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How to speed up my code?

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Benson Gou
Benson Gou il 18 Giu 2021
Commentato: Benson Gou il 21 Giu 2021
Dear All,
I found my code spent a lot of cpu time on the following functions:
  1. intersect, took 20.11 seconds, called 232074 times in my code.
My code:
C = intersect(A,B); where A and B are two column arrays.
2. unique, took 8.845 seconds, called 251736 times in my code.
My code:
BadMeas = unique([A; red2(B)']); where A and B are two column arrays, red2 is another row array.
3. ismember, took 7.117 seconds, called 269278 times in my code.
My code:
if ismember(selectedBus, InjBus_moreZeroLines)
for i = 1 : m
do the calculation
4. setdiff, called 13103 times in my code.
My code:
red = setdiff([1:length(A)],B);
A is an array and B is an array formed by integers.
I am wondering if there are faster functions to replace the above ones. Thanks.
  5 Commenti
dpb il 18 Giu 2021
Modificato: dpb il 18 Giu 2021
Cant' disagree w/ Stephen -- you would need, I think, to build a test case that has just the guts of the algorithm for anybody to be able to tell much...or describe the problem to be solved and perhaps somebody will have an alternate algorithm. Generally, real gains in speed are obtained by better/smarter algorithms than just trying to optimize existing code.
One thing I do note, however is
BadMeas = unique([A; red2(B)']);
has a set of square brackets and catenation inside what is described as a tight loop -- that may be a major part of the time there rather than unique() itself. Move that out if at all possible or do a direct assignment to a preallocated array instead of catenation.
Benson Gou
Benson Gou il 18 Giu 2021
Hi, dpb,
Thanks a lot for your reply. I will do as you suggested. You have a good weekend!

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Jan il 19 Giu 2021
if ismember(selectedBus, InjBus_moreZeroLines)
ismember replies a vector, if the inputs are vectors. The if command requires a scalar condition. So I guess, this can be abbreviated. Maybe this is faster:
if any(selectedBus == InjBus_moreZeroLines)
In the line:
red = setdiff([1:length(A)],B)
the square brackets are a waste of time. [ ] is the concatenation operator, but here you concatenate the vector 1:length(A) with nothing.
Maybe this is more efficient:
red = true(size(A));
red(B) = false;
Then use red for logical indexing, instead of numerical indices.
I assume there are faster repalcements for the called functions, but without knowing what the inputs are and the context, it is not possible to suggest modifications.
  1 Commento
Benson Gou
Benson Gou il 21 Giu 2021
Hi, Jan,
Thanks a lot for your great help. I will test your suggestions. Once they work, I will accept your answers.
Best regards,

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