listing/enumerating numbers

I have the following matrix, named interact:

interact = [1 2;
            1 3; 
            2 1;
            2 3;
            3 1;
            3 4; 
            4 1]

This matrix means that 1 and 2 interact, 1 and 3 interact, and so on. (just a matrix full of indices.)

Now I want to perform this operation/task.

All that 1 interacts will be added and stored in a matrix. Meaning the interaction between 1 and 2 + interaction between 1 and 3 = interaction of 1.

Similarly, separate matrices for interaction of 2, 3, 4, etc.

How do I do this in matlab?

Answer 1

[Thorsten]

 for val=1:4
   iv{val} = interact(ismember(interact(:,1), val), 2); 
 end

Answer 2

[Guillaume]

Simply use accumarray:

interact = [1 2; 1 3; 2 1; 2 3; 3 1; 3 4; 4 1];
interactwith = accumarray(interact(:, 1), interact(:, 2), [], @(v) {v})

accumarray uses the subs argument (here your first column) to group together the val argument (here your 2nd column) into a vector which at the end it passes to the fun function argument. Here that function is an anonymous function that just returns the vector as a cell array.