Can someone explain to me why this sub-array behaves this way?

I'm new to Matlab and don't understand why an array

arr = [1 2 3; 4 5 6; 7 8 9]

gives an answer of

ans = [6 6; 6 6]

when you ask for

arr([2 2], [3 3])

If someone could explain this to me it would be greatly appreciated.

Answer 1

[Walter Roberson]

When you index, you get every combination of indices. For example, arr(1,:) gives every combination of 1 in the first index and all the valid column numbers in the second index -- all of row 1. arr(1:2,:) would give every combination of either 1 or 2 for the first index and all the valid column numbers for the second index -- all of rows 1 and 2.

It is every combination -- not every unique combination. So arr([1 1],:) would give all of row 1 twice.

When you index arr([2 2], [3 3]), treat it like arr([A B], [C D]), where you get every combination arr(A,C), arr(B, C), arr(A,D), arr(B,D), even though some of those are the same as the others. With A and B both being 2 and C and D both being 3, you end up selecting arr(2,3) four times

[Kevin H]

Oh I get it now thank you so much.

Answer 2

[James Tursa]

The (i,j)'th element of the result is made up of the (row_index(i),col_index(j)) element of the source. Since you have two row indexes (both equal to 2), and two column indexes (both equal to 3), you get a 2x2 result where all of the elements are made up of the (2,3) element of the source. E.g.,

arr([a b],[c d])

will be a 2x2 matrix as follows

[ arr(a,c) arr(a,d)
  arr(b,c) arr(b,d) ]

Answer 3

[Peter]

Great answer, James! Kevin, perhaps you intended this?

>> arr([2 3],[2 3])

ans =

     5     6
     8     9