MATLAB: Get the index from the comparison of two different lenght vectors

Question

Hi,

I would like to compare two vectors, and get the indexes of the positions where the two vectors are matching. In such a way I could create a sort of lookup table with the correspondences between the vectors indexes. For instance, if I have:

A = [1 2 3 5 6 8 9 7 10] B = [1 5 7 9]

indexes = compare(B,A);

I would like to have as a result: indexes = [1 4 9 8] I would have this way a sort of lookup table keeping track of the correspondences between the two vectors.

If I would then obtain the second element of B then I could just say that is A(4) = 5….or…. B(2) = 4;

how could I do this without looping over the whole vector? (which in my case is very huge….)

Answer 1

You can use function INTERSECT for this:

>> A = [1 2 3 5 6 8 9 7 10] ;B = [1 5 7 9];
>> [c, ia, ib] = intersect(A,B)

c =

     1     5     7     9

ia =

     1     4     8     7

ib =

     1     2     3     4