Assiging elements into the lower triangular part of a matrix

Question

I'm creating a diagonal matrix in MATLAB using eye(3). How can I assign the number "2" only to the elements under the main diagonal?

Shai · Accepted Answer · 2012-12-25T11:48:39.003

5

The command tril has an extra argument that controls which lower triangular exactly to use.

A = eye(3) + 2*tril(ones(3), -1);

edited Dec 25 '12 at 11:48

answered Dec 25 '12 at 11:39

Shai

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I removed it because it completely falls short when compared to yours. – Eitan T Dec 25 '12 at 11:52
@EitanT "completely" is a very strong word. Thanks for the support. – Shai Dec 25 '12 at 12:01
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Hmm... okay then. I've posted a different approach, depending on the OP's desire. – Eitan T Dec 25 '12 at 12:18

Eitan T · Answer 2 · 2012-12-25T13:44:48.527

3

If you're interested in assigning elements into an already existent matrix, you can use tril in similar fashion to Shai's answer and combine it with logical indexing. For example:

A = eye(3);
idx = tril(true(size(A)), -1); % # Lower triangular half
A(idx) = 2

Which should yield the desired result:

A =

     1     0     0
     2     1     0
     2     2     1

If you're at the stage of creating such a matrix, then you should generate it like Shai suggests.

edited Dec 25 '12 at 13:44

answered Dec 25 '12 at 11:37

Eitan T

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I copied and paste your code: A = eye(3); idx = tril(true(size(A), -1)); A(idx) = 2 But I still get the identity matrix. What do i do wrong? – Shakedk Dec 25 '12 at 13:38
@user1928113 sorry, I forgot to close the parentheses after `size(A)`, and you misplaced them. Please try the fixed answer. – Eitan T Dec 25 '12 at 13:45

Assiging elements into the lower triangular part of a matrix

2 Answers2