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

The easiest approach would be to use max function of TraversableOnce trait, as follows,

val list = (1 to 10).toList
list.max

to guard against the emptiness you can do something like this,

if(list.empty) None else Some(list.max)

Above will give you an Option[Int]

My second approach would be using foldLeft

(list foldLeft None)((o, i) => o.fold(Some(i))(j => Some(Math.max(i, j))))

or if you know a default value to be returned in case of empty list, this will become more simpler.

val default = 0
(list foldLeft default)(Math.max)

Anyway since your requirement is to do it in recursive manner, I propose following,

def recur(list:List[Int], i:Option[Int] = None):Option[Int] = list match {
  case Nil => i
  case x :: xs => recur(xs, i.fold(Some(x))(j => Some(Math.max(j, x))))
}

or as default case,

val default = 0
def recur(list:List[Int], i:Int = default):Int = list match {
  case Nil => i
  case x :: xs => recur(xs, i.fold(x)(j => Math.max(j, x)))
}

Note that, this is tail recursive. Therefore stack is also saved.