Why is the output of this code only even numbers between 1 and 121?

Question

A=list(range(1,121))
for i in A:
    A.remove(i)
print(A)

Shouldn't it be empty? I really don't get this..

@JohnZwinck when I was editing it, I made a mistake. Now it should work — , Sep 14 '17 at 12:19
https://stackoverflow.com/questions/1207406/remove-items-from-a-list-while-iterating — John Zwinck, Sep 14 '17 at 12:21
I understand it is bad. But if you don't tell him why he won't every understand WHY it's bad. Us millennials need to know WHY. ;) <3 — Billy Ferguson, Sep 14 '17 at 12:28

score 2 · Accepted Answer · answered Sep 14 '17 at 12:22

You typically don't want to modify lists you're iterating over because you will get weird results like what you're running into.

You have a list of numbers, 1-121. You remove the first one, everything shifts down in memory so 2 is now in the zeroeth position. Do range 2-122 and you'll get only odd numbers.

 x = [1, 2, 3, 4, 5, 6]
 for i in x:
     x.remove(i)
     #first time through x = [2, 3, 4, 5, 6]
                           #  ^ i is still pointing here though and the next time
                           #    through it will be pointing at 3 (i = 2)

Why is the output of this code only even numbers between 1 and 121?

1 Answers1