Find a row in a matrix in faster way
1 view (last 30 days)
Show older comments
Hi,
I have to compare two matrices: in detail, I have to find if each row of a matrix1 is contained in a matrix2. Then I have to store the corresponding positions in a vector.
The following is a trivial method and seems to be faster than ismember version.
if true
t1=tic;
for m=1:size(matrix1,1)
for l=1:size(matrix2,1)
if(sum(xor(matrix1(m,:), matrix2(l,:)))==0)
vector(m)=l;
end
end
end
toc(t1)
end
ismember version.
if true
t2=tic;
for m=1:size(matrix1,1)
index=ismember(matrix2,matrix1(m,:),'rows');
l=find(index);
vector(m)=l;
end
toc(t2)
end
Is there a way to speed up such computation? Thank
2 Comments
Image Analyst
on 29 Nov 2012
Are they integer or floating point? http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F
Matt Fig
on 29 Nov 2012
IA, the methods do not return the same results unless both matrices are binary....
Accepted Answer
Matt Fig
on 29 Nov 2012
Edited: Matt Fig
on 29 Nov 2012
Here is a faster method, in the script I use to compare:
M1 = rand(1000,8)>.5;
M2 = rand(1000,8)>.5;
tic % Method 1, use XOR
V1 = zeros(1,size(M1,1));
for m=1:size(M1,1)
for n=1:size(M2,1)
if(sum(xor(M1(m,:), M2(n,:)))==0)
V1(m)=n;
end
end
end
toc
tic % Method 2, use ISMEMBER
V2 = zeros(1,size(M1,1));
for m=1:size(M1,1)
index=ismember(M2,M1(m,:),'rows');
n = find(index,1,'last');
if ~isempty(n)
V2(m)=n;
end
end
toc
tic % Method 3, use BSXFUN
V3 = zeros(1,size(M1,1));
for m=1:size(M1,1)
index=all(bsxfun(@eq,M1(m,:),M2),2);
n = find(index,1,'last');
if ~isempty(n)
V3(m)=n;
end
end
toc
isequal(V1,V2,V3)
3 Comments
More Answers (0)
See Also
Categories
Find more on Matrix Indexing in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!