Trying to extract the rows from a matrix where the values from the first two columns match the values from another matrix

Hi, I am trying to extract the rows from a matrix where the values from the first two columns of that matrix match the values from another matrix.

Basically, if the values in column 1 AND column 2 match the values for column 1 AND column 2 from another matrix, I want to extract the entire row. I want to do this for each row where the values match, essentially making a new matrix of the extracted rows.

Any help would be appreciated! Thanks.

Answer 1

[Dyuman Joshi]

%Sample data for example
y1 = magic(5)

y1 = 5×5

17 24 1 8 15 23 5 7 14 16 4 6 13 20 22 10 12 19 21 3 11 18 25 2 9

y2 = y1;
y2(randi(25,1,5)) = 0

y2 = 5×5

17 0 1 8 15 0 5 7 14 16 0 6 13 20 22 10 12 19 21 3 11 18 25 0 0

%Comparison
idx = all(y1(:,1:2)==y2(:,1:2), 2)

idx = 5×1 logical array
   0
   0
   0
   1
   1

out = y1(idx, :)

out = 2×5

10 12 19 21 3 11 18 25 2 9

If you are working with floating point numbers, use a tolerance to compare instead of equality

tol = 1e-6;
idx = all(abs(y1(:,1:2)-y2(:,1:2))<tol,2)

[Michael Bochert]

Ah I forgot to mention, the matrices are not the same size (they have the same number of columns, but not the same number of rows). When using the above code, it says that the arrays have incompatible sizes for this operation. Any advice on how to get around this? Thanks!

[Dyuman Joshi]

In that case

%Sample data for example
y1 = magic(5)

y1 = 5×5

17 24 1 8 15 23 5 7 14 16 4 6 13 20 22 10 12 19 21 3 11 18 25 2 9

y2 = [y1; randi(25, 2, 5)];
k = randi(numel(y2),1,14); 
y2(k) = k*5

y2 = 7×5

17 24 75 8 15 10 5 80 14 16 4 50 13 20 22 10 55 19 125 3 11 18 95 2 9 21 22 100 15 170 15 70 14 140 175

%Comparison
[idx1,idx2] = ismember(y1(:,1:2),y2(:,1:2),'rows')

idx1 = 5×1 logical array
   1
   0
   0
   0
   1

idx2 = 5×1

1 0 0 0 5

out1 = y1(idx1, :)

out1 = 2×5

17 24 1 8 15 11 18 25 2 9

out2 = y2(idx2(idx1),:)

out2 = 2×5

17 24 75 8 15 11 18 95 2 9

For floating point numbers use ismembertol.

[Michael Bochert]

Worked perfectly. Thank you so much!!