MATLAB: Sub2ind problem for matrix

Question

I have a matrix

r =

     6     1
     5     2
     4     3
     3     3
     2     3
     1     2
     2     3
     3     3
     4     3
     5     2
     6     1

and a matrix I.

I applied

idx=sub2ind(size(I),r)

It shows me the following:

idx =

     6     1
     5     2
     4     3
     3     3
     2     3
     1     2
     2     3
     3     3
     4     3
     5     2
     6     1

how to get the correct one easily? thanks

Answer 1

idx=sub2ind(size(I),r(:,1),r(:,2))