Scala in Impatient 习题解答12 高阶函数L1

看到现在，直到这一章才讲到函数式的强项啊

1. def values(fun: (Int) => Int, low: Int, high: Int) = {
     low.to(high).map(x => x -> fun(x))
   }
   

2. 如何用reduceLeft得到数组中的最大元素。 只要知道reduceLeft是以参数1和参数2作为入参，结果最为下一次的参数1的特性，就好办了。

    Scala> (1 to 10).reduceLeft(math.max(_,_))
    res25: Int = 10
   

3. 用to和reduceLeft实现递归。此题与题2类似

    def factorial(n: Long) = {
      (1L to n).reduceLeft(_ * _)
    }
   

4. def factorial(n: Long) = {
     (1L to n).foldLeft(1L)(_ * _)
   }
   

5. def largest(fun: (Int) => Int, inputs:Seq[Int]) = inputs.map(fun(_)).max
   

6. def largest(fun: (Int) => Int, inputs:Seq[Int]) = {
       inputs.reduceLeft((x, y) => if (fun(x) > fun(y)) x else y)
   }
   

7. def adjustToPair(f: (Int,Int) => Int) = {
      x: Product2[Int, Int] => f(x._1,x._2)
   }
   

   scala> val pairs = (1 to 10) zip (11 to 20)
   pairs: scala.collection.immutable.IndexedSeq[(Int, Int)] = Vector((1,11),
   (2,12), (3,13), (4,14), (5,15), (6,16), (7,17), (8,18), (9,19), (10,20))
   
   //Tuple不接受两个两个参数运算
   scala> pairs.map(_ + _)
   <console>:9: error: wrong number of parameters; expected = 1
                 pairs.map(_ + _)
   //通过adjustToPair转换成功
   scala> pairs.map(adjustToPair(_ + _))
   res22: scala.collection.immutable.IndexedSeq[Int] = Vector(12, 14, 16, 18, 20,
   22, 24, 26, 28, 30)
   

8. scala> val a = Array(1,2,3)
   a: Array[Int] = Array(1, 2, 3)
   
   scala> val b = Array("j","jm","jmu")
   b: Array[java.lang.String] = Array(j, jm, jmu)
   
   scala> b.corresponds(a)(_.length == _)
   res23: Boolean = true
   

9. import scala.collection.mutable.ArraySeq
   
   trait MyArray[A] extends ArraySeq[A]{
     def corresponds[B] (that: Seq[B], p: (A, B) => Boolean): Boolean = {
       super.zip(that).map(x => if (p(x._1, x._2) != true) return false)
       true
     }
   }
   

   scala> val b1 = new ArraySeq[String](3) with MyArray[String]
   b1: scala.collection.mutable.ArraySeq[String] with MyArray[String] = (null,
   null, null)
   
   scala> b1(0) = "j"
   
   scala> b1(1) = "jm"
   
   scala> b1(2) = "jmu"
   
   scala> b1.corresponds(a,_.length == _)
   <console>:18: error: missing parameter type for expanded function ((x$1: String,
   x$2) => x$1.length.$eq$eq(x$2))
                 b1.corresponds(a,_.length == _)
   
   scala> b1.corresponds(a,_.length == (_:Int))
   res49: Boolean = true
   

使用非柯里化时，遇到如上所示类型无法判断出的错误。与柯里化相比，此时that和 p失去了联系。程序无法推断经过运算后的结果的类型。

1. 第一个参数需要换名调用，柯里化后更美观。只是block返回值如何处理。

   def unless(condition: => Boolean)(block: => Unit) {
     if (!condition) block
   }