Make matrix from input size

I have an input of zeros n x n and I would like to output a matrix with an index like this:

function [ out ] = test3( mat )
[r c]= size(mat)
end

for input [0 0 0; 0 0 0] output [1 1; 1 2; 1 3; 2 1; 2 2; 2 3]

for input [0 0 0 0; 0 0 0 0; 0 0 0 0] output [1 1; 1 2; 1 3; 1 4; 2 1; 2 2; 2 3; 2 4; 3 1; 3 2; 3 3; 3 4]

and so on for all size of 0 matrix input. I tried something like

out=[1:r 1:c]

But I get the wrong output. How can I achieve something like this?

Answer 1

[Stephen23]

If you want all pairs of subscript indices, then you can use ind2sub:

>> mat = zeros(2,3)
mat =
     0     0     0
     0     0     0
>> [R,C] = ind2sub(size(mat),1:numel(mat));
>> [R;C]'
ans =
     1     1
     2     1
     1     2
     2     2
     1     3
     2     3

And use sortrows if you really need that exact order:

>> sortrows([R;C]')
ans =
     1     1
     1     2
     1     3
     2     1
     2     2
     2     3

Note that in MATLAB there are three main ways of indexing, and using subscript indices is not the most efficient: logical indexing is usually the fastest indexing method.

Answer 2

[James Tursa]

doc ndgrid

Look at combining the results with your 1:r and 1:c inputs.

EDIT:

Using your two inputs of 1:r and 1:c, it appears you want to get all combinations of pairs of them. So ndgrid can be used to do this.

[X,Y] = ndgrid(1:r,1:c);
out = [X(:) Y(:)];

The (:) notation turns a variable into a column vector.

[Millone]

Thanks but that looks quite complex for me. I am just starting using Matlab on my own.

[Walter Roberson]

In that case use a nested for loop.

[Millone]

Thanks a lot. But for input [0 0; 0 0] I should have 1 1; 1 2; 2 1; 2 2 and I get instead 1 1; 1 2; 1 3; 1 4;

[Walter Roberson]

Then you aren't using a nested for. A nested for is like

for J = 1 : 17
  for K = 53 : 88
    ...
  end
end

Answer 3

[Andrei Bobrov]

sortrows(fullfact(size(mat)),1)

[Stephen23]

+1 An excellent solution.

Note that fullfact requires the Statistics Toolbox.