Question

Is there a better way to do this for-loop?

for i = find(A > 42)
    B(A(i), i) = B(A(i), i) + 1;
end

A is an integer array. B is a max(A)×length(A) matrix.

Example:

A = reshape(magic(3), 1, 9); %# 8 3 4 1 5 9 6 7 2
B = zeros(max(A), length(A));
for i = find(A > 3)
    B(A(i), i) = B(A(i), i) + 1;
end

B = [
    0     0     0     0     0     0     0     0     0
    0     0     0     0     0     0     0     0     0
    0     0     0     0     0     0     0     0     0
    0     0     1     0     0     0     0     0     0
    0     0     0     0     1     0     0     0     0
    0     0     0     0     0     0     1     0     0
    0     0     0     0     0     0     0     1     0
    1     0     0     0     0     0     0     0     0
    0     0     0     0     0     1     0     0     0
]

Answer 1

I’d recommend linear indexing for this case. Convert your row/column subindices into linear indices with sub2ind.

i = find(A > 3);
si = sub2ind(size(B),A(i),i);
B(si) = B(si) + 1;

You can combine this into a one-liner if you want, I left it as multiple lines for clarity.