MATLAB: Nested loop problem with the second index

Question

Dear all,

the following code does not work well for index j > 1. I have two vectors cpx = 1-by-n and cpy = 1-by-m.

I would a new matrix cp with n-by-m rows and 2 columns with all the possible permutations of the elements in cpx and cpy. Where is the mistake?

for i = 1:length(cpx)
    for j = 1:length(cpy)
        cp(i.*j,:) = [cpx(i) cpy(j)];
    end
end

The results are fine till row 55, so that the first cicle for i = 1:55 and j = 1 works well, then the results are incomprehensible.

Answer 1

The problem of you code was cp(i.*j,:). i*j is not a growing index. In the 1st iteration 1*1 is fine. For i=1 it works for the complete loop over j. But in the next iteration for i=2, j=1 you overwrite the value written by i=1, j=2. Better:

nx = numel(cpx);
ny = numel(cpy);
cp = zeros(nx * ny, 2);  % Pre-allocation!!!
for i = 1:nx
    for j = 1:ny
        cp((i-1) * ny + j,:) = [cpx(i) cpy(j)];
    end
end

But this can be written without loops:

cp = [repelem(cpx(:), numel(cpy), 1), repmat(cpy(:), numel(cpx), 1)];