《快学Scala》的笔记-第190页

  // 习题1
  def indexes(str: String) = {
    str.zipWithIndex.groupBy(_._1).map { tuple =>
      tuple._1 -> tuple._2.map(_._2).toSet
    }
  }

  // 要求mutable map和mutable set以及排序集
  import scala.collection.mutable
  def indexesMutable(str: String) = {
    var res = mutable.HashMap[Char, mutable.LinkedHashSet[Int]]()
    for ((c, i) <- str.zipWithIndex) {
      val set = res.getOrElseUpdate(c, mutable.LinkedHashSet())
      set += i
    }

    res
  }

  // 习题2
  def indexes2(str: String) = {
    str.zipWithIndex.groupBy(_._1).map { tuple =>
      tuple._1 -> tuple._2.map(_._2).toList
    }
  }

  // 习题3
  def filterZero(l: List[Int]) = l.filter(_ != 0)

  // 习题4
  def getMapValue(a: Array[String], m: Map[String, Int]) = a.map(m.get(_)).flatMap(x => x)

  // 习题5
  def mkString[T](s: Seq[T], sep: String =",") = s.map(_.toString).reduceLeft(_.toString + sep + _.toString)

  // 习题6
  // (lst :\ List[Int]())(_::_) 得到和lst一样的整数List
  // (List[Int]() /: lst)(_:+_) 得到和lst一样的整数List
  // 要反序排列，可以
  // (lst :\ List[Int]())((elem, coll) => coll :+ elem) 或者
  // (List[Int]() /: lst)((coll, elem) => elem +: coll)

  // 习题7
  val prices = List(5.0, 20.0, 9.95)
  val quantities = List(10, 2, 1)
  prices.zip(quantities).map(Function.tupled(_*_))

  // 习题8
  def mkTwoDimArray(a: Array[Double], cols: Int) = a.grouped(cols).toArray

  // 并发相关部分没看
  // 习题9
  // 习题10