Intersection indices by rows

indexing, intersection, matlab, matrix, vectorization

Solution

One possible solution:

function [Index] = GetIntersectionIndicies(m1, m2)
[~, I1] = intersect(m1, m2, 'rows');
Index = zeros(size(m1, 1), 1);
Index(I1) = 1;

By the way, I love the inventive solution of @Shai, and it is much faster than my solution if your matrices are small. But if your matrices are large, then my solution will dominate. This is because if we set `T = size(m1, 1)`, then the `tmp` variable in the answer of @Shai will be T*T, ie a very large matrix if `T` is large. Here's some code for a quick speed test:

%# Set parameters
T = 1000;
M = 10;

%# Build test matrices
m1 = randi(5, T, 2);
m2 = randi(5, T, 2);

%# My solution
tic
for m = 1:M
[~, I1] = intersect(m1, m2, 'rows');
Index = zeros(size(m1, 1), 1);
Index(I1) = 1;
end
toc

%# @Shai solution
tic
for m = 1:M
tmp = bsxfun( @eq, permute( m1, [ 1 3 2 ] ), permute( m2, [ 3 1 2 ] ) );
tmp = all( tmp, 3 ); % tmp(i,j) is true iff m1(i,:) == m2(j,:)
imdices = any( tmp, 2 );
end
toc

Set `T = 10` and `M = 1000`, and we get:

Elapsed time is 0.404726 seconds. %# My solution
Elapsed time is 0.017669 seconds. %# @Shai solution

But set `T = 1000` and `M = 100` and we get:

Elapsed time is 0.068831 seconds. %# My solution
Elapsed time is 0.508370 seconds. %# @Shai solution

Problem

Given these two matrices: ``` m1 = [ 1 1; 2 2; 3 3; 4 4; 5 5 ]; m2 = [ 4 2; 1 1; 4 4; 7 5 ]; ``` I'm looking for a function, such as: ``` indices = GetIntersectionIndecies (m1,m2); ``` That the output of which will be ``` indices = 1 0 0 1 0 ``` How can I find the intersection indices of rows between these two matrices without using a loop ?

Original source