bartonc  6,596
Recognized Expert Expert
I was chasing a bug last night. I actually saw it before I inserted the trace (print) statements. This: -
>>> l=range(10)
-
>>> for i in l:
-
... l.remove(i)
-
... print i
-
...
0
2
4
6
8
shows interesting side effects of removing items while looping on a list's items.
However, it works working backwards in the list: -
>>> l=range(10)
-
>>> for i in l[::-1]:
-
... l.remove(i)
-
... print i
-
...
9
8
7
6
5
4
3
2
1
0
4 1867  -
>>> l=range(10)
-
>>> for i in l[:]:
-
... l.remove(i)
-
... print i
-
...
-
-
1
-
2
-
3
-
4
-
5
-
6
-
7
-
8
-
9
-
>>>
-
-
I was chasing a bug last night. I actually saw it before I inserted the trace (print) statements. This: -
>>> l=range(10)
-
>>> for i in l:
-
... l.remove(i)
-
... print i
-
...
0
2
4
6
8
shows interesting side effects of removing items while looping on a list's items.
However, it works working backwards in the list: -
>>> l=range(10)
-
>>> for i in l[::-1]:
-
... l.remove(i)
-
... print i
-
...
9
8
7
6
5
4
3
2
1
0
My explanation is that in l[::-1]: a temporary shallow copy of the list is created and this is where the values of i are read from, but for
The values of i are skiping one each time because after every removal, the elements' positions shift left by one.
bartonc 6,596
Recognized Expert Expert
My explanation is that in l[::-1]: a temporary shallow copy of the list is created and this is where the values of i are read from, but for
The values of i are skiping one each time because after every removal, the elements' positions shift left by one.
Exactly right! That's why ghostdog74's code works going forward. It was 4:20 AM PST when I posted that. By 9:45 that thought had occurred to me, but I was away from the computer.
Exactly right! That's why ghostdog74's code works going forward. It was 4:20 AM PST when I posted that. By 9:45 that thought had occurred to me, but I was away from the computer.
And i'm still a Python newbie.
Wait till I can give it more time.
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