# About Preallocating for speed

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Hari on 14 Feb 2017
Answered: Alexandra Harkai on 14 Feb 2017
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
.
.
##### 1 CommentShowHide None
Adam on 14 Feb 2017
Edited: Adam on 14 Feb 2017
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.

Alexandra Harkai on 14 Feb 2017
NumberedOD = nchoosek(1:N, 2);

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