How can I write this to run more quickly, without a loop?

What would be the proper Matlab-y way to write this? It's a bottleneck in my code.

% IDX_full is a n_items x 1 double

    M = zeros(n_items,n_items);

    for i = 1:length(IDX_full)
        for j = 1:length(IDX_full)
            if (IDX_full(i) == IDX_full(j)) && (IDX_full(i) > 0)
                M(i,j) = 1;
            end
        end
    end

Answer 1

[Image Analyst]

How big is your array? I tried 10 thousand by 10 thousand, and got it down from 1.05 seconds to 0.67 seconds with this vectorization:

% IDX_full is a n_items x 1 double
n_items = 10000;
IDX_full = randi([0, 2], n_items, 1);
M = zeros(n_items,n_items);
tic
for i = 1:length(IDX_full)
	if IDX_full(i) > 0
		indexes = IDX_full(i) == IDX_full;
		M(i, indexes) = 1;
	end
end
toc
% M
% imshow(M, []);

[Daniel Drucker]

Thanks, that definitely helps.