Change diagonal of a non square matrix

Question

Suppose I have this kind of matrix:

How do I add a diagonal of ones into this? Is there an easy way? To show what I mean:

so you mean 'replace' and not an `+` operation, right? – Autonomous Nov 14 '13 at 19:04 — Autonomous, Nov 14 '13 at 19:04

Stewie Griffin · Accepted Answer · 2013-11-14T20:02:11.130

5

You can do this quite easily with linear indexing, you don't even need reshape!

[r, c] = size(m);
m(1:c:end) = 1;
m =

     1     2     3
     4     1     6
     7     8     1
     1    11    12
    13     1    15

If r < c, this is the best I got:

if r < c
   n = m';
   n(1:r:end) = 1;
   m = n';
else
   m(1:c:end) = 1;
end

edited Nov 14 '13 at 20:02

answered Nov 14 '13 at 19:32

Stewie Griffin

14,889
11
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@jkshah: Honestly, I can't find a way to do that *and* avoid `if` using just linear indexing. I updated the answer. If you have a better suggestion, *shoot*! =) – Stewie Griffin Nov 14 '13 at 20:02
1

@RobertP. +1. seems perfect! I didn't have better solution, but was trying to make it complete. Appreciate your thinking. – jkshah Nov 15 '13 at 05:10
@RobertP. You don't even need the row number: why not just `m(1:size(m,2):end) = 1`? Note, however, that your solution only works for r >= c – Luis Mendo Nov 15 '13 at 12:18

Luis Mendo · Answer 2 · 2013-11-14T21:45:11.180

This is a general solution, using linear indexing and modulo operations:

[R C] = size(m);
ii = 1:R;
jj = mod(ii,C);
jj(jj==0) = C;
m(ii+(jj-1)*R) = 1; %// or m(sub2ind([R C],ii,jj)) = 1;

For example,

m =

     1     2     3
     4     5     6
     7     8     9
    10    11    12
    13    14    15
    16    17    18
    19    20    21

gets transformed into

m =

     1     2     3
     4     1     6
     7     8     1
     1    11    12
    13     1    15
    16    17     1
     1    20    21

score 0 · Answer 3 · answered Nov 14 '13 at 19:07

0

s=min(size(m))
m(1:s,1:s)=eye(s)+(~eye(s).*m(1:s,1:s))

answered Nov 14 '13 at 19:07

Daniel

36,610
3
36
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I tried this on m=[1,2,3;4,5,6;7,8,9;10,11,12;13,14,15] but only the upper diagonal was replaced. – Paulinchen2 Nov 14 '13 at 19:17

score 0 · Answer 4 · answered Nov 16 '13 at 14:26

0

If you want a shorter version without comparing the number of rows to the number of columns you can try this (Assuming that you want to make the diagonal elements of an arbitrary matrix M equal to 1):

M(logical(speye(size(M)))) = 1

answered Nov 16 '13 at 14:26

Chris

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Change diagonal of a non square matrix

4 Answers4