assign value to the diagnal of a matrix in matlab

Question

diag(M) = c(1,2,3)

say I have a matrix M 3*3, then I want to assign value to its diagnal elements, but above command line does not work why?

The errors says Subscript indices must either be real positive integers or logicals.

see http://stackoverflow.com/questions/3963565/how-to-assign-values-on-the-diagonal — Nasser, Apr 21 '14 at 03:16
For the [generic approach to troubleshoot this error](http://stackoverflow.com/a/20054048/983722), see [this question](http://stackoverflow.com/q/20054047/983722). — Dennis Jaheruddin, May 26 '14 at 15:38

score 1 · Answer 1 · answered Apr 21 '14 at 03:33

1

you can just use linear indexing, for example, ifM is 3x3:

 M(1:(size(M,1)+1):end)=[10 20 30]

answered Apr 21 '14 at 03:33

bla

25,846
10
70
101

That'd be linear indexing – tmpearce Apr 21 '14 at 03:36
right... sorry a Freudian slip... lol – bla Apr 21 '14 at 03:37

score 1 · Accepted Answer · answered Apr 21 '14 at 05:36

You can use diag this way -

%%// Given matrix M
M = randi(10,3,3)

%%// Assign the diagonal elements as 1,2,3
M(diag(ones(size(M,1),1),0)>0) = 1:3

Output -

M =
     3     1     2
     3     5     8
     6     2     3

M =
     1     1     2
     3     2     8
     6     2     3

score 0 · Answer 3 · answered Apr 21 '14 at 15:27

You can do it with the identity matrix:

>> M = rand(3)

M =

    0.3922    0.7060    0.0462
    0.6555    0.0318    0.0971
    0.1712    0.2769    0.8235

>> M(eye(size(M)) == 1) = [1 2 3]

M =

    1.0000    0.7060    0.0462
    0.6555    2.0000    0.0971
    0.1712    0.2769    3.0000

assign value to the diagnal of a matrix in matlab

3 Answers3