how to correctly modify the iterator of a loop in python from within the loop

Question

what I basically need is to check every element of a list and if some criteria fit I want to remove it from the list.

So for example let's say that

list=['a','b','c','d','e']

I basically want to write (in principle and not the actual code I try to implement)

If an element of the list is 'b' or 'c' remove it from the list and take the next.

But

for s in list:
    if s=='b' or s=='c':
        list.remove(s)

fails because when 'b' is removed the loop takes 'd' and not 'c' as the next element. So is there a way to do that faster than storing the elements in a separate list and removing them afterwards?

Thanks.

jsbueno · Accepted Answer · 2018-01-05 12:45:56Z

13

The easier way is to use a copy of the list - it can be done with a slice that extends "from the beginning" to the "end" of the list, like this:

for s in list[:]:
    if s=='b' or s=='c':
        list.remove(s)

You have considered this, and this is simple enough to be in your code, unless this list is really big, and in a critical part of the code (like, in the main loop of an action game). In that case, I sometimes use the following idiom:

to_remove = []
for index, s in enumerate(list):
    if s == "b" or s == "c":
         to_remove.append(index)

for index in reversed(to_remove):
    del list[index]

Of course you can resort to a while loop instead:

index = 0
while index < len(list):
   if s == "b" or s == "c":
       del list[index]
       continue
   index += 1

edited Jan 5, 2018 at 12:45

answered Dec 17, 2011 at 12:00

jsbueno

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2 Comments

tst Over a year ago

jsbueno thank you very much. I feel a bit ashamed for not thinking that. Thanks again!

tst Over a year ago

I actually wanted a loop within a loop and your last example was perfect. thanks again.

Abhijit · Accepted Answer · 2014-08-11 09:10:19Z

9

Its better not to reinvent things which are already available. Use filter functions and lambda in these cases. Its more pythonic and looks cleaner.

filter(lambda x:x not in ['b','c'],['a','b','c','d','e'])

alternatively you can use list comprehension

[x for x in ['a','b','c','d','e'] if x not in ['b','c']]

edited Aug 11, 2014 at 9:10

answered Dec 17, 2011 at 12:04

Abhijit

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2 Comments

jsbueno Over a year ago

Actually, the list comprehension in your second example is "more Pythonic" than the filter and lambda combination.

tst Over a year ago

thanks but I needed to put one such loop into an other and this way things can become really messy.

Charles Haynes · Accepted Answer · 2013-08-05 01:04:25Z

3

This is exactly what itertools.ifilter is designed for.

from itertools import ifilter

ifilter(lambda x: x not in ['b', 'c'], ['a', 'b', 'c', 'd', 'e'])

will give you back a generator for your list. If you actually need a list, you can create it using one of the standard techniques for converting a generator to a list:

list(ifilter(lambda x: x not in ['b', 'c'], ['a', 'b', 'c', 'd', 'e']))

or

[x for x in ifilter(lambda x: x not in ['b', 'c'], ['a', 'b', 'c', 'd', 'e'])]

answered Aug 5, 2013 at 1:04

Charles Haynes

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Comments

gecco · Accepted Answer · 2011-12-17 12:04:25Z

1

If you are ok with creating a copy of the list you can do it like this (list comprehension):

[s for s in list if s != 'b' and s != 'c']

answered Dec 17, 2011 at 12:04

gecco

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