Python：如何仅使用while循环返回列表而不修改原始列表？

Question

Say I have a function called everythird that takes a list as its parameter and returns a new list containing every third element of the original list, starting from index 0. I know how to do this using slice notation (return everythird[0::3]), but we have to use a while loop only. If I type in everythird([1, 2, 3, 4, 5, 6, 7, 8]), I want it to return [1, 4, 7]. I tried a few different ways, but I'm not getting a list back, or I only get one value back. How do I return a list? Also how do you know for certain whether something modifies or doesn't modify an original list?

Thank you.

This is one of the ways I attempted this: every_third([1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11])

def everythird(l):
    '''(list) -> list
    Returns every third element of original list, starting at index 0'''

    i = 0
    while i < len(l):
        print(l[i])
        i += 3

This prints

1
4
7

Answer 1

If you need to do this with a while loop, you could do it by appending each element to a list rather than printing it, and then returning that list:

def everythird(l):
    i = 0
    ret = []
    while i < len(l):
        ret.append(l[i])
        i += 3
    return ret

Though as you note, it would certainly be preferably to do

def everythird(l):
    return l[0::3]

Or if you were allowed to use a for loop:

def everythird(l):
    ret = []
    for i in range(0, len(l), 3):
        ret.append(l[i])
    return ret

Finally, if you were allowed to use a list comprehension:

def everythird(l):
    return [l[i] for i in range(0, len(l), 3)]

The slice indexing is certainly the best, but in any case a while loop might be the worst way to do it.