Filter matrix rows depending on values in a second matrix

Question

Given a 2x3 matrix x and a 4x2 matrix y, I\'d like to use each row of y to index into x. If the value in x i

Answer 1

>> x = [1, 2, 3; -1, 2, -1];
>>y = [1, 1; 
    1, 2; 
    1, 3; 
    2, 1;
    2, 2;
    2, 3];
>>row_idx = reshape((x == -1)',1,6);
>>y = y(row_idx,:);

I think you didn't include all the index of x in y. I included all of them in y. Have a look..

Generalized version:

>> x = [1, 2, 3; -1, 2, -1];
>>y = [1, 1; 
    1, 2; 
    1, 3; 
    2, 1;
    2, 2;
    2, 3];
>>row_idx = reshape((x == -1)',1,size(x,1)*size(x,2));
>>y = y(row_idx,:);

Answer 2

A raw approach to what sub2ind follows (as used by this pretty nice-looking solution posted by Luis) inherently would be this -

y = y(x((y(:,2)-1)*size(x,1)+y(:,1))==-1,:)

---

Benchmarking

Benchmarking Code

N = 5000;
num_runs = 10000;

x = round(rand(N,N).*2)-1;
y = zeros(N,2);
y(:,1) = randi(size(x,1),N,1);
y(:,2) = randi(size(x,2),N,1);

disp('----------------- With sub2ind ')
tic
for k = 1:num_runs
    y1 = y(x(sub2ind(size(x), y(:,1), y(:,2)))==-1,:);
end
toc,clear y1

disp('----------- With raw version of sub2ind ')
tic
for k = 1:num_runs
    y2 = y(x((y(:,2)-1)*size(x,1)+y(:,1))==-1,:);
end
toc

Results

----------------- With sub2ind 
Elapsed time is 4.095730 seconds.
----------- With raw version of sub2ind 
Elapsed time is 2.405532 seconds.

Answer 3

This can be easily vectorized as follows (see sub2ind):

y = y(x(sub2ind(size(x), y(:,1), y(:,2)))==-1,:);