computeCentroids.m

function centroids = computeCentroids(X, idx, K)
  %COMPUTECENTROIDS returns the new centroids by computing the means of the 
  %data points assigned to each centroid.
  %   centroids = COMPUTECENTROIDS(X, idx, K) returns the new centroids by 
  %   computing the means of the data points assigned to each centroid. It is
  %   given a dataset X where each row is a single data point, a vector
  %   idx of centroid assignments (i.e. each entry in range [1..K]) for each
  %   example, and K, the number of centroids. You should return a matrix
  %   centroids, where each row of centroids is the mean of the data points
  %   assigned to it.

  [m n] = size(X);
  centroids = zeros(K, n);
  frequency = zeros(K, 1);
  for i = 1:m
    frequency(idx(i))++;
    centroids(idx(i), :) += X(i, :);
  endfor
  mask = centroids == 0;
  frequency = maskZeroAsOne(frequency);
  centroids = centroids ./ frequency;
  
  function mat = maskZeroAsOne(mat)
    mask = mat == 0;
    mat += mask;
  endfunction
end