# [SciPy-User] masking an array ends up flattening it

Zachary Pincus zachary.pincus@yale....
Tue Feb 28 16:35:11 CST 2012

```Hi Johann,

> In [146]: mask
> Out[146]:
> array([[ True,  True,  True, False],
>        [ True,  True,  True, False],
>        [ True,  True,  True, False],
>        [False, False, False, False]], dtype=bool)
>
> Naively, I thought I would end up with a (3,3) shaped array when
> applying the mask to m

So that would make some sense for the above mask, but obviously doesn't generalize... what shape output would you expect if 'mask' looked like the following?

array([[ True,  True,  True, False],
[ True,  True,  True, False],
[ True,  True,  True, False],
[False, False, False,  True]], dtype=bool)

Flattening turns out to be the most-sensible general-case thing to do. Fortunately, this is generally not a problem, because often one winds up doing things like:
a[mask] = b[mask]
where a and b can both be n-dimensional, and the fact that you go through a flattened intermediate is no problem.

If, on the other hand, your task requires slicing square regions out of arrays, you could do that directly by other sorts of fancy-indexing or using programatically-generated slice objects, or some such. Can you describe the overall task? Perhaps then someone could suggest the "idiomatic numpy" solution?

Zach

> , but instead I get :
>
> In [147]: m[mask]
> Out[147]:
> array([  1.82243247e-23,  -5.53103453e-14,   4.32071039e-13,
>         -5.52425949e-14,   6.26697129e-02,  -5.12076585e-02,
>          4.31598429e-13,  -5.12102340e-02,   6.27539118e-02])
>
> In [148]: m[mask].shape
> Out[148]: (9,)
>
> Is there another way to proceed and get directly the (3,3) shaped masked
> array, or do I need to reshape it by hand?
>
> thanks a lot in advance,
> Johann
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```

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