Recommended steps to deal with a slow loop

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Danielle Leblanc
Danielle Leblanc el 27 de Jul. de 2012
Hi,
If a "for loop" is slow and each iteration is taking 2 hours what are the recommended steps that I should follow to deal with this issue?

Respuestas (2)

Conrad
Conrad el 27 de Jul. de 2012
I would recommend that you first focus on what happens inside the for loop... why is it taking so long? Get a complete breakdown of how much time is spend at every step, then you can decide where you can possibly optimise.
Further details would be great.

Sean de Wolski
Sean de Wolski el 27 de Jul. de 2012
  • Listen to mlint. If it is warning you about a variable changing size within a loop, that will slow it down immensely
  • Remove every calculation from every loop that does not depend on the loop.
  3 comentarios
Sean de Wolski
Sean de Wolski el 27 de Jul. de 2012
Use the profiler to identify the bottlenecks. Just run two or three files with profiler and it will show you where to focus your efforts. Some obvious things:
  • try/catch is slow. I would try and write the code to avoid those.
  • Skip the call to find here:
x0=find(ismember(s2(:,1),ID(k,1)));
  • regexp can be slow, perhaps csvread or dlmread?
  • Conversions to and from cell is slow (s2=num2cell(nan(size(V,1)-1,size(scol,2)));) avoid this if possible.
Danielle Leblanc
Danielle Leblanc el 27 de Jul. de 2012
the profiler is showing that ismember and unique functions are th emost time consuming functions

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