python codecademy 学习2

 The Variance.  A very large variance means that the students' grades were all over the place, while a small variance (relatively close to the average) means that the majority of students did fairly well.

code:

grades = [100, 100, 90, 40, 80, 100, 85, 70, 90, 65, 90, 85, 50.5]

def print_grades(grades):
    for grade in grades:
        print grade

def grades_sum(grades):
    total = 0
    for grade in grades: 
        total += grade
    return total
    
def grades_average(grades):
    sum_of_grades = grades_sum(grades)
    average = sum_of_grades / float(len(grades))
    return average


def grades_variance(scores):
    average=grades_average(scores)
    variance=0
    for score in scores:
        variance+=(average-score)**2
    res=variance/len(scores)
    return res
print grades_variance(grades)