Scala :foldLeft foldRight min max

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Scala的foldLeft和foldRight

1.foldLeft和foldRight

1.foldLeft

定义：

def foldLeft[B](z:B)(f:(B, A) => B): B={     var acc = z     var these = this     while (!these.isEmpty) {     acc = f(acc, these.head)     these = these.tail    }    acc}

z : the start value
f : 操作函数（累积器， these.head）

2.foldRight

把输入的数组逆序，再调用foldLeft

3.简写

def /:[B](z:B)(op:(B,A)=>B):B = foldLeft(z)(op)def :\[B](z:B)(op:(A,B)=>B):B = foldRight(z)(op)

即：

/:（这两个字符是个整体）是foldLeft的简写:\ (这两个字符也是个整体)是foldRight的简写

4.例子

# 简单的例子1List(1,2,3).foldLeft(10)((sum,i)=> sum+i)res1:Int = 16acc = 10acc = 10 + List.headList = List.tailacc = acc + List.head((1 to 5):\100((i,sum)=>sum-i)100-5-4-3-2-1=85

# 简单的例子2(0/:(1 to 100))(_+_)等价于 (1 to 100).foldLeft(0)(_+_)res2:Int = 5050

# 复杂的例子def times(chars:List[Char]):List[(Char,Int)]={def incr(pairs:List[(Char,Int)],C:Char):List[(Char,Int)]=    pairs match{        case Nil => List((C, 1))        case (C, n) :: ps =>(C, n+1)::ps        case p::ps => p ::incr(ps, C)    }    chars.foldLeft(List[(Char,Int)]())(incr)}times(List('a','b','a'))----> List(('a',2),('b',1))

2.min max

def minMax(a: Array[Int]) : (Int, Int) = {  if (a.isEmpty) throw new java.lang.UnsupportedOperationException("array is empty")  a.foldLeft((a(0), a(0)))  { case ((min, max), e) => (math.min(min, e), math.max(max, e))}}

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