Why can't I write this code in the usual for statement?

Question

I just read a post here about how to filter values from two lists.python: How to remove values from 2 lists based on what's in 1 list

I tried to write some code myself using for and zip, but the result is wrong, can anyone help me understand this?

xVar = [1, 2, 3, 4, 5, 6, 7]
yVar = [11, 22, 33, 44, 55, 66, 77]

z = zip(xVar, yVar)
zFiltered = [(x, y) for x, y in z if y < 50]
print zFiltered # Correct, prints [(1, 11), (2, 22), (3, 33), (4, 44)]

for (x, y) in z:
    if y > 50:
        z.remove((x, y))

print z # Wrong, prints [(1, 11), (2, 22), (3, 33), (4, 44), (6, 66)]

Why is this happening? I tried to use pdb and found that after z.remove((5, 55)), (x, y)'s value is (7, 77), (6, 66) is skiped, I assume that for is assigning (x, y) via indexs, removing (5, 55) will violate the original index, if so, how can I do the same thing here with for statements? and is there any other pitfalls here?

use copy module : import copy, for (x,y) in z ---> for (x,y) in copy.copy(z). — luxiaolu, Sep 26 '13 at 06:02

Shashank · Answer 1 · 2013-09-26T06:10:38.717

3

Try this list comprehension which will select elements in your zipped list based on a condition (basically the opposite of removing elements from the list that do not satisfy this condition, but it will yield the same results).

z = [(x, y) for (x, y) in z if y <= 50]

If you want to delete elements in a list while iterating over it, you must iterate backwards because every time you delete an item in a list, all of the elements that have higher indexes in the list get shifted downward to lower indexes.

You can do such an iteration like so:

for i in xrange(len(z) -1, -1, -1):
    if z[i][1] > 50:
        z.remove(z[i])

edited Sep 26 '13 at 06:10

answered Sep 26 '13 at 06:03

Shashank

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The first approach is pythonic =D – justhalf Sep 26 '13 at 06:36
@justhalf yup, it's clean...but I still like to be a complete dork and do it the second way sometimes xD – Shashank Sep 26 '13 at 06:38

score 2 · Accepted Answer · answered Sep 26 '13 at 06:24

The problem is that when you remove an item from a list while iterating over it, its internal structure gets out of whack, and items are skipped. The easy way around this issue is to make a copy of the list by adding [:].

for (x, y) in z[:]:
    ...

Now you can make changes to the original list as it is not being iterated over.

Why can't I write this code in the usual for statement?

2 Answers2