Recursion

Exercise 1 sum

Write a function that takes a positive integer n and returns the sum of all the integers from 1 to n.

def sum(n: Int): Int = 
  if n <= 1 then n 
  else n + sum(n - 1)

// example usage
sum(5) // returns 15

Exercise 2 sumBetween

Write a function that takes two integers and returns the sum of all the integers between them, including the endpoints.

def sumBetween(x: Int, y: Int): Int = 
  if x == y then x 
  else x + sumBetween(x + 1, y)

// example usage
sumBetween(1, 5) // returns 15

Exercise 3 sumList

Write a function that takes a list of integers and returns the sum of all the integers in the list.

def sumList(list: List[Int]): Int = {
	if list.isEmpty then 0
	else list.head + sumList(list.tail)
}

// example usage
sumList(List(1, 2, 3, 4, 5)) // returns 15

Exercise 4 filter even

Write a function that takes a list of integers and returns a new list with all the even numbers.

def filterEven(list: List[Int]): List[Int] =
	if list.isEmpty then List()
	else if list.head % 2 == 0 then
		list.head :: filterEven(list.tail)
	else
		filterEven(list.tail)

// example usage
filterEven(List(1, 2, 3, 4, 5, 6)) // returns List(2, 4, 6)

Exercise 5 longest string

Write a function that takes a list of strings and returns the length of the longest string in the list.

def longestString(list: List[String]): Int =
  if list.isEmpty then 0
  else
    list.head.length.max(longestString(list.tail))

// example usage
longestString(List("apple", "banana", "orange", "pear")) // returns 6