rand_index.m

function ri = rand_index(p1, p2)
%RAND_INDEX Computes the rand index between two partitions.
%   RAND_INDEX(p1, p2) computes the rand index between partitions p1 and
%   p2.
%   
%   RAND_INDEX(p1, p2, 'adjusted'); computes the adjusted rand index
%   between partitions p1 and p2. The adjustment accounts for chance
%   correlation.

    % Parse the input and throw errors
    adj = 0;
    if nargin == 0
    end
    if nargin > 3
        error('Too many input arguments');
    end
    adj = 1;
    if length(p1)~=length(p2)
        error('Both partitions must contain the same number of points.');
    end
    
	% Preliminary computations and cleansing of the partitions
    N = length(p1);
    [~, ~, p1] = unique(p1);
    N1 = max(p1);
    [~, ~, p2] = unique(p2);
    N2 = max(p2);
    
    % Create the matching matrix
    for i=1:1:N1
        for j=1:1:N2
            G1 = find(p1==i);
            G2 = find(p2==j);
            n(i,j) = length(intersect(G1,G2));
        end
    end
    
    % If required, calculate the basic rand index
    if adj==0
        ss = sum(sum(n.^2));
        ss1 = sum(sum(n,1).^2);
        ss2 =sum(sum(n,2).^2);
        ri = (nchoosek2(N,2) + ss - 0.5*ss1 - 0.5*ss2)/nchoosek2(N,2);
    end
    
    
    % Otherwise, calculate the adjusted rand index
    if adj==1
        ssm = 0;
        sm1 = 0;
        sm2 = 0;
        for i=1:1:N1
            for j=1:1:N2
                ssm = ssm + nchoosek2(n(i,j),2);
            end
        end
        temp = sum(n,2);
        for i=1:1:N1
            sm1 = sm1 + nchoosek2(temp(i),2);
        end
        temp = sum(n,1);
        for i=1:1:N2
            sm2 = sm2 + nchoosek2(temp(i),2);
        end
        NN = ssm - sm1*sm2/nchoosek2(N,2);
        DD = (sm1 + sm2)/2 - sm1*sm2/nchoosek2(N,2);
        ri = NN/DD;
    end 
    

    % Special definition of n choose k
    function c = nchoosek2(a,b)
        if a>1
            c = nchoosek(a,b);
        else
            c = 0;
        end
    end
end