On Wed, 2004-09-01 at 14:51, Darren Dale wrote:
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 needs to be sum(a(find(b)).
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.
I don't think that's bad, and in fact it is substantially better than MATLAB. Consider the following clip from MATLAB Version 7:
a=randn(10000000,1); t=cputime;sum(a);e=cputime()-t
e = 0.1300
f=rand(10000000,1)<0.1; t=cputime;sum(a(find(f)));e=cputime()-t
e = 0.2200 In other words, masking off all but 10% of the elements of a 1e7 element array actually increased the CPU time required for the sum by about 50%. In addition, I doubt you can measure CPU time for only a 10 element array. I had to use 1e7 elements in MATLAB on a 2.26MHz P4 just to get the CPU time large enough to measure reasonably accurately. Also recall that it is a known characteristic of numarray that it is slow on small arrays in general. -- Stephen Walton <stephen.walton@csun.edu> Dept. of Physics & Astronomy, Cal State Northridge