Finding (multiset) difference between two arrays

Question

Given arrays (say row vectors) A and B, how do I find an array C such that merging B and C will give A?

For example, given

A = [2, 4, 6, 4, 3, 3, 1         


        

Answer 1

You can use the second output of ismember to find the indexes where elements of B are in A, and diff to remove duplicates:

This answer assumes that B is already sorted. If that is not the case, B has to be sorted before executing above solution.

For the first example:

A = [2, 4, 6, 4, 3, 3, 1, 5, 5, 5];
B = [2, 3, 5, 5];
%B = sort(B); Sort if B is not sorted.
[~,col] = ismember(B,A);
indx = find(diff(col)==0);
col(indx+1) = col(indx)+1;
A(col) = [];
C = A;

>>C

4     6     4     3     1     5

For the second example:

A = [2, 4, 6, 4, 3, 3, 1, 5, 5, 5];
B = [2, 4, 5, 5];
%B = sort(B); Sort if B is not sorted.
[~,col] = ismember(B,A);
indx = find(diff(col)==0);
col(indx+1) = col(indx)+1;
A(col) = [];
C = A;
>>C

6     4     3     3     1     5