MATLAB: Count frequency of each unique occurence in an Nx2 matrix

2-dimensionalcountmatrixsum

I have a data set where each point is described by two unique numbers (call it a and b), and I need to count the number of times each combination of these two numbers occurs. For example if I had 6 points that were described by
mat = [1 1 ; 1 3 ; 2 3; 2 1; 1 2; 1 3]
where a is the first column and b is the second column, I want to know how many times a specific combo occurs. I know I can get the count of one number in a column doing something like this:
sum(mat(:,1)==1)
sum(mat(:,2)==3)
which would return 4 and 3, respectively, but I want to count how many times the combo (1, 3) occurs, which would be 2 for this list.

Best Answer

You could iteratively test each row for the combo you are looking for, adding 1 to the total each time a match is found, like so:
mat = [1 1; 1 3; 2 3; 2 1; 1 2; 1 3];
totalOccurences = 0;
for i = 1:length(mat(:,1))
    if mat(i,:)==[1 3]
        totalOccurences = totalOccurences+1;
    end
end
At the end of this code, totalOccurences = 2, which is the result you wanted to get in your example.
Related Question