csci5521/assignments/hwk02/Classify.m

% implements Classify, return the predicted class for each row (we'll call each row x) in data
% by computing the posterior probability that x is in class 1 vs. class 2 then 
% these posterior probabilities are compared using the log odds.
function [predictions] = Classify(data, m1, m2, S1, S2, pc1, pc2)

  [num_rows, d] = size(data);
  % calculate P(x|C) * P(C) for both classes
    
  % pxC1 = 1/(power(2*pi, d/2) * power(det(S1), 1/2)) * exp(-1/2 * (data-m1) * inv(S1) * (data-m1)');
  % pxC2 = 1/(power(2*pi, d/2) * power(det(S2), 1/2)) * exp(-1/2 * (data-m2) * inv(S2) * (data-m2)');
  
  pxC1 = zeros(num_rows,1);
  pxC2 = zeros(num_rows,1);
  for i = 1:num_rows
    x = data(i,:);
    pxC1(i) = 1/(power(2*pi, d/2) * power(det(S1), 1/2)) * exp(-1/2 * (x-m1) * inv(S1) * (x-m1)');
    pxC2(i) = 1/(power(2*pi, d/2) * power(det(S2), 1/2)) * exp(-1/2 * (x-m2) * inv(S2) * (x-m2)');
  end

  % pxC1 = mvnpdf(data, m1, S1);
  % pxC2 = mvnpdf(data, m2, S2);

  % P(C|x) = (P(x|C) * P(C)) / common factor
  pC1x = pxC1 * pc1;
  pC2x = pxC2 * pc2;

  % calculate log odds, if > 0 then data(i) belongs to class c1, else, c2
  log_odds = log(pC1x) - log(pC2x);
  
  % get predictions from log odds calculation
  predictions = zeros(num_rows,1);
  for i = 1:num_rows
    if log_odds(i) > 0
      predictions(i) = 1;
    else
      predictions(i) = 2;
    end
  end
  
end % Function end