Logical indexing and sums

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Fabs
Fabs 2014-5-28
评论： Fabs 2014-5-28
Hi,
first of: I know I can do what I want in a loop. However, I'm performing something similar to my problem thousands of times on much bigger matrices, so if there is a solution with logical indexing and without any loop, that would be preferred.
OK, let's assume I have a matrix
a=[23 45; 65 13];
and I want the values of a to written to new positions defined in
b=[2 4;1 2]; % matrix with new positions for values in a
Obviously b is referencing one position (2) twice and
c=nan(size(a));
c(b) = a % matrix with values from a at positions from b
results in
c = [65 13; NaN 45]; % location two is written twice and only the second value shows up
Is there a way to get matrix c to be
c = [65 36; NaN 45]; % location 2 is sum of 13+23
?
Thanks y'all, I appreciate your help, F

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Kelly Kearney
Kelly Kearney 2014-5-28
Meet accumarray, my favorite powerful-and-useful-but-terribly-named function (and lately, seemingly the answer to every question I respond to):
c = reshape(accumarray(b(:), a(:), [numel(a) 1], @sum, NaN), size(a))
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Fabs
Fabs 2014-5-28
That's what I call a quick response! Thank you very much!

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