[Numpy-discussion] Selecting columns of a matrix
Erin Sheldon
erin.sheldon at gmail.com
Tue Jun 20 23:10:06 CDT 2006
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/NumPy_for_Matlab_Users
>
> >>> 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]])
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