Need help removing this for loop
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xtremecheez
on 14 Nov 2017
Commented: Andrei Bobrov
on 14 Nov 2017
This is the last step in vectorizing my code, but I just can't see how to get rid of the pesky for loop. Any assistance is appreciated.
I've replaced the variables with numbers for a more readable example.
%size(Cp) = [5,3,100]
%size(Tr) = [2e4,100]
A = zeros(5,2e4,100);
for ii = 1:100
A(:,:,ii) = Cp(:,Tr(:,ii),ii);
end
1 Comment
Stephen23
on 14 Nov 2017
Edited: Stephen23
on 14 Nov 2017
"I just can't see how to get rid of the pesky for loop. Any assistance is appreciated."
Don't get rid of the loop. Using a loop is probably the neatest, clearest, and most efficient way to solve that task. Anything else will just pointlessly obfuscate your code and waste your time later when you forget what it does.
Accepted Answer
Walter Roberson
on 14 Nov 2017
If I got my calculations right, then:
A = Cp( bsxfun(@plus, (1:size(Cp,1)).', bsxfun(@plus, (Tr - 1) * size(Cp,1), ((1:size(Cp,3)-1) * size(Cp,1) * size(Cp,2)) ) );
This is difficult to read and difficult to write. I cannot recommend it unless perhaps run time is very important -- and even then considering the size of the internal matrices involved and the number of operations used to construct them, I would tend to doubt that it is more efficient.
Your existing code with a loop is easier to read and probably nearly as efficient, possibly more efficient.
This is one of those cases where the goal to "vectorize" requires constructing large intermediate arrays and use calculations that have no obvious meaning. Since typically more time is spent programming and debugging and documenting than is spent in execution of a program, it is unlikely that you would be gain anything meaningful by vectorization.
2 Comments
More Answers (1)
Andrei Bobrov
on 14 Nov 2017
Edited: Andrei Bobrov
on 14 Nov 2017
[k,m,n] = size(Cp);
A = Cp((1:k)' + k*(permute(Tr,[3 1 2]) - 1)+reshape(0:n-1,1,1,[])*k*m);
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