low speed of 'ismember' function

Question

Ehsan 2014-8-29

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https://ww2.mathworks.cn/matlabcentral/answers/152723-low-speed-of-ismember-function

评论： Ehsan 2014-8-29

hi. in my code there is a for loop like the following:

for i=1:n
N=find(~ismember(Vector1,Vector2))
do something...
end

This loop is repeated several thousand times. 'ismember' function is very slow. So that most of the runtime for 'ismember' function lost. Is there any alternative to this?

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Answer 1

Johan Löfberg 2014-8-29

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If data is sorted, using ismembc could be a quick fix.

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Ehsan 2014-8-29

No. This loop used for programming the Traveling Salesman Problem. this vectors is not sorted and can't be like that.

Johan Löfberg 2014-8-29

Then I would scratch ismember and do sort+ismembc (ismember contains a lot of overhead. look into it and you will see that it basically does sort+ismembc once all data is checked etc)

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Answer 2

David Sanchez 2014-8-29

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在 MATLAB Online 中打开

Depending on your data, you might try to code your own function avoiding some conditions and calls to sub-functions within the ismember function.

You can take a look at ismember code with:

type ismember

and try to make it easier.

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José-Luis 2014-8-29

The way you use ismember() seems to be inefficient. You are going to be processing similar things over and over. You could instead loop through the variable of interest, do an exhaustive search, save that somewhere and then %do something based on those results.

Ehsan 2014-8-29

I just do not understand. What is your suggestion? Please change the code according to your suggestion.

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Answer 3

Titus Edelhofer 2014-8-29

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Hi,

does "Vector1" and "Vector2" changes in every iteration? Often you see something like

for i=1:n
  v = ismember(x(i), y);
  % do something
end

which can be changed to

vAll = ismember(x, y);
for i=1:n
  % now do something with vAll(i)
end

Titus

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Ehsan 2014-8-29

在 MATLAB Online 中打开

this is my code:

        for k=1:nAnt
            ant(k).Tour=randi([1,nCity],1,1);
        end
        for i=2:nCity  
            for k=1:nAnt 
                N=find(~ismember(1:nCity,ant(k).Tour)); 
               % do somthing
            end
        end