// Task 1 

List(7,2,3,4,5,6).find(_ < 4)  // => Some(3)

List(5,6,7,8,9).find(_ < 4)    // => None

List(5,6,7,8,9).min            // => 5

List(5,6,7,8,9).minOption      // => Some(5)

List[Int]().minOption          // => None 

// Task 2

Try(Some(List(5,6,7,8,9).min)).getOrElse(None)

Try(Some(List[Int]().min)).getOrElse(None)

// Task 3

import scala.util._

import io.Source

val my_url = "https://nms.kcl.ac.uk/christian.urban/"

// fails if there is no file with that name

Source.fromFile("test.txt")("ISO-8859-1").mkString

Source.fromFile("test.txt")("ISO-8859-1").getLines().toList

// encapsulates the failure case as None

Try(Some(Source.fromFile("test.txt")("ISO-8859-1").mkString)).getOrElse(None)

Try(Source.fromFile("test.txt")("ISO-8859-1").mkString).toOption // same but shorter

// for files with propper closing of the file after reading

Using(Source.fromFile("test.txt")("ISO-8859-1"))(_.mkString).toOption

// Task 4 (Higher-Order Functions)

List(7,2,3,4,5,6).find(_ < 4)

List(7,2,3,4,5,6).count(_ % 2 == 0)

List(7,2,3,4,5,6).sortWith(_ > _)

List(7,2,3,4,5,6).filter(_ > 4)

// Task 5 (Maps)

List(7,2,3,4,5,6).map(n => n * n)

for (n <- List(7,2,3,4,5,6)) yield n * n

// The advantages of for-comprehensions is that they

// can be nested and also can contain guards. In such

// cases the translations to maps and filters is a bit

// involved. 

// Task 6 (Pattern-Matching)

def my_map(lst: List[Int], f: Int => Int) : List[Int] = {

 if (lst == Nil) Nil

 else f(lst.head) :: my_map(lst.tail, f)

}

def my_map(lst: List[Int], f: Int => Int) : List[Int] = lst macth {

 case Nil => Nil

 case x::xs => f(x) :: my_map(xs, f)

}

// Task 7 (Web-Crawler, hard)

// see lecture2.scala

// requires an accumulator that records all pages that have 

// already been visited, for example

def crawl(url: String, n: Int, acc : Set[String] = Set()) : Unit = {

  if (n == 0) ()

  else {

    println(s"  Visiting: $n $url")

    val urls = get_all_URLs(get_page(url))

    for (u <- urls) crawl(u, n - 1, acc | urls)

  }

}