list traversal and remove

[digisatori]

I supposed the below code will print seven 2 and generate the list li
without 2.
Strangely it only print four 2. If you change the number of 2 in the
list, the results are all beyond expectation.
I know the other way to achieve the expected goal, but why this is
happening? Could somebody enlight me?

li= [2,2,2,2,2,2,2,3,4,5,6,7,8,9]
for x in li:
if x == 2:
print x
li.remove(x)

[Arnaud Delobelle]

On Jan 31, 7:49*am, "[e-mail]" <[e-mail]>
wrote:

I supposed the below code will print seven 2 and generate the list li
without 2.
Strangely it only print *four 2. If you change the number of 2 in the
list, the results are all beyond expectation.
I know the other way to achieve the expected goal, but why this is
happening? Could somebody enlight me?

li= [2,2,2,2,2,2,2,3,4,5,6,7,8,9]
for x in li:
* * if x == 2:
* * * * print x
* * * * li.remove(x)

There's just been a thread on this:

http://groups.google.com/group/comp....623baaa46e72a/

--
Arnaud

[Steven D'Aprano]

On Wed, 30 Jan 2008 23:49:46 -0800, di********@gmail.com wrote:

I supposed the below code will print seven 2 and generate the list li
without 2.
Strangely it only print four 2. If you change the number of 2 in the
list, the results are all beyond expectation. I know the other way to
achieve the expected goal, but why this is happening? Could somebody
enlight me?

Do not modify a list at the same time that you are traversing it.

If you look at the archives (say, on Google Groups, or any number of
other places), this topic was just discussed yesterday and earlier today.

See the thread with subject line:

"Removal of element from list while traversing causes the next element
to be skipped"

As for why it is happening... you have code that looks like this:

for x in li:
if x == 2:
print x
li.remove(x)
Consider that "under the hood", the for-loop looks something like this:

i = 0
while i < the length of the list:
set x equal to the item in the i-th position
execute the loop block of code
i += 1

If you start deleting or inserting items in the middle of the loop, the
index won't be pointing where you expect.

--
Steven

[digisatori]

Thank you very much for the great anwsers. You guys save my sleep
today.