Thanks Chris! Thanks for the quick reply. Indeed this is the case! I have
now written out a new list, instead of modifying the list I am iterating
over.
Logged at my blog:
http://learnwithhelvin.blogspot.com/2009/09/python-loop-and-modify-list.html
Regards,
Helvin =)
On Tue, Sep 15, 2009 at 1:55 PM, Chris Rebert <c...@rebertia.com> wrote:
> On Mon, Sep 14, 2009 at 6:49 PM, Helvin <helvin...@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
>
--
Helvin
"Though the world may promise me more, I'm just made to be filled with the
Lord."
--
http://mail.python.org/mailman/listinfo/python-list
