[Numpy-discussion] product of arrays of different lengths

SimonPalmer simon.palmer@gmail....
Mon Sep 15 04:55:23 CDT 2008


I have two 1D arrays of different lengths and I want the calculate sum
of the product of the two to the extent of the smaller of them (make
sense?).  I don't know which of the two will be the longer ahead of
time.

I expected (hoped) to be able to do this:

sp = (A * B).sum()

and imagined that the trailing end of the longer of the two arrays
might just be ignored, but instead I get an error "objects cannot be
broadcast to a single shape".

Both arrays are generally very short (<100 elements) so I can tolerate
a copy, however this calculation is right in the heart of the
innermost loop in my code, so I have to tread a bit carefully.

Here's what I do at the moment, which is ugly, but works

        max_idx = min(len(A), len(B))
        total_product = 0
        for idx in range(0, max_idx):
            total_product += A[idx] * B[idx]

...but this is numpy, so I'm looking for a neat way.


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