[Numpy-discussion] getting position index from array

Benjamin Root ben.root@ou....
Thu Jan 19 22:33:02 CST 2012


On Thursday, January 19, 2012, Ruby Stevenson <ruby185@gmail.com> wrote:
> hi, all
>
> I am a newbie on numpy ... I am trying to figure out, given an array,
> how to get back position value based on some conditions.
> Say, array([1, 0, 0, 0 1], and I want to get a list of indices where
> it is none-zero, [ 0 , 4 ]
>
> The closest thing I can find from the doc is select(), but I can't
> figure out how to use it properly.
>
> Thanks for your help.
>
> Ruby
>

np.nonzero()

Note that you typically use it with a Boolean array result like "a >= 4".
 Also note that it returns a tuple of index lists, on for each dimension.
 This can the be feed back into the array to get the values as a flat array.

Ben Root
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