[Numpy-discussion] efficient summation

Darren Dale dd55 at cornell.edu
Wed Sep 1 14:52:04 EDT 2004

I am trying to effieciently sum over a subset of the elements of a 
matrix. In Matlab, this could be done like:
b = [1,0,0,0,0,0,0,0,0,1]
res=sum(a(b)) %this sums the elements of a which have corresponding 
elements in b that are true

Is there anything similar in numarray (or numeric)? I thought masked 
arrays looked promising, but I find that masking 90% of the elements 
results in marginal speedups (~5%, instead of 90%) over the unmasked array.


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