade
    Returns    : the letter grade as a string
    """
    if percentage < 39.9:
        letter_grade = 'F'
    elif percentage < 44.0:
        letter_grade = 'D-'
    elif percentage < 49.0:
        letter_grade = 'D'
    elif percentage < 54.0:
        letter_grade = 'D+'
    elif percentage < 59.0:
        letter_grade = 'C-'
    elif percentage < 64.0:
        letter_grade = 'C'
    elif percentage < 69.0:
        letter_grade = 'C-'
    elif percentage < 74.0:
        letter_grade = 'B-'
    elif percentage < 79.0:
        letter_grade = 'B'
    elif percentage < 84.0:
        letter_grade = 'B+'
    elif percentage < 89.0:
        letter_grade = 'A-'
    else:
        letter_grade = 'A'
    return letter_grade

if __name__ == '__main__':
    """ Main Entry """

    # call a function to get the total marks
    total_marks = read_total_marks()

    # call a function to read the mark of the user
    mark = read_users_mark(total_marks)

    # call a function to calculate the percentage 
    percentage = compute_percentage(total_marks, mark)

    # call a function to get the appropriate letter grade
    letter_grade = find_letter_grade(percentage)

    # print to the console with proper formatting 
    print 'You got %.2f%c on the test which gives you a letter grade of %s' % \
          (percentage, '%', letter_grade)