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# Problem with Matrix indexing using vector

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Sebastian Raabe on 20 Nov 2019
Edited: Jan on 20 Nov 2019
Hello, I have a problem with indexing matrix elements. I use the following code to describe my problem:
a = blkdiag(ones(3),ones(3));
[r,c] = find(a);
b = zeros(size(a));
b(r,c) = 1;
I assume that the matrix a and the matrix b are equal, but they are not. I can solve this problem with a for-loop, but for a very large matrix this solution is too slow. Are there any other solutions? Thank yoou for your help!

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### Accepted Answer

Jan on 20 Nov 2019
Edited: Jan on 20 Nov 2019
This is a job for sub2ind:
a = blkdiag(ones(3),ones(3));
[r,c] = find(a);
b = zeros(size(a));
index = sub2ind(size(a), r, c);
b(index) = 1;
isequal(a, b) % TRUE
Maybe using logical indexing is more efficient than find():
mask = (a == 1);
b = zeros(size(a));
b(mask) = 1;

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