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Substitue a diagonal matrix

Asked by AnnaMaria Accardo on 26 Feb 2015
Latest activity Edited by sandhya
on 27 Feb 2015
Hi all, I'm trying to create a square matrix 9x9 where all the values are 9 and the diagonal is 0. Example:
0 9 9
9 0 9
9 9 0
I found a way to do it which is:
s(logical(eye(size(s)))) = 0
Is there an easy way to substitute a diagonal of a matrix (possibly without using [m n] = size(s) --> s[1:m]....) Thanks

  3 Comments

Sean de Wolski
on 26 Feb 2015
Fun question, well written, etc.
John D'Errico
on 26 Feb 2015
Except, I find myself wondering if the problem is to create a 9x9 matrix of 9's, or a 3x3 matrix of 9's. The example shoed a 3x3 matrix, the words 9x9. :)
The difference is not important, just a matter of confusion.
@John D'Errico is a 9x9 matrix but it will be very long if I had to write the entire matrix. It was just for an example. To be sure that the people will understand ;) :)

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3 Answers

Answer by sandhya
on 26 Feb 2015
 Accepted Answer

The one line solution to your problem that strikes me (without using size function) is as under
9*(~eye(9,9))
Copy the above line of code and run in your command window. It gives the result you want.
Vote my answer if it was helpful....

  2 Comments

Ok amazing answer. You will be the accepted answer. But can you explain to me what '~' means? Thanks
sandhya
on 27 Feb 2015
it is tilde operator whose fuction is negating the operation , that is convert 0' sto 1's and 1's to zeros. hope this answers your question

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Answer by John D'Errico
on 26 Feb 2015
Edited by John D'Errico
on 26 Feb 2015

There are many ways of doing most anything in a tool like MATLAB.
To create the matrix you want to create, the very simplest one liner to do so is...
s = 9*~eye(3)
s =
0 9 9
9 0 9
9 9 0
Clearly this relies on the fact that your matrix was a very simple one, with constant off-diagonal elements, as well as constant (but different) diagonal elements. But the general idea is one that will work in other places too, so it is good to remember.
You did find a very reasonable way to zero out the diagonal of a matrix, or indeed to replace it too. I'll replace the diagonal of a 3x3 matrix of 9's with the numbers [1,2,3], just to show that you can use it for any general vector of elements.
s = 9*ones(3);
s(logical(eye(3))) = 1:3
s =
1 9 9
9 2 9
9 9 3
(Of course, vote for this answer if you found it useful.)

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Answer by Sean de Wolski
on 26 Feb 2015

And another:
~diag(1:3)*9

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