Find the elements in an array that is not in another array

Question

I have two arrays lets say A = [1;2;4;7;10;20]; B = [1;4;8];

Now I want to find the elements of A, that are not in B i.e; [2;7;10;20]. I just need their index that is the index of the elements [2;7;10;20] in A. How can I implement this in matlab. I can use loops and all. But that is not what I want. I want an elegant solution. Suggestions?

Answer 1

You can do that using the ismember function.

A = [1;2;4;7;10;20]; 
B = [1;4;8];
ismem = ismember(A,B);

will give you

[1 0 1 0 0 0]'

If you really need the indices, you can use find.

find(ismem==0)

Just as a reminder, you can always use logical indexing like so:

A(~ismem)

will give you

[2 7 10 20]'

Answer 2

If you want the elements of A which are not in B you can use setdiff.

If you want the indices of the elements rather than their values, you can use ismember and negate the result.