# [Numpy-discussion] Consider improving numpy.outer's behavior with zero-dimensional vectors

Neil Girdhar mistersheik at gmail.com
Sat Apr 11 12:06:16 EDT 2015

```>
> On Wed, Apr 8, 2015 at 7:34 PM, Neil Girdhar <mistersheik at gmail.com
> <http://mail.scipy.org/mailman/listinfo/numpy-discussion>> wrote:
> >
> * Numpy's outer product works fine with vectors. However, I seem to always
> *>
> * want len(outer(a, b).shape) to be equal to len(a.shape) + len(b.shape). *
> >
> * Wolfram-alpha seems to agree *>
> * https://reference.wolfram.com/language/ref/Outer.html
> <https://reference.wolfram.com/language/ref/Outer.html> with respect to
> matrix *>
> * outer products. * You're probably right that this is the correct
> definition of the outer
> product in an n-dimensional world. But this seems to go beyond being
> just a bug in handling 0-d arrays (which is the kind of corner case
> we've fixed in the past); np.outer is documented to always ravel its
> inputs to 1d.
> In fact the implementation is literally just:
> a = asarray(a)
> b = asarray(b)
> return multiply(a.ravel()[:, newaxis], b.ravel()[newaxis,:], out)
> Sebastian's np.multiply.outer is much more generic and effective.
> Maybe we should just deprecate np.outer? I don't see what use it
> serves. (When and whether it actually got removed after being
> deprecated would depend on how much use it actually gets in real code,
> which I certainly don't know while typing a quick email. But we could
> start telling people not to use it any time.)
>

+1 with everything you said.

(And thanks Sebastian for the pointer to np.multiply.outer!)

> -n

On Wed, Apr 8, 2015 at 7:34 PM, Neil Girdhar <mistersheik at gmail.com> wrote:

> Numpy's outer product works fine with vectors.  However, I seem to always
> want len(outer(a, b).shape) to be equal to len(a.shape) + len(b.shape).
> Wolfram-alpha seems to agree
> https://reference.wolfram.com/language/ref/Outer.html with respect to
> matrix outer products.  My suggestion is to define outer as defined below.
> I've contrasted it with numpy's current outer product.
>
> In [36]: def a(n): return np.ones(n)
>
> In [37]: b = a(())
>
> In [38]: c = a(4)
>
> In [39]: d = a(5)
>
> In [40]: np.outer(b, d).shape
> Out[40]: (1, 5)
>
> In [41]: np.outer(c, d).shape
> Out[41]: (4, 5)
>
> In [42]: np.outer(c, b).shape
> Out[42]: (4, 1)
>
> In [43]: def outer(a, b):
>     return a[(...,) + len(b.shape) * (np.newaxis,)] * b
>    ....:
>
> In [44]: outer(b, d).shape
> Out[44]: (5,)
>
> In [45]: outer(c, d).shape
> Out[45]: (4, 5)
>
> In [46]: outer(c, b).shape
> Out[46]: (4,)
>
> Best,
>
> Neil
>
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