Scala List foldLeft用法

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def sum(list: List[Int]): Int = list.foldLeft(0)((r,c) => r+c)def sum(list: List[Int]): Int = list.foldLeft(0)(_+_)

def product(list: List[Int]): Int = list.foldLeft(1)(_*_)

def count(list: List[Any]): Int =  list.foldLeft(0)((sum,_) => sum + 1)

def average(list: List[Double]): Double =  list.foldLeft(0.0)(_+_) / list.foldLeft(0.0)((r,c) => r+1)def average(list: List[Double]): Double = list match {  case head :: tail => tail.foldLeft( (head,1.0) )((r,c) =>    ((r._1 + (c/r._2)) * r._2 / (r._2+1), r._2+1) )._1  case Nil => NaN}

def last[A](list: List[A]): A =  list.foldLeft[A](list.head)((_, c) => c)

def get[A](list: List[A], idx: Int): A =  list.tail.foldLeft((list.head,0)) {    (r,c) => if (r._2 == idx) r else (c,r._2+1)  } match {    case (result, index) if (idx == index) => result    case _ => throw new Exception("Bad index")}

def mimicToString[A](list: List[A]): String = list match {  case head :: tail => tail.foldLeft("List(" + head)(_ + ", " + _) + ")"  case Nil => "List()"}

def reverse[A](list: List[A]): List[A] =  list.foldLeft(List[A]())((r,c) => c :: r)

def unique[A](list: List[A]): List[A] =  list.foldLeft(List[A]()) { (r,c) =>    if (r.contains(c)) r else c :: r  }.foldLeft(List[A]())((r,c) => c :: r)

def group[A](list: List[A], size: Int): List[List[A]] =  list.foldLeft( (List[List[A]](),0) ) { (r,c) => r match {    case (head :: tail, num) =>      if (num < size)  ( (c :: head) :: tail , num + 1 )      else             ( List(c) :: head :: tail , 1 )    case (Nil, num) => (List(List(c)), 1)    }  }._1.foldLeft(List[List[A]]())( (r,c) => c.reverse :: r)

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