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

Bill Baxter wbaxter at gmail.com
Wed Jun 21 00:48:48 EDT 2006

On 6/21/06, Erin Sheldon <erin.sheldon at gmail.com> wrote:

On 6/20/06, Bill Baxter <wbaxter at gmail.com> wrote: > I think that one's on the NumPy for Matlab users, no? > > http://www.scipy.org/NumPyforMatlabUsers > > >>> import numpy as num > >>> a = num.arange (10).reshape(2,5) > >>> a > array([[0, 1, 2, 3, 4], > [5, 6, 7, 8, 9]]) > >>> v = num.rand(5) > >>> v > array([ 0.10934855, 0.55719644, 0.7044047 , 0.19250088, 0.94636972]) > >>> num.where(v>0.5) > (array([1, 2, 4]),) > >>> a[:,num.where(v>0.5)] > array([[[1, 2, 4]], > > [[6, 7, 9]]]) > > Seems it grows an extra set of brackets for some reason. Squeeze will get > rid of them. > > >>> a[:,num.where(v>0.5)].squeeze() > array([[1, 2, 4], > [6, 7, 9]]) > > Not sure why the squeeze is needed. Maybe there's a better way. where returns a tuple of arrays. This can have unexpected results so you need to grab what you want explicitly: >>> (w,) = num.where(v>0.5) >>> a[:,w] array([[1, 2, 4], [6, 7, 9]])

Ah, yeh, that makes sense. Thanks for the explanation. So to turn it back into a one-liner you just need:

a[:,num.where(v>0.5)[0]] array([[1, 2, 4], [6, 7, 9]])

I'll put that up on the Matlab->Numpy page.

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