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

David Huard david.huard@gmail....
Fri Jan 29 11:32:44 CST 2010


On Fri, Jan 29, 2010 at 12:10 PM,  <josef.pktd@gmail.com> wrote:
> 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)
>

Hi josef,

Where then do you specify the shape of the A array ?  Maybe an example
would be clearer:

Let's say A's shape is (10, 1, 5, 20)
and the index is [::2, ..., 0]

A[::2, ..., 0] shape would be (5, 1, 5)

The broadcast idea has potential, I'll toy with it.

David



> Josef
>
>
>
>> Thanks.
>>
>> David
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