快学scala学习练习题摘记

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Scala

1.提供操作符用于构造HTML表格，例如
Table() | "java" | "scala" || "Gosling" | "Ordersky" ||"jvm" | "dot Net"
应该产出　同

<table>
<tr><td>java</td><td>scala</td></tr>
<tr><td>Gosling</td><td>Ordersky</td></tr>
<tr><td>jvm</td><td>dot net</td></tr>
</table>

1. 一行代码完成一个字符串中字母出现的次数，如字符串"hello world"
2. 编写一个函数，给定字符串，产出一个包含所有字符下标的映射，
   例如　indexes("Mississippi")应返回一个映射,M 对应{0} i对应　{1,4,7,10}

//Table() | "java" | "scala" || "Gosling" | "Ordersky" ||"jvm" | "dot Net"
class Table {
  private var sb = new StringBuilder("<table><tr>")
  final val TR_END = "</tr>"
  final val TR_START = "<tr>"
  final val TB_END = "</table>"
  def |(col: String) = {
    sb.append("<td>")
    sb.append(col)
    sb.append("</td>")
    this
  }
  def ||(row: String) = {
    sb.append(TR_END)
    sb.append(TR_START)
    |(row)
  }
  def print = println(sb.toString() + TR_END + TB_END)
}
object Table extends App {
  def apply() = new Table()
  // Table() | "java" | "scala" || "Gosling" | "Ordersky" || "jvm" | "dot Net" print
  //一行代码完成一个字符串中字母出现的次数
  val str = "hello world"
  println(str.aggregate(Set[Char]())(_ + _, _ ++ _))
  println(str.foldLeft(Map[Char, Int]())((m, c) => m + (c -> (m.getOrElse(c, 0) + 1))))
  def numsFrom(n: Int): Stream[Int] = n #:: numsFrom(n + 1)
  val tenMore = numsFrom(10)
  tenMore.take(5).foreach(println)
  import math._
  val powers = (0 until 1000).view.map(pow(10, _))
  println(powers(3))
  def indexes(str: String) = {
    str.zipWithIndex.foldLeft(Map[Char, List[Int]]()) {
      (m, ci) =>
        m + (ci._1 -> (m.getOrElse(ci._1, List[Int]()):::List(ci._2)))
    }
  } 
  println(indexes("Mississippi"))
}