Logical indexing and sums

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

Answer 1

[Kelly Kearney]

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))

[Fabs]

That's what I call a quick response! Thank you very much!