MATLAB: Code vectorization problem

vectorization

Hi
I have a piece of code with a for loop that I am trying to vectorize, but cannot find any easy way to do it. The code is
NG = 1e4;
N = 1e6;
a = floor(NG*rand(N, 1)) + 1;
b = rand(N, 1);
nn = sparse(NG, N);
for k = 1 : N
      i = a(k);
      nn(i, k) = b(k);
end
Does anyone have any suggestions? I tried to use linear indexing, but the matrix size is too large.
Thanks

Best Answer

nn=sparse(NG,N);
k=1:N;
nn(sub2ind([NG,N],a(k),k'))=b(k);
%Timing for NG = 10000 and N = 10000
%Elapsed time is 0.103679 seconds. -> with the for loop 
%Elapsed time is 0.003603 seconds. -> with the code of my answer
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