ismember function too slow

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Salvatore Mazzarino
Salvatore Mazzarino on 17 Oct 2012
I have to improve the speed of my simulation. Well my code is quite simple. I have a vector A=[1 2 3 4 5 6 7 8 9 10] and another vector B=[3 4 9]. I use ismember to check if every element is into A matrix.
for n=1:length(A)
if ismember(B(n),A)
do-something
end
end
this part of my code is executed so many time in my simulation and my matrices are really big.some idea to improve my code?
  1 Comment
Matt J
Matt J on 17 Oct 2012
Since this is speed-critical, it would be wise for you to elaborate on "do-something". That could point the way to eliminating the for-loop altogether, or some other kind of performance optimization.

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Answers (5)

Jonathan Epperl
Jonathan Epperl on 17 Oct 2012
Don't you want to actually INTERSECT A and B? If so, then here's a very fast intersect function that intersects two sets of positive integers only. If you have negative integers you could obviously shift. If you have reals you're SOL. If your integers are greater than ~1e5 the speed gain is negligible.
C = fastintersect(A,B)
if ~isempty(A)&&~isempty(B)
P = zeros(1, max(max(A),max(B)) ) ;
P(A) = 1;
C = B(logical(P(B)));
else
C = [];
end
Obviously you could leave out the check for emtpy sets if you know you won't have empty sets.
  4 Comments
Shengtao Wang
Shengtao Wang on 20 Feb 2013
Note this may produce the same number multiple times if they are not unique in the arrays. not a problem at all though.

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Matt Fig
Matt Fig on 17 Oct 2012
BinA = B(ismember(B,A));
for n=1:length(BinA)
do-something with BinA(n)
end
  2 Comments
Matt Fig
Matt Fig on 17 Oct 2012
If your B vector contains ONLY elements of A, then why were YOU checking if each element was a member in your code?? That is just weird.
If you know that every element of B is a member of A, then what is the point of your question?? Why not just get rid of the IF statement in your FOR loop altogether??

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Azzi Abdelmalek
Azzi Abdelmalek on 17 Oct 2012
Edited: Azzi Abdelmalek on 17 Oct 2012
use ismember out of the loop
A=[1 2 3 4 5 6 7 8 9 10]
B=[3 4 11 9 22]
idx=find(ismember(B,A)==1)
for k=idx
%do
end
  2 Comments
Azzi Abdelmalek
Azzi Abdelmalek on 17 Oct 2012
what do you mean?

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Robert Cumming
Robert Cumming on 17 Oct 2012
if you want to keep it in the loop you can do:
for n=1:length(B)
if min (abs( A-B(n)) ) == 0;
% do something
end
end;
Or use logical indexing
tic
A=[1 2 3 4 5 6 7 8 9 10];
B=[3 4 11 9 22];
idx=find(ismember(B,A)==1);
toc
tic
flags = false(1,length(B));
for i=1:length(A)
check = B==A(i);
flags(check)=1;
end
idx2 = find(flags==1);
toc
A for loop can be faster!!

Omar Ali Muhammed
Omar Ali Muhammed on 3 Mar 2021
A=[1 2 3 4 5 6 7 8 9 10];
B=[3 4 9];
tt=ismember(B,A);
if sum(tt)==3
Do Something;
else
Do Something;
end

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