Speed up a for loop in my programme?

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:

How Can i speed up "for" loop ? - MATLAB Answers - MATLAB Central (mathworks.com)

but I don't know if I can apply to my problem

[Yongjian Feng]

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]

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

Answer 1

[Eike Blechschmidt]

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.