[Numpy-discussion] How to get the shape of an array slice without doing it

josef.pktd@gmai... josef.pktd@gmai...
Fri Jan 29 11:10:42 CST 2010


On Fri, Jan 29, 2010 at 11:49 AM, David Huard <david.huard@gmail.com> wrote:
> Hi,
>
> I have a 4D "array" with a given shape, but the array is never
> actually created since it is large and distributed over multiple
> binary files. Typical usage would be to take slices across the 4D
> array.
>
> I'd like to know what the shape of the resulting array would be if I
> took a slice out of it.
> That is, let's say my 4D array is A, I'd like to know
>
> A[ndindex].shape
>
> without actually creating A.
>
> ndindex should support all numpy constructions (integer, boolean,
> array, slice, ...). I am guessing something already exists to do this,
> but I just can't put my finger on it.

trying out some things, just because it's a puzzling question

>>> indi= (slice(2,5), np.arange(2), np.arange(3)[:,None])
>>> np.broadcast(*indi).shape
(3, 2)

I don't know if this is ok for all possible cases, (and there are some
confusing things with reordering axis, when slices and fancy indexing
is mixed)

Josef



> Thanks.
>
> David
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