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Modify off diagonal elements of Matrix without looping

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I have a MXM matrix. Is there a way to set off diagonal elements to a specified value (.e.g. I want all off-diagonal elements to be some constant "k'), without looping through the matrix? I know there is a function "diag"....was hoping to find something that does the opposite....not sucessful so far.

Accepted Answer

James Tursa
James Tursa on 17 Aug 2022
You could use logical indexing to get at the off-diagonal elements. E.g.,
A(~eye(size(A))) = skalar;

More Answers (1)

Bruno Luong
Bruno Luong on 17 Aug 2022
M = 4;
A = rand(M);
A(1:size(A,1)+1:end) = 10, % This is how you change the diagonal of A using linear indexing
A = 4×4
10.0000 0.8772 0.1202 0.5331 0.9790 10.0000 0.6722 0.3879 0.1196 0.8373 10.0000 0.2409 0.2357 0.9320 0.4094 10.0000
Chet Sharma
Chet Sharma on 17 Aug 2022
Found a way without the loop....but gosh it's not pretty:
My original matrix is 5x5.
So I create two idx matrices - upper and lower triangular. Then I replace the off-diagonal elements of the original A with the intended constant (skalar). If there's something better than this, I'll take it.....else this is it for now (found it on one of the QA's on the forum).
idx1 = tril(ones(5,5), -1);
idx2 = triu(ones(5,5), 1);
A(idx1==1) = skalar;
A(idx2== 1) = skalar;

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