[Numpy-discussion] efficient summation

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


I am trying to effieciently sum over a subset of the elements of a 
matrix. In Matlab, this could be done like:
a=[1,2,3,4,5,6,7,8,9,10]
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.

Thanks!
Darren





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