indexing and performing operations in matrix

I have an nx2 matrix A. I also have an interaction matrix of A, which looks like this:

interact = [1 2; 
            1 4;
            1 5;
            2 1; 
            2 3;
            3 4;
            3 6]

These numbers are just the index. I want to perform the following operation: (A(1,:) - A(2,:))+(A(1,:)-A(4,:))+ (A(1,:)-A(5,:)), then store result into some matrix R(1,:)

Likewise, A(2,:) - A(1,:) + A(2,:) - A(3,:) = R(2,:) etc.

How do I do this?

[Walter Roberson]

Could you confirm that all of the entries with the same value in column 1 should be treated as a group? And if that is the case, then what should happen if the entrees are not consecutive, such as if there was another entry beginning with 1 after the [3 6] ?

[cgo]

I got it already!!!

for val=1:n
iv=interact(ismember(interact(:,1),val),2)

repmat(A(val,:),size(iv,1),1) 
A(iv,:)
end

Thanks for your help though.