python 'for' loop logic

Question

New to python - don't understand why the following seems to skip x=2 ?

a = [1, 2, 3]

for x in a:
    print("x=", x)
    print("a=", a)
    a.remove(x)
    print("new a=", a)

returns:

x= 1
a= [1, 2, 3]
new a= [2, 3]
x= 3
a= [2, 3]
new a= [2]

Process finished with exit code 0

In general: if you mutate a sequence while iterating over it, the effects are usually somewhat undefined and/or unexpected. — deceze, Sep 20 '17 at 13:57
It's because the loop walks through the indices not the values and I see that is a bug in conception — Abr001am, Sep 20 '17 at 14:02
Possible duplicate of [Remove items from a list while iterating](https://stackoverflow.com/questions/1207406/remove-items-from-a-list-while-iterating) — wwii, Sep 20 '17 at 14:09

score 0 · Accepted Answer · answered Sep 20 '17 at 14:02

0

You're changing the list as you iterate over it.

On its first loop, it's looking at the first element of a. Then you remove the first element, changing a to [2,3]. On its second loop, its looking at the second element of a. Because a has changed, its second element is now 3, which is what you get.

answered Sep 20 '17 at 14:02

Andrew Hows

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Yes but it is unexpected behavior since the loop is not index-based, I think it as a bug. – Abr001am Sep 20 '17 at 14:03
1

You can think whatever you like. – Andrew Hows Sep 20 '17 at 14:04
@Idle001 Python for loop (not foreach) is based on C for loop and therefore index-based. When iterated, list object yields indexes, as seen in xrange. – Kotauskas Sep 20 '17 at 14:54
@VladislavToncharov I didn't know there is a `foreach` item iterator in python sorry both of you – Abr001am Sep 20 '17 at 15:00
@Idle001 There's no for loop in Python. There's foreach loop which is called for in Python. – Kotauskas Sep 20 '17 at 18:36

score 0 · Answer 2 · answered Sep 20 '17 at 14:04

The for loop is iterating over the memory location of the list a. After you delete the first element in the list. The second element is stored in the memory location of the first. Thus the loop loops over the second memory location next which currently is storing the third element.

python 'for' loop logic

2 Answers2