gistfile1.scala

 
  /**
   * The correlation between two vectors A, B is
   *   cov(A, B) / (stdDev(A) * stdDev(B))
   *
   * This is equivalent to
   *   [n * dotProduct(A, B) - sum(A) * sum(B)] / 
   *     sqrt{ [n * norm(A)^2 - sum(A)^2] [n * norm(B)^2 - sum(B)^2] }
   */
  def correlation(size : Double, dotProduct : Double, ratingSum : Double, 
    rating2Sum : Double, ratingNormSq : Double, rating2NormSq : Double) = {
      
    val numerator = size * dotProduct - ratingSum * rating2Sum
    val denominator = scala.math.sqrt(size * ratingNormSq - ratingSum * ratingSum) * scala.math.sqrt(size * rating2NormSq - rating2Sum * rating2Sum)
    
    numerator / denominator
  }