[Numpy-discussion] Selecting columns of a matrix

Simon Burton simon at arrowtheory.com
Thu Jun 22 07:19:05 EDT 2006

On Wed, 21 Jun 2006 10:50:26 -0600
Travis Oliphant <oliphant.travis at ieee.org> wrote:

> 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.

But just for the record, we should note how to
do the operation that this used to do, eg.

>>> a=numpy.array([1,2])
>>> a[[numpy.bool_(1)]]

This could be a way of, say, maping a large
boolean array onto some other values (1 or 2 in the
above case).

So, with the new implementation, is it possible to cast
the bool array to an integer type without incurring a copy overhead ?

And finally, is someone keeping track of the performance
of array getitem ? It seems that as travis overloads it more and
more it might then slow down in some cases.

I must admit my vision is blurring and head is spining as numpy 
goes through these growing pains. I hope it's over soon. Not
because I have trouble keeping up (although i do) but it's
my matlab/R/numarray entrenched co-workers who cannot
be exposed to this unstable development (they will run
screaming to the woods).



Simon Burton, B.Sc.
Licensed PO Box 8066
ANU Canberra 2601
Ph. 61 02 6249 6940

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