low speed of 'ismember' function

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?

Answer 1

[Johan Löfberg]

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

[Ehsan]

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

[Johan Löfberg]

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)

Answer 2

[David Sanchez]

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.

[Ehsan]

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

[José-Luis]

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]

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

Answer 3

[Titus Edelhofer]

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

[Ehsan]

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