fold()操作和reduce()操作的区别

reduce()——规约操作，包含reduceLeft()和reduceRight()两种操作。

fold()——折叠操作，包含foldLeft()和foldRight()两种操作。

两者功能相似，不同之处在于：

fold()操作需要从一个初始值开始，并以该值作为上下文，处理集合中的每个元素。

reduce()操作举例：

1.  
   scala> val list = List(1,2,3,4,5)
2.  
   list: List[Int] = List(1, 2, 3, 4, 5)
3.  
    
4.  
   scala> list.reduceLeft((x:Int,y:Int)=>{println(x,y);x+y})
5.  
   (1,2)
6.  
   (3,3)
7.  
   (6,4)
8.  
   (10,5)
9.  
   res0: Int = 15
10.  
     
11.  
    scala> list.reduceRight((x:Int,y:Int)=>{println(x,y);x+y})
12.  
    (4,5)
13.  
    (3,9)
14.  
    (2,12)
15.  
    (1,14)
16.  
    res1: Int = 15

1.  
   scala> val list = List(1,2,3,4,5)
2.  
   list: List[Int] = List(1, 2, 3, 4, 5)
3.  
    
4.  
   scala> list.reduceLeft((x:Int,y:Int)=>{println(x,y);x-y})
5.  
   (1,2)
6.  
   (-1,3)
7.  
   (-4,4)
8.  
   (-8,5)
9.  
   res2: Int = -13
10.  
     
11.  
    scala> list.reduceRight((x:Int,y:Int)=>{println(x,y);x-y})
12.  
    (4,5)
13.  
    (3,-1)
14.  
    (2,4)
15.  
    (1,-2)
16.  
    res3: Int = 3

fold()操作举例：

1.  
   scala> val list = List(1,2,4,3,5)
2.  
   list: List[Int] = List(1, 2, 4, 3, 5)
3.  
    
4.  
   scala> list.fold(10)(_*_)
5.  
   res0: Int = 1200

1.  
   scala> list.foldLeft(0)((x:Int,y:Int)=>{println(x,y);x+y})
2.  
   (0,1)
3.  
   (1,2)
4.  
   (3,4)
5.  
   (7,3)
6.  
   (10,5)
7.  
   res1: Int = 15
8.  
    
9.  
   scala> list.foldRight(0)((x:Int,y:Int)=>{println(x,y);x+y})
10.  
    (5,0)
11.  
    (3,5)
12.  
    (4,8)
13.  
    (2,12)
14.  
    (1,14)
15.  
    res2: Int = 3

上面fold(10)(_*_)，fold(0)中，10和0就是进行fold()操作所附的初值。