# Speed up a for loop in my programme?

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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 CommentsShowHide 1 older comment
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

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.