Compute centroid means (vectorized)

ex7/computeCentroids.m

@@ -27,17 +27,9 @@ centroids = zeros(K, n);
 %
 
 for k = 1:K
-  count = 0;
-
-  % XXX vectorize
-  for i = 1:m
-    if idx(i) == k
-      centroids(k, :) += X(i, :);
-      count += 1;
-    end
-  end
-  if count > 0
-    centroids(k, :) /= count;
+  X_in_k = X(idx == k, :);
+  if size(X_in_k, 1) > 0
+    centroids(k, :) = sum(X_in_k)/size(X_in_k, 1);
   end
 end