内部有条件 function 的列表理解

Question

I want to write list comprehension with conditional function inside. This what i have so far

more_grades = [0.0, 50.0, 49.9, 79.0, 101.0, 65.0, 54.2, 48.2, 78.9]
def grade_classification(grade):
        if grade < 40:
            result = 'fail'
        elif grade >= 40 and grade < 50:
            result = 'Pass'
        elif grade >= 50 and grade < 60:
            result = '2:2'
        elif grade >= 60 and grade < 70:
            result= '2:1'
        elif grade >= 70:
            result = 'First'
        else:
            result = 'unknown grade'
        return result
studen_classficatin = [result for result in more_grades if grade_classification(result)]
print(studen_classficatin)

The output should be = ['Fail', '2:2', 'Pass', 'First', 'First', '2:1', '2:2', 'Pass', 'First'] but from the code above its giving me = [0.0, 50.0, 49.9, 79.0, 101.0, 65.0, 54.2, 48.2, 78.9] I dont know what i am doing wrong. Please any suggestion?

Answer 1

Change it to this instead:

studen_classficatin = [grade_classification(result) for result in more_grades]

The trailing if will determine whether or not an element should be added to the list, not "transform" the element itself

Answer 2

You don't really need to do list comprehension, instead you can apply map to your list to call a function on each entry of the list. The result will be an iterator that consists of the returned value for each value in your previous list:

studen_classficatin = map(grade_classification, more_grades)

map returns an iterator, so it doesn't actually do anything until you iterate over the list. If you want to print it, call list on the iterator to exhaust it:

studen_classficatin = list(map(grade_classification, more_grades))

Answer 3

Your code is this:

studen_classficatin = [result for result in more_grades if grade_classification(result)]

Which, because non-empty strings are truthy , is the same as:

studen_classficatin = [result for result in more_grades if True]

Which is the same as:

studen_classficatin = [result for result in more_grades]

which is the same as just copying more_grades .