Here is a very straightforward way to get your values of B_prime and Idx.
A = [1 5 7];
B = [1 2 3 6 9 10];
NA = length(A);
B_prime = zeros(1,NA);
B_orig = B;
for n = 1:NA
[~, j] = min(abs(A(n)-B));
B_prime(n) = B(j);
B(j) = [];
end
[~,Idx] = ismember(B_prime,B_orig);
I expect this code will run into problems if the values in B are not unique (but I did not test that for you).
After admittedly very little thought, I could not think of any easy way to vectorize this, given your requirement of not replicating elements of B.
