# [Tutor] Aschenputtel problem

Pierre Barbier de Reuille pierre.barbier at cirad.fr
Fri Sep 16 11:30:07 CEST 2005

```Well, in the specific case of numeric arrays, you can use Numeric or
numarray :

from Numeric import array, compress

ol = array(original_list)
selection = array([condition(i) for i in original_list)
list1 = compress(selection, ol)
list2 = compress(!selection, ol)

Even better, if condition can be expressed directly on the matrix itself
(which is pretty common with Numeric/numarray):

ol = array(original_list)
selection = condition(ol)
list1 = compress(selection, ol)
list2 = compress(!selection, ol)

Remember: if you have complex index operations on arrays of numbers,
Numeric (and even more numarray) contains a lot of very usefull
efficient functions.

Pierre

Christopher Arndt a écrit :
> Hi,
>
> I wonder if there is a shorter form of the following idiom:
>
> list1 = []
> list2 = []
> for item in original_list:
>     if condition(item):
>         list1.append(item)
>     else:
>         list2.append(item)
>
> (optional step:)
>
> original_list[:] = list1
>
>
> I call this the "Aschenputtel" problem, because it is described by the famous
> quote from the fairy tale as told by the Grimm brothers:
>
> "Die guten ins Töpfchen, die schlechten ins Kröpfchen."
>
> (The good ones in the pot, the bad ones in the crop)
>
> Chris
>
>
>
>
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>
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--
Pierre Barbier de Reuille