Simple newbie question
fumanchu at amor.org
Thu Dec 4 17:29:02 CET 2003
Or, simply iterate through the list backwards.
> -----Original Message-----
> From: Duncan Booth [mailto:duncan at NOSPAMrcp.co.uk]
> Sent: Thursday, December 04, 2003 7:51 AM
> To: python-list at python.org
> Subject: Re: Simple newbie question
> Angelo Secchi <secchi at sssup.it> wrote in
> news:mailman.107.1070547952.16879.python-list at python.org:
> > The code i'm using is the following:
> > for line in lista:
> > if sum(line == 0) > 0:
> > lista.remove(line)
> > The problem is that the loop stops at the first line with
> zeros and it
> > doesn't remove the other lines with zeros (at least if I do
> not re-run
> > the loop). Probably I do not understand how the loop works.
> The loop will assign the first element of lista to line, then
> the second,
> then the third, and so on.
> The problem is that, if you remove an element everything
> after it shifts up
> one place. Say you are looking at the first element and
> remove it, what was
> the second element now becomes the first element, but the
> next time round
> the loop you will look at the new second element and the one that was
> originally second (and is now first) will be skipped.
> The solution is never to work on the list you are modifying.
> Just make a
> copy of the list and iterate over the copy, that means you can safely
> modify the original. To copy a list, use the 'list' builtin.
> for line in list(lista):
> if condition:
> Alternatively turn your brain around and instead of deleting specific
> elements from the list just build a new list with the
> elements you want to
> keep. In this case you could do that using a list comprehension:
> lista = [ line for line in lista if not condition ]
> (Obviously in both these cases replace condition with the correct
> expression, which as Gerhard pointed out may not be the
> condition in your
> original posting.)
> The list comprehension rebinds the lista variable, whereas
> your original
> attempt was modifying the list inplace. This might be
> significant if there
> are other references to the same list elsewhere in your code.
> If it does
> matter, then you can still use a list comprehension to mutate
> the original
> lista[:] = [ line for line in lista if not condition ]
> Unlike the original 'for' loop, this works by building a complete
> replacement list then mutating the original list in one go.
> Duncan Booth
> duncan at rcp.co.uk
> int month(char
> "\6\7\xb\1\x9\xa\2\0\4"];} // Who said my code was obscure?
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