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

Here is my code (I am a newbie in functional programming) and I'm assuming whoever lands up under this question will be folks like me. The top answer, while great, is bit too much for newbies to take! So, here is my simple answer. Note that I was asked (as part of a Course) to do this using only head and tail.

/**
   * This method returns the largest element in a list of integers. If the
   * list `xs` is empty it throws a `java.util.NoSuchElementException`.
   *
   * @param xs A list of natural numbers
   * @return The largest element in `xs`
   * @throws java.util.NoSuchElementException if `xs` is an empty list
   */
    @throws(classOf[java.util.NoSuchElementException])
    def max(xs: List[Int]): Int = find_max(xs.head, xs.tail)

    def find_max(max: Int, xs: List[Int]): Int = if (xs.isEmpty) max else if (max >= xs.head) find_max(max, xs.tail) else find_max(xs.head, xs.tail)

Some tests:

test("max of a few numbers") {
    assert(max(List(3, 7, 2)) === 7)
    intercept[NoSuchElementException] {
      max(List())
    }
    assert(max(List(31,2,3,-31,1,2,-1,0,24,1,21,22)) === 31)
    assert(max(List(2,31,3,-31,1,2,-1,0,24,1,21,22)) === 31)
    assert(max(List(2,3,-31,1,2,-1,0,24,1,21,22,31)) === 31)
    assert(max(List(Int.MaxValue,2,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,222,3,-31,1,2,-1,0,24,1,21,22)) === Int.MaxValue)
  }