[Numpy-discussion] somewhat less stupid problem with 0-d arrays

Sebastian Berg sebastian at sipsolutions.net
Sat May 11 05:22:28 EDT 2013

On Fri, 2013-05-10 at 19:57 -0400, Neal Becker wrote:
> It would be convenient if in arithmetic 0-d arrays were just ignored - it would
> seem to me to be convenient in generic code where a degenerate array is treated
> as "nothing"

Small naming detail. A 0-d array is an array with exactly one element
and no dimensions, i.e. np.array(0), and behaves mostly like a scalar.
What you have is an empty array with no elements.

> np.zeros ((0,0)) + np.ones ((2,2))
> ---------------------------------------------------------------------------
> ValueError                                Traceback (most recent call last)
> <ipython-input-17-27af0e0bbc6f> in <module>()
> ----> 1 np.zeros ((0,0)) + np.ones ((2,2))
> ValueError: operands could not be broadcast together with shapes (0,0) (2,2)

I am not sure in what general code you need that, it seems weird to me,
since np.zeros((N, N)) + np.ones((2,2)) would also only work if N=1. And
if N=1, it looks like it might be a reduction result.
Empty arrays *do* support most reductions (making them not empty, like
summing them gives 0). And they do broadcast under the normal
broadcasting rules, such that np.zeros((0,0)) + np.zeros((10,1,1)) gives
np.zeros((10,0,0)).  For the most part, they are not a special case and
just work the same as non-empty arrays, which seems right to me.

- Sebastian

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