Run function for paired data?

Hi,

Let's say I have a 10x10 matrix A. I wish to run each unique pair of elements from that matrix (e.g. (1,1),(1,2),...,(1,10) - not to repeat (2,1) for instance) in a function that I have. Any ideas how this can be possible?

Thanks!

Answer 1

[Geoff Hayes]

Fred - if you just want to loop over each unique pair of elements in the matrix, then consider doing something like

 % assume A is 2D
 [r,c] = size(A);

 for m=1:r
     for n=m:c
         % do something with pair (m,n)
     end
 end

Try the above and see what happens!

[Fred John]

Thanks. But would it be:

for m=1:r
    for n=1:r

      end

end

to loop over EVERY pair? And how would I write the output matrix? Something like this? :

outputm(m,n)=myfunction(a,b,c) % function and inputs
end

[Geoff Hayes]

Yes, if you assume that the matrix is square and you want loop over each pair (and not the unique ones) then you can do as above (though use c for the number of columns for the case where your matrix is not square). And yes, you can write your output matrix as described above (consider pre-sizing it since you know the dimensions).

Answer 2

[Guillaume]

To get your pair indices:

maxpair = 10; %? a 10x10 matrix has 100 elements.                                    
indices  = [repmat([1:maxpair]', 1, 2); nchoosek(1:maxpair, 2)]

You can then use each row of indices as indices into your matrix:

m = randi(1000, 10, 10); %for example
maxpair = numel(m);
indices  = [repmat([1:maxpair]', 1, 2); nchoosek(1:maxpair, 2)];
pairs = cellfun(@(row) m(row), num2cell(indices, 2), 'UniformOutput', false) %is this what you want?