How make this code execute faster?

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Andrea Abbate
Andrea Abbate 2019-6-9
编辑： Adam Danz 2019-6-9
Hi, I'm just trying to figure out how to make this code faster:
N = size(X);
temp = 1:N;
S1 = temp(~ismember(temp,S));%this runs slow
M = X(S,S1); %this runs slos
m = min(M);
cost = sum(m);
X is a square matrix.
Given array of indexes S, I want to extract the submatrix M of S rows, and S1 columns, where S1 are the indexes which are not in S.
Hope I explained it well.
There is a way to achieve the same results faster ?
  2 个评论
Andrea Abbate
Andrea Abbate 2019-6-9
编辑：Andrea Abbate 2019-6-9
X is a square matrix, NxN, it is variable. What does you mean with size? I don't know which exact size will it be

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Adam Danz
Adam Danz 2019-6-9
编辑：Adam Danz 2019-6-9
Sidx = ismember(1:size(X,1),S);
M = X(Sidx,~Sidx);
cost = sum(min(M,[],1));
This versions is slightly faster. I ran your and my versions 100,000 times with X set as a 32x32 matrix and compared the median speeds. This version is 1.11 times faster (p<0.001, Wilcoxon Signed Rank test).
  2 个评论
Andrea Abbate
Andrea Abbate 2019-6-9
Thank you for your answer!
I tried using your suggestion and I came up to this:
S1 = ~ismember(1:size(X,1),S);
M = X(S,S1);
cost = sum(min(M,[],1));
Which seems to run even faster, but I'm not sure it is the same. The results seems to be good
Adam Danz
Adam Danz 2019-6-9
编辑：Adam Danz 2019-6-9
" but I'm not sure it is the same"
That's easy to check. Run both versions on the same inputs and compare the results. Spoiler alert: they are the same ;)
I ran you're new version vs my version 100,000 times and their difference in run time has a factor of 1.0 (meaning no difference).

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dpb
dpb 2019-6-9
Try
S1 = setdiff(1:N;S));
What is the precise definition of "runs slow(ly)"? In what context and how have you deduced/concluded this is so?
  1 个评论
Andrea Abbate
Andrea Abbate 2019-6-9
编辑：Andrea Abbate 2019-6-9
I used the profiler and that is the part of code that runs slower than the others.
However, I tried setdiff before, but it runs slower

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