CSC 347 - Concepts of Programming Languages

Iteration: Loops and Recursion

Instructor: Stefan Mitsch

Learning Objectives

How to implement iteration?

  • Understand loops vs. recursion

Loops by Recursion

  • Loop forever printing hello (press Control-C to quit)
    while true do println("Hello")
  • Recursive implementation
    def loop() = println("Hello"); loop()
  • With a loop counter
    def loop(int n) = println(s"Hello $n"); loop(n+1)

Translating Loop to Recursion

Loop (mutable data)

def factorial (n:Int) : Int =
  val result = 1
  var m = n
  while m > 1 do
    result = result * m
    m = m - 1
  result

  • Recursive (mutable)

    def factorial (n:Int) : Int =
  var result = 1
  var m = n
  def loop () : Unit =
    if m > 1 then 
      result = result*m
      m = m-1
      loop()
  loop()
  result

  • Recursive (mutable)

    def factorial (n:Int) : Int =
  var result = 1
  def loop (m:Int) : Unit =
    if m > 1 then 
      result = result*m
      loop(m-1)
  loop(n)
  result

  • Recursive (immutable)

    def factorial (n:Int) : Int =
  def loop (m:Int) : Int =
    if m > 1 then m * loop(m-1)
    else 1
  loop(n)

  • Tail-recursive

    def factorial (n:Int) : Int =
  def loop (m:Int, result:Int) : Int =
    if m > 1 then loop(m-1, m*result)
    else result
  loop(n,1)

Summary

  • Iteration with loops often uses mutable data
  • Iteration with recursion often uses immutable data
  • Can translate between loops and recursion