How to count the number of times a pair of values occurs in an N by 2 matrix?

I have 2 vectors A and B consisting of 1 and 2 and I'd like to count the number of times each pair (1,1), (1,2), (2,1) and (2,2) occurs.

I tried the following A = [ 1 2 2 1 1 ]'; B = [ 2 2 1 2 1 ]'; C = [A , B]; x = [1 1; 1 2; 2 1; 2 2]; count = zeros(4,1); for k = 1:4 count(k) = sum(C==x(k,:)); end

It returns an error saying dimensions don't match. The code works if C is one-dimensional, for example: C = [ 1 2 2 1 1 ]'; x = [1 ; 2]; count = zeros(2,1); for k = 1:2 count(k) = sum(C==x(k)); end

Why does it not work if C is now 2 columns and I'm asking if a particular row is equal to a row of x?

Can you suggest a fix? Thank you.

[wy6622]

UPDATE:

This works c = accumarray([A(:), B(:)],1);

 c(1,1);
 c(1,2);
 c(2,1);
 C(2,2);

I'm guessing the == only works for scalars?

Answer 1

[Star Strider]

See if this does what you want:

A = [ 1 2 2 1 1 ]';
B = [ 2 2 1 2 1 ]';
C = [A , B];

[Cu,~,ic] = unique(C, 'rows');
count_mtx = accumarray(ic,1);

fprintf(1, '\t Pair\t  Count\n')
fprintf(1, '\t%d\t%d\t\t%d\n', [Cu  count_mtx]')

      Pair                Count
    1  1    1
    1  2    2
    2  1    1
    2  2    1

[wy6622]

Thank you!

[Star Strider]

My pleasure!