[Numpy-discussion] taking a 2D uneven surface slice

Bradley M. Froehle brad.froehle@gmail....
Tue Apr 16 15:35:42 CDT 2013


Hi Bryan:

On Tue, Apr 16, 2013 at 1:21 PM, Bryan Woods <bwoods@aer.com> wrote:

> I'm trying to do something that at first glance I think should be simple
> but I can't quite figure out how to do it. The problem is as follows:
>
> I have a 3D grid Values[Nx, Ny, Nz]
>
> I want to slice Values at a 2D surface in the Z dimension specified by
> Z_index[Nx, Ny] and return a 2D  slice[Nx, Ny].
>
> It is not as simple as Values[:,:,Z_index].
>
> I tried this:
> >>> values.shape
> (4, 5, 6)
> >>> coords.shape
> (4, 5)
> >>> slice = values[:,:,coords]
> >>> slice.shape
> (4, 5, 4, 5)
> >>> slice = np.take(values, coords, axis=2)
> >>> slice.shape
> (4, 5, 4, 5)
> >>>
>
> Obviously I could create an empty 2D slice and then fill it by using
> np.ndenumerate to fill it point by point by selecting values[i, j,
> Z_index[i, j]]. This just seems too inefficient and not very pythonic.
>

The following should work:

>>> values.shape
(4,5,6)
>>> coords.shape
(4,5)
>>> values[np.arange(values.shape[0])[:,None],
...        np.arange(values.shape[1])[None,:],
...        coords].shape
(4, 5)

Essentially we extract the values we want by values[I,J,K] where the
indices I, J and K are each of shape (4,5) [or broadcast-able to that
shape].
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