因为一直在搞算法开发，所以还是对算法比较感兴趣，学习Spark的过程，也顺带练习一下自己没用过的小算法。没有Java经验，没有Hadoop经验，没有SQL经验，对一些运行机制的理解还是不深，系统学习太枯燥，通过例程慢慢感悟吧！这次的参考书目是《Spark MLlib机器学习实践》，这本书也很好，没有太深奥的理论和繁琐的解释，有很多实用小程序，初学者上手很棒！

之前做相似度用的比较多的是欧式距离，余弦相似性也是相似度的一种度量，更适合稀疏数据，经常应用于协同过滤算法中。具体公式很简单，随便查一查好了。代码如下：

import org.apache.spark.SparkContext
import org.apache.spark.SparkContext._
import org.apache.spark.SparkConf
import scala.collection.mutable.Map

object collaborativeFilter {
  val conf = new SparkConf().setAppName("collaborativeFilter").setMaster("local")
  val sc = new SparkContext(conf)
  val users = sc.parallelize(Array("Rami Said Malek", "Olivia Colman", "Mahershala Ali", "Regina King", "Brian"))  // 用户名
  val films = sc.parallelize(Array("Green Book","Roma", "Bohemian Rhapsody", "The Favourite", "BlacKkKlansman"))  // 电影名
  val source = Map[String, Map[String, Int]]()  // 第一个String为用户名，嵌套的Map为电影名和打分值的存储
  val filmSource = Map[String, Int]()  // 电影名和分值的存储

  def getSource : Map[String, Map[String, Int]] = {
    val user1FilmSource = Map("Green Book" -> 2, "Roma" -> 3, "Bohemian Rhapsody" -> 1, "The Favourite" -> 0, "BlacKkKlansman" -> 1)
    val user2FilmSource = Map("Green Book" -> 1, "Roma" -> 2, "Bohemian Rhapsody" -> 2, "The Favourite" -> 1, "BlacKkKlansman" -> 4)
    val user3FilmSource = Map("Green Book" -> 2, "Roma" -> 1, "Bohemian Rhapsody" -> 0, "The Favourite" -> 1, "BlacKkKlansman" -> 4)
    val user4FilmSource = Map("Green Book" -> 3, "Roma" -> 2, "Bohemian Rhapsody" -> 0, "The Favourite" -> 5, "BlacKkKlansman" -> 3)
    val user5FilmSource = Map("Green Book" -> 5, "Roma" -> 3, "Bohemian Rhapsody" -> 1, "The Favourite" -> 1, "BlacKkKlansman" -> 2)
    source += ("Rami Said Malek" -> user1FilmSource)
    source += ("Olivia Colman" -> user2FilmSource)
    source += ("Mahershala Ali" -> user3FilmSource)
    source += ("Regina King" -> user4FilmSource)
    source += ("Brian" -> user5FilmSource)
    source
  }
  // 两两计算分值，采用余弦相似性
  def getCollaborateSource(user1: String, user2: String): Double = {
    val user1FlimSource = source.get(user1).get.values.toVector  // 获得第一个用户评分
    val user2FlimSource = source.get(user2).get.values.toVector  // 获得第二个用户评分
    // 对余弦相似度公式分子部分进行计算
    val member = user1FlimSource.zip(user2FlimSource).map(d => d._1 * d._2).reduce(_ + _).toDouble
    // 对余弦相似度公式分母部分进行计算
    val temp1 = math.sqrt(user1FlimSource.map(num => {
      math.pow(num, 2)
    }).reduce(_ + _))
    val temp2 = math.sqrt(user2FlimSource.map(num => {
      math.pow(num, 2)
    }).reduce(_ + _))
    val denominator = temp1 * temp2
    member / denominator
  }

  def main(args: Array[String]): Unit = {
    getSource
    val name = "Brian"
    users.foreach(user => {println(name + "相对于" + user + "的相似性分数是：" + getCollaborateSource(name, user))})
  }
}

输出结果：

Brian相对于Rami Said Malek的相似性分数是：0.8981462390204985
Brian相对于Olivia Colman的相似性分数是：0.6821910402406466
Brian相对于Mahershala Ali的相似性分数是：0.7416198487095662
Brian相对于Regina King的相似性分数是：0.738024966423108
Brian相对于Brian的相似性分数是：0.9999999999999998