intersect function returns only one index why ?

Hi,

I have:

A = [1 2 3 4;
     1 2 3 4;
     5 6 1 2]

B = [1 2]
[~, index] = intersect(A(:,1:2),B(1,:),'rows')

The result I get is:

index = 2

Why is that while I actually have 2 rows in A equal to B

Why I don't get index = 1 and 2

Answer 1

[Walter Roberson]

C = intersect(A,B) returns the data common to both A and B with no repetitions.

Notice the "no repetitions". The two entries in A become the same after extracting the first two columns, so outputting the entry twice would be a repetition.

If A and B are numeric arrays, logical arrays, character arrays, categorical arrays, or cell arrays of strings, then C = A(ia) and C = B(ib).

Notice there that since C is the same size either way, it follows that ia and ib must be the same length. As only one intersection is being returned for C (because the other is a repetition) it follows that only one index is being returned in ia and ib.

You should be considering ismember()

Answer 2

[the cyclist]

The slightly snarky answer is, "That is not what the intersection function returns."

The slightly nicer answer is that if you read the documentation, you'll see that intersect does not return the indices to all rows that are in the intersection.

To do what you want is easy, though:

C = intersect(A(:,1:2),B(1,:),'rows')
indices = find(ismember(A(:,1:2),C,'rows'))