Speed up a for loop in my programme?

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Alberto Paniate
Alberto Paniate on 29 Jul 2021
Answered: Eike Blechschmidt on 30 Jul 2021
Hi, one of the part of my programme contains this piece of code:
size2=2500;
gran=3;
A=ones(size2,size2);
for k=1:gran:(size2-gran)
for j=1:gran:(size2-gran)
X=rand*2*pi-pi;
for h=1:gran
for l=1:gran
A(k+l-1,j+h-1) = A(k+l-1,j+h-1) *exp(+1i*X); %phase in the square gran x gran
end
end
end
end
My pc runs this code in 0.60 seconds but I would like to know if it is possible to speed up this process.
I have already looked similar questions like:
but I don't know if I can apply to my problem
  2 Comments
Yongjian Feng
Yongjian Feng on 29 Jul 2021
Can this be written as matrix operations? If so, then use the corresponding matlab matrix operation. Then Matlab might be able to optimize the process for you.
Alberto Paniate
Alberto Paniate on 29 Jul 2021
I don't know, I have to create a matrix X of random numbers each square of sides gran x gran. In these squares the randoms numbers are equal

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Answers (1)

Eike Blechschmidt
Eike Blechschmidt on 30 Jul 2021
If I understood you right this could do the trick and is about 2.5x faster on my machine.
tic();
X=rand(size2*size2,1)*2*pi-pi;
[row,col] = ind2sub([size2, size2],1:size2*size2);
blockRow = ceil(row/gran);
blockCol = ceil(col/gran);
idx = sub2ind([size2/gran, size2/gran], blockRow, blockCol);
A = ones(size2, size2);
A(1:size2*size2) = exp(+1i*X(idx));
toc();
Important to not is that size2 needs to be a multiple of gran to for this solution to work.

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