MATLAB: Can this be vectorized

Question

Dear all,

I have two vectors A and B of respective sizes na and nb, with na>=nb. All elements in B are unique while the elements in A are not. But all elements in A are in B. I would like to create a mapping between A and B and one way to do that is as follows:

A=nan(na,1);
for ii=1:nb
    A(A==B(ii))=ii;
end

This works well be becomes prohibitively expensive for large na. And so my question is whether there is a way of getting rid of the for loop and/or creating this mapping differently.

Thanks,

P.

Answer 1

[lc,ii] = ismember(A,B);
A(lc) = ii(lc);

or with nan

out = ii;
out(~lc) = nan;

or jast (with zeros)

out = ii;