list partition

[Moosebumps]

Is there a function that takes a list and predicate, and returns two
lists -- one for which the predicate is true and one for which it is false?

I know I can call filter(p, list) and filter( not p, list) or something like
that, but I assume it would be more efficient to do it in one pass.

Actually I probably can't call "not p", I would have to wrap p in a function
notP or something. Or is there a shortcut for lambda(x) : not p(x)? I
guess it isn't that long : )

MB

[Peter Otten]

Re: list partition

Moosebumps wrote:
[color=blue]
> Is there a function that takes a list and predicate, and returns two
> lists -- one for which the predicate is true and one for which it is
> false?[/color]

I take a slightly different approach, which is not limited to boolean
predicates:
[color=blue][color=green][color=darkred]
>>> def predicate(v): return v & 1[/color][/color][/color]
....[color=blue][color=green][color=darkred]
>>> d = {}
>>> for i in range(10):[/color][/color][/color]
.... d.setdefault(pr edicate(i), []).append(i)
....[color=blue][color=green][color=darkred]
>>> d[True][/color][/color][/color]
[1, 3, 5, 7, 9][color=blue][color=green][color=darkred]
>>> d[False][/color][/color][/color]
[0, 2, 4, 6, 8][color=blue][color=green][color=darkred]
>>>[/color][/color][/color]

Peter

[David M. Cooke]

Re: list partition

At some point, "Moosebumps " <Moosebumps@Moo sebumps.Moosebu mps> wrote:
[color=blue]
> Is there a function that takes a list and predicate, and returns two
> lists -- one for which the predicate is true and one for which it is false?
>
> I know I can call filter(p, list) and filter( not p, list) or something like
> that, but I assume it would be more efficient to do it in one pass.[/color]

Not too hard to write your own:

def filtersplit(p, iterable):
ptrue = []
pfalse = []
for x in iterable:
if p(x):
ptrue.append(x)
else:
pfalse.append(x )
return ptrue, pfalse
[color=blue]
> Actually I probably can't call "not p", I would have to wrap p in a function
> notP or something. Or is there a shortcut for lambda(x) : not p(x)? I
> guess it isn't that long : )[/color]

itertools.ifilt erfalse negates the condition for you.

--
|>|\/|<
/--------------------------------------------------------------------------\
|David M. Cooke
|cookedm(at)phy sics(dot)mcmast er(dot)ca