我有两个几乎相同的“for循环”函数，但只有一个有效。 这是为什么？

Question

I'm learning loops in Python and I'm trying to make a function that takes 2 lists of the same length and returns True if lst1 is the same as lst2 reversed (and false otherwise).

For example: reversed_list([1, 2, 3], [3, 2, 1]) should return True.

Below there are 2 nearly identical ways of writing this function, I think they should both work correctly but the second one doesn't work even though it is practically identical to the first.

def reversed_list(lst1, lst2):
    for index in range(len(lst1)):
        if lst1[index] != lst2[len(lst2) - 1 - index]:
            return False
    else: return True
#tests
print(reversed_list([1, 2, 3], [3, 2, 1]))
#output = True (correct)
print(reversed_list([1, 2, 3], [4, 2, 1]))
#output = False (correct)






def reversed_list(lst1, lst2):
    for index in range(len(lst1)):
        if lst1[index] == lst2[len(lst2) - 1 - index]:
            return True
    else: return False
#tests
print(reversed_list([1, 2, 3], [3, 2, 1]))
#output = True (correct but it outputs True even if it should be False)
print(reversed_list([1, 2, 3], [4, 2, 1]))
#output = True (incorrect)

The only difference between the two functions is that I changed != to == and swapped True/False accordingly. However, the second function always returns True even if it should be False. I suspect this has something to do with the loop. Please explain why the second function doesn't work properly.

Answer 1

The first one stops early and returns False if it finds two entries that don't match; otherwise it returns True . The second one stops early and returns True as soon as it finds two entries that do match - but your task is to verify all the entries, while this stops after verifying only one. It's basically checking whether there's at least one match, rather than checking whether the whole list is a match.

Answer 2

The problem with the second function is that the function will stop running as soon as it finds a match, rather than check all the values and stop running if finds a pair that doesn't match.

In your case of [1, 2, 3] vs [4, 2, 1]

When these two lines run:

if lst1[index] == lst2[len(lst2) - 1 - index]:
    return True

The second function got to the first pair of items when index == 0 and saw that lst[0] has a value of 1 and that lst2[len(lst2) - 1 - 0] also has a value of 1 . They do match, so the function returns true. It never actually makes it far enough to compare 3 vs 4

Look into what is called a fail fast algorithm if you want to know more about this.