Adding elements of matrix according to criteria

Hi there,

I have a quick question which should be trivial to solve but I cannot figure out how to solve it.

I have 2 matrices:

• Matrix A with dimensions (2x40x2x1000)
• Matrix B with dimensions (2x40x2)

I want to add matrix A elements along the 4th dimension if the element is above the one on matrix B.

So if elements (1,1,1,:) of matrix A are bigger than element (1,1,1) of matrix B, I want to add them up. If elements (2,1,1,:) of matrix A are bigger than element (2,1,1) of matrix B, I want to add them up. etc

What is the easiest way to do it?

Answer 1

[Matt J]

   mask=bsxfun(@gt,A,B);

   result=sum(A.*mask,4);

Answer 2

[Jan]

The term "add them up" is not clear. A small example would be nice. I guess it could mean something like this:

S = zeros(size(B));
for k = 1:1000
  Ak   = A(:, :, :, k);
  M    = Ak > B;
  S(M) = S(M) + Ak(M);
end

Or:

S = zeros(size(B));
for k = 1:1000
  Ak = A(:, :, :, k);
  S  = S + Ak .* (Ak > B);
end