[Numpy-discussion] Programmatically contracting multiple tensors

Eric Wieser wieser.eric+numpy at gmail.com
Fri Mar 12 19:32:01 EST 2021

Einsum has a secret integer argument format that appears in the Examples
section of the `np.einsum` docs, but appears not to be mentioned at all in
the parameter listing.


On Sat, 13 Mar 2021 at 00:25, Michael Lamparski <diagonaldevice at gmail.com>

> Greetings,
> I have something in my code where I can receive an array M of unknown
> dimensionality and a list of "labels" for each axis.  E.g. perhaps I might
> get an array of shape (2, 47, 3, 47, 3) with labels ['spin', 'atom',
> 'coord', 'atom', 'coord'].
> For every axis that is labeled "coord", I want to multiply in some
> rotation matrix R.  So, for the above example, this could be done with the
> following handwritten line:
> return np.einsum('Cc,Ee,abcde->abCdE', R, R, M)
> But since I want to do this programmatically, I find myself in the awkward
> situation of having to construct this string (and e.g. having to
> arbitrarily limit the number of axes to 26 or something like that).  Is
> there a more idiomatic way to do this that would let me supply integer
> labels for summation indices?  Or should I just bite the bullet and start
> generating strings?
> ---
> Michael
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