list iteration if statement
Steve Holden
steve at holdenweb.com
Fri Jan 2 23:04:27 EST 2009
alex goretoy wrote:
> Thank you Steve and MRAB,
>
> This is what I was looking for:
>
> [[v.append(j) for j in i if j != 0] for i in self.value]
>
No, it wasn't. You should *not* be modifying v in the list comprehension!
> the value is actually stored as a string so I would need to check if it
> is "0". I do have one more question about list comprehension though.
If the value is stored as a string them you should be testing for "0",
not 0.
> After doing this I get an unwanted list of None how do I make it
> disappear or not return this list. Would I have to be modifying it in
> place for this to happen? Wouldn't modifying it in place potentially
> overwrite some other values? I just don't want it to return [[None],
> [None, None], [None, None], [None]]
>
If you just want to append all those lists to v then you should just say
v += [[j for j in i if j != "0"] for i in self.value]
You are getting the None values because the append method returns None.
regards
Steve
> You guys rock, thanks for helping me learn python.
>
> On Sat, Jan 3, 2009 at 2:09 AM, Steve Holden <steve at holdenweb.com
> <mailto:steve at holdenweb.com>> wrote:
>
> alex goretoy wrote:
> > Hello All,
> >
> > I'm doing this in my code
> >
> > [[v.append(j) for j in i] for i in self.value]
> >
> > if works and all, but I need to add a if statement in the mix. Can't
> > seem to remember the syntax to do so and everything I've tried
> seems to
> > fail. How do I add a check to see if j is not int("0") then append
> to v
> > list? Thank you in advance. -A
>
> Who, tiger. It "works" for a value of "works" that involves creating two
> lists. One is the one you want, referenced by v, and the other is the
> value if the list comprehension, which will be a list full of lists of
> all the None values returned by those append() calls. But I presume you
> are throwing that second list away ...
>
> See, a list comprehension is intended to create a list. So what you
> should have used (assuming v was the empty list before you started) was
>
> v = [[j for j in i] for i in self.value]
>
> Further, when you say 'j is not int("0")', do you actually mean that a
> is not in integer with the value 0? Assuming you do then what you
> need is
>
> v = [[j for j in i if not j] for i in self.value]
>
> or, more pedantically
>
> v = [[j for j in i if j==0] for i in self.value]
>
> regards
> Steve
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