About Preallocating for speed

Question

Hari 2017-2-14

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https://ww2.mathworks.cn/matlabcentral/answers/324934-about-preallocating-for-speed

回答： Alexandra Harkai 2017-2-14

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:

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Adam 2017-2-14

编辑：Adam 2017-2-14

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.

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