How to create 3D array out of 2D, with one row step without using for loop?

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Mantas Vaitonis
Mantas Vaitonis 2018-7-3
评论： Mantas Vaitonis 2018-7-4
Hello,
Lets say I have 2D array x(1100x5), n1=1100,d=501 and w=500. I can create 3D array, with one row step using for loop, however it is very slow working with bigger arrays:
for k=1:n1-d
a(:,:,k)=x(d-w+k-1:d+k-2,:);
end
What would be the way without using for loop?
  2 个评论
John D'Errico
John D'Errico 2018-7-3
It is slow mainly because you do not understand the importance of preallocating your array, thus a. When you grow an array as you did, you force MATLAB to reallocate that memory each time in the loop. This gets slow, because the memory needed increases in size each time through.
Mantas Vaitonis
Mantas Vaitonis 2018-7-3
Thank You for Your answer, regarding preallocation, I do this a=zeros(500,5,599,'gpuArray'); and accordingly when working with bigger arrays, but its little help.

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Matt J
Matt J 2018-7-3
xt=x.';
idx=(d-w+1:d-2).' + (1:n1-d);
a=permute( reshape( xt(:,idx(:)) ,5,[],n1-d) ,[2,1,3]) ;

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