how is Matrix Multiplication with Numpy

Question

I have a 2D matrix for example X=np.ones((97,2)) and another vector for example Theta=np.zeros((2,1)) when I multiply the vector Theta and the Matrix X, it raises an error

'operands could not be broadcast together with shapes (2,1) (97,2)'

but when I multiply Theta.T* x, it output a new (97, 2) matrix.

How did it work? How (1,2) *(97,2) works and if the shapes don't matter why it raises an error when Theta * X could anyone explain it, please.

Matrix multiplication should be X @ Theta. Theta.T * X is element-wise multiplication. — Quang Hoang
– Quang Hoang, Commented Jul 22, 2020 at 13:50

Chris Schmitz · Accepted Answer · 2020-07-22 13:51:34Z

1

It tries to broadcast ( = replicate) the smaller array across the larger one when a matrix multiplication doesn't work by dimensions, which is why your second example works but the first one doesn't.

https://numpy.org/doc/stable/user/basics.broadcasting.html

answered Jul 22, 2020 at 13:51

Chris Schmitz

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