About Preallocating for speed

What is a more efficient way of writing this code?

   NumberedOD = [];
for a = 1:(N-1)
    for b = (a+1) :N
        NumberedOD = [NumberedOD;a, b];

    end
end

My aim is to get a list of numbers like this:

1 2
1 3
 .
 . 
1 N
2 3
2 4
 .
 .

[Adam]

Purely from a pre-allocation perspective you can work out the number of elements there will be in that matrix - it is fixed and calculable so you can preallocated a zeros matrix. I'm sure there is a quicker vectorized way to do it though, I just don't have time to think about it myself right now. I'm sure someone will suggest one though.

Answer 1

[Alexandra Harkai]

NumberedOD = nchoosek(1:N, 2);