[Numpy-discussion] Selecting columns of a matrix (original) (raw)

Travis Oliphant oliphant.travis at ieee.org
Wed Jun 21 13:22:04 EDT 2006

Pau Gargallo wrote:

On 6/21/06, Travis Oliphant <oliphant.travis at ieee.org> wrote:

Johannes Loehnert wrote:

Hi,

I'm not sure why bool arrays cannot be used as indices. The "natural" solution to the original problem seemed to be: M[:,V>0] but this is not allowed. I started a thread on this earlier this year. Try searching the archive for "boolean indexing" (if it comes back online somewhen). Travis had some reason for not implementing this, but unfortunately I do not remember what it was. The corresponding message might still linger on my home PC, which I can access this evening.... I suspect my reason was just not being sure if it could be explained consistently. But, after seeing this come up again. I decided it was easy enough to implement. So, in SVN NumPy, you will be able to do a[:,V>0] a[V>0,:] The V>0 will be replaced with integer arrays as if nonzero(V>0) had been called. does it work for a[,] ? Sure, it will work. Basically all boolean arrays will be interpreted as nonzero(V>0), everywhere. what about a[ix( nonzero(), nonzero() )] ? maybe the to nonzero() conversion would be more coherently done by the ix function than by the [] I've just added support for inside ix_ so that the nonzero will be done automatically as well.

So

a[ix_(,)] will give the cross-product selection.

-Travis

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