how to create pairwise label matrix ?

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Sarah A on 26 Jul 2018
Edited: dpb on 31 Jul 2018
Hello,
I aim to create a label matrix that works as the following:
suppose I have the samples (1,2,3,4) as vectors and I want to say if any two are neighbors so the label is (1). if any two are not neighbors so the label is (-1). Note that neighbors means these two samples are of the same class. for example, the samples 1 and 3 belong to the same class and the samples 2 and 4 belong to the same class. Now, since the samples 1 and 3 of the same class so the label is (1). The samples 2 and 4 of the same class so the label is (1). The samples 1 and 4 is not of the same class so the label is (-1). The samples 3 and 2 is not of the same class so the label is (-1). In the matrix if the pair like (1,1) or (2,2).. so put zero. finally, the matrix should be:
[0 -1 1;-1 0 -1;1 -1 0 ]
So, any ideas to do that please.
Regards,

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Sarah A on 26 Jul 2018
Thank you for your comment. I edited my question, can you read it again and give me your feedback.
Guillaume on 26 Jul 2018
yes, it's a lot clearer now. The only thing missing is how do you know the sample class?
Sarah A on 31 Jul 2018
this input information which means I previously know these samples classes.

dpb on 26 Jul 2018
Edited: dpb on 26 Jul 2018
One possible solution--
>> class=categorical({'A','B','A','B'});
>> nk=nchoosek(1:4,2);
>> arrayfun(@(ix1,ix2) isequal(class(ix1),class(ix2)),nk(:,1),nk(:,2))
ans =
6×1 logical array
0
1
0
0
1
0
>>
>> ~arrayfun(@(ix1,ix2) isequal(class(ix1),class(ix2)),nk(:,1),nk(:,2))*2-1
ans =
1
-1
1
1
-1
1
>>
You can fill in the square array if desired, of course.

Sarah A on 31 Jul 2018
can you explain your solution for me please, I totally new in Matlab.
dpb on 31 Jul 2018
nk is the list (by row) of four things taken two at a time so arrayfun just passes each pair to the anonymous function that is the functional form of the equality expression class(i)==class(j).
arrayfun is the one-line equivalent of writing an explicit for loop over the range of the indices 1:numel() of the variables in the argument list.
NB: I stuck with previous variable names; @G's correct in that 'class' isn't good to alias as general practice.

Guillaume on 26 Jul 2018
Going with dpb's example (with slightly better variable names, don't use class it's too useful a function to shadow!), to generate the label matrix:
varclass = categorical({'A','B','A','B'});
labelmatrix = 2 * (varclass == varclass') - 1;
labelmatrix(logical(eye(numel(varclass)))) = 0

dpb on 26 Jul 2018
" class it's too useful a function..."
Very good point; being a dinosaur I don't write OO code so didn't even think of it... :)
Guillaume on 31 Jul 2018
class hasn't got much to do with OOP actually. Too often do we ask the OP, "what is class(thevariable_that_generate_the_error) ?". Case in point, here, it would be useful to know what the 'sample class' actually is.
dpb on 31 Jul 2018
Well, there's that use of class, too... :)
Had been in a conversation over on comp.lang.fortran comparing/contrasting newer features in it to C++ and had those kinds of classes on the brain at the time...

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