How would I use the 'count' function but to only count instances of both columns being specified?

If I have a matrix of 2 columns with independant data in them, how would I count the number of instances of a specific combination occuring. For examples if the matrix is 2 columns of integers from 1-10

x_y_= [1 4; 5 3;9 8; 2 7;1 8;7 2; 2 7;1 5;4 3]

how would I count the number of lines that read as 2,7 (but not 7,2) for example?

Answer 1

[Jan]

x = [1 4; 5 3;9 8; 2 7;1 8;7 2; 2 7;1 5;4 3];
n = sum(all(x == [2, 7], 2))  % Alternative: nnz() instead of sum()

n = 2

If the values are positive integers and smaller than 1000, this is slightly faster:

n = sum((x * [1000; 1]) == [2, 7] * [1000; 1])

n = 2

A speed comparison:

x = randi([1, 100], 1e6, 2);
r = 100;  % Repeat to reduce noise
tic; for k = 1:r, n = nnz(ismember(x,[2,7],'rows')); end, toc

Elapsed time is 15.483181 seconds.

tic; for k = 1:r, n = sum(all(x == [2, 7], 2)); end, toc

Elapsed time is 0.219182 seconds.

tic; for k = 1:r, n = sum((x * [1000;1]) == 2007); end, toc

Elapsed time is 0.159192 seconds.

tic; for k = 1:r, n = sum(x(:, 1)==2 & x(:, 2)==7); end, toc

Elapsed time is 0.263742 seconds.

tic; for k = 1:r  % And a dull not matlabish loop:
    n = 0; for i=1:size(x, 1), if x(i,1)==2 & x(i,2)==7, n=n+1; end, end
end, toc

Elapsed time is 0.218550 seconds.

Answer 2

[Torsten]

x_y_= [1 4; 5 3;9 8; 2 7;1 8;7 2; 2 7;1 5;4 3];
n = nnz(ismember(x_y_,[2,7],'rows'))

n = 2