Remove empty strings from list

[Chris Rebert]

On Mon, Sep 14, 2009 at 6:49 PM, Helvin <helvinlui at gmail.com> wrote:
> Hi,
>
> Sorry I did not want to bother the group, but I really do not
> understand this seeming trivial problem.
> I am reading from a textfile, where each line has 2 values, with
> spaces before and between the values.
> I would like to read in these values, but of course, I don't want the
> whitespaces between them.
> I have looked at documentation, and how strings and lists work, but I
> cannot understand the behaviour of the following:
>                        line = f.readline()
>                        line = line.lstrip() # take away whitespace at the beginning of the
> readline.
>                        list = line.split(' ') # split the str line into a list
>
>                        # the list has empty strings in it, so now,
> remove these empty strings
>                        for item in list:
>                                if item is ' ':
>                                        print 'discard these: ',item
>                                        index = list.index(item)
>                                        del list[index]         # remove this item from the list
>                                else:
>                                        print 'keep this: ',item
> The problem is, when my list is :  ['44', '', '', '', '', '',
> '0.000000000\n']
> The output is:
>    len of list:  7
>    keep this:  44
>    discard these:
>    discard these:
>    discard these:
> So finally the list is:   ['44', '', '', '0.000000000\n']
> The code above removes all the empty strings in the middle, all except
> two. My code seems to miss two of the empty strings.
>
> Would you know why this is occuring?

Block quoting from http://effbot.org/zone/python-list.htm
"""
Note that the for-in statement maintains an internal index, which is
incremented for each loop iteration. This means that if you modify the
list you’re looping over, the indexes will get out of sync, and you
may end up skipping over items, or process the same item multiple
times.
"""

Thus why your code is skipping over some elements and not removing them.
Moral: Don't modify a list while iterating over it. Use the loop to
create a separate, new list from the old one instead.

Cheers,
Chris
--
http://blog.rebertia.com