Matlab: Remove diagonal from matrix and reform

Question

I would like to remove the diagonal of the following matrix;

 [0 1 1
  0 0 0
  0 1 0]

and put this in a vector as such

[1 1 0 0 0 1]

Is there a one-way function to do this? Most other solutions I found on Stack Overflow delete all zeros.

Is it always going to be a square matrix or can dimensions be different? — Dennis Klopfer, Nov 15 '15 at 12:55

score 7 · Answer 1 · answered Nov 15 '15 at 12:55

7

If two lines are fine...

x = x.'; %'// transpose because you want to work along 2nd dimension first
result = x(~eye(size(x))).'; %'// index with logical mask to remove diagonal

answered Nov 15 '15 at 12:55

Luis Mendo

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It's as simple as that! Thank you very much! – Jill Nov 15 '15 at 13:01
Why the transpose? I can see that it works by testing, but the `~eye` makes a mask over the original `x` as far as I can see. Does the second line of code use linear indices in column-major order? – Adriaan Nov 15 '15 at 13:55
2

The first transpose is because the OP wants the result returned row major but indexing with `~eye(size(x))` returns column major. The second transpose is because the OP wants a row vector instead of a column vector that is returned by the logical mask. – IKavanagh Nov 15 '15 at 14:07

Divakar · Answer 2 · 2015-11-15T14:22:50.737

Here's an almost one-liner -

[m,n] =  size(x);
x(setdiff(reshape(reshape(1:numel(x),m,n).',1,[]),1:m+1:numel(x),'stable'))

And I will put up my fav bsxfun here -

xt = x.';    %//'
[m,n] =  size(x);
out = xt(bsxfun(@ne,(1:n)',1:m)).'

Sample run -

>> x
x =
    52    62    37    88
    23    68    98    91
    49    40     4    79
>> [m,n] =  size(x);
>> x(setdiff(reshape(reshape(1:numel(x),m,n).',1,[]),1:m+1:numel(x),'stable'))
ans =
    62    37    88    23    98    91    49    40    79
>> xt = x.';
>> xt(bsxfun(@ne,(1:n)',1:m)).'
ans =
    62    37    88    23    98    91    49    40    79

Matlab: Remove diagonal from matrix and reform

2 Answers2