MATLAB: About Preallocating for speed

preallocation

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:

Best Answer

NumberedOD = nchoosek(1:N, 2);

Related Solutions

MATLAB: Generate matrices of the form [a b c d]

Of course you could just do a trivial, brute force way of 4 nested "for" loops, which I'm sure you can do. (If not, let us know.) But I'm thinking that you're wanting some more clever method, right? Or not right, and the trivial way is fine?

Oh well, here's the trivial way, just to save time:

for a = 2:4
  for b = 1:2
    for c = 3:4
      for d = 2:4
        out = [a,b,c,d]
      end
    end
  end
end

MATLAB: About preallocating for speed

This depends on what the size and class of the matrix returned by build_matrix( ) is. E.g., suppose it returns an MxN double matrix. Then the pre-allocation and the assignments would look like this:

A = zeros(size(P,2)*M,N);  % pre-allocate result
for i = 1:size(P,2)
    A(1+(i-1)*M:i*M,:) = build_matrix(P(:,i));  % modify the way you do the assignment
end

Best Answer

Related Solutions

MATLAB: Generate matrices of the form [a b c d]

MATLAB: About preallocating for speed

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