[Numpy-discussion] Selecting columns of a matrix

[Pau Gargallo]

On 6/21/06, Travis Oliphant <oliphant.travis at ieee.org> wrote:
> 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[<boolean>,<boolean>] ?
> >
> Sure, it will work.  Basically all boolean arrays will be interpreted as
> nonzero(V>0), everywhere.
> > 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 []
> >
> >
> I've just added support for <boolean> inside ix_  so that the nonzero
> will be done automatically as well.
>
> So
>
> a[ix_(<boolean>,<boolean>)]  will give the cross-product selection.
>


ok so: a[ b1, b2 ] will be different than a[ ix_(b1,b2) ] just like
with integer indices.
Make sense to me.

also, a[b] will be as before (a[where(b)]) ?
maybe a trailing coma could lunch the new behaviour?
a[b] -> a[where(b)]
a[b,] -> a[b,...] -> a[nonzero(b)]

Thanks,

pau