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.

Answer 1

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