[Numpy-discussion] Selecting columns of a matrix

[Pau Gargallo]

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[<boolean>,<boolean>] ?

what about a[ix_( nonzero(<boolean>), nonzero(<boolean>) )] ?

maybe the <boolean> to nonzero(<boolean>) conversion would be more
coherently done by the ix_ function than by the []


pau