How to find the largest element in a list of integers recursively?

Question

I\'m trying to write a function which will recursively find the largest element in a list of integers. I know how to do this in Java, but can\'t understand how to do this at Sca

Answer 1

You could use pattern matching like that

def max(xs: List[Int]): Int = xs match {
  case Nil => throw new NoSuchElementException("The list is empty")
  case x :: Nil => x
  case x :: tail => x.max(max(tail)) //x.max is Integer's class method
}

Answer 2

 def max(xs: List[Int]): Int = xs match {
    case Nil => throw new NoSuchElementException("empty list!")
    case x :: Nil => x
    case x :: tail => if (x > max(tail)) x else max(tail)
  }

Answer 3

If you want functional approach to this problem then use reduceLeft:

def max(xs: List[Int]) = {
  if (xs.isEmpty) throw new NoSuchElementException
  xs.reduceLeft((x, y) => if (x > y) x else y)
}

This function specific for list of ints, if you need more general approach then use Ordering typeclass:

def max[A](xs: List[A])(implicit cmp: Ordering[A]): A = {
  if (xs.isEmpty) throw new NoSuchElementException
  xs.reduceLeft((x, y) => if (cmp.gteq(x, y)) x else y)
}   

reduceLeft is a higher-order function, which takes a function of type (A, A) => A, it this case it takes two ints, compares them and returns the bigger one.

Answer 4

1. Folding can help:

   if(xs.isEmpty)
     throw new NoSuchElementException
   else
     (Int.MinValue /: xs)((max, value) => math.max(max, value))
   

2. List and pattern matching (updated, thanks to @x3ro)

   def max(xs:List[Int], defaultValue: =>Int):Int = {
     @tailrec
     def max0(xs:List[Int], maxSoFar:Int):Int = xs match {
       case Nil => maxSoFar
       case head::tail => max0(tail, math.max(maxSoFar, head))
     }
     if(xs.isEmpty)
       defaultValue
     else
       max0(xs, Int.MinValue)
   }
   

(This solution does not create Option instance every time. Also it is tail-recursive and will be as fast as an imperative solution.)