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Appendix G: Chapter 7 Exercise & Challenge Solutions Written by Massimo Carli

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Exercise 7.1

Implement the extension function isEmpty(), which returns true if FList<T> is empty and false otherwise.

Exercise 7.1 solution

You can solve this exercise a few different ways. The first uses what you implemented in the chapter and the size function.

fun <T> FList<T>.isEmpty(): Boolean = size() == 0
fun <T> FList<T>.isEmpty(): Boolean = match(
  whenNil = { true },
  whenCons = { _, _ -> false }
)
fun main() {
  println(FList.empty<Int>().isEmpty())
  println(FList.of(1).isEmpty())
  println(FList.of(1, 2, 3).isEmpty())
}
true
false
false

Exercise 7.2

Implement the extension function tail(), which returns the tail of a given FList<T>.

Exercise 7.2 solution

You can easily implement the tail function like this:

fun <T> FList<T>.tail(): FList<T> = match(
  whenNil = { FList.empty() },
  whenCons = { _, tail -> tail }
)
fun main() {
  println(FList.empty<Int>().tail())
  println(FList.of(1).tail())
  println(FList.of(1, 2, 3).tail())
}
com.raywenderlich.fp.Nil@27c170f0
com.raywenderlich.fp.Nil@27c170f0
FCons(head=2, tail=FCons(head=3, tail=com.raywenderlich.fp.Nil@27c170f0))

Exercise 7.3

Kotlin provides forEachIndexed for the Iterable<T> interface, which accepts as input a lambda of type (Int, T) -> Unit. The first Int parameter is the index of the item T in the collection. To test forEachIndexed, run the code:

listOf("a", "b", "c").forEachIndexed { index, item ->
  println("$index $item")
}
 0 a
 1 b
 2 c

Exercise 7.3 solution

The solution, in this case, is a little more complicated because you need to keep track of the index for the current element. The following implementation is a possible option:

fun <T> FList<T>.forEachIndexed(fn: (Int, T) -> Unit) { // 1

  fun FList<T>.loop(i: Int = 0): Unit = match( // 2
    whenNil = {}, // 3
    whenCons = { head, tail ->
      fn(i, head) // 4
      tail.loop(i + 1) // 5
    }
  )

  loop() // 6
}
fun main() {
  FList.of("a", "b", "c").forEachIndexed { index, item ->
    println("$index $item")
  }
}
0 a
1 b
2 c

Exercise 7.4

Another option to implement forEachIndexed is to make FList<T> an Iterable<T>. How would you do that? To make all the code coexist in the same codebase, call the Iterable<T> version IFList<T> with INil and ICons<T>.

Exercise 7.4 solution

As mentioned in the problem statement, start the exercise with the existing FList<T> definition in FList.kt and rename it in IFList<T> along with INil and ICons<T>. A possible solution could be:

sealed class IFList<out T> : Iterable<T> { // 1

  companion object {
    @JvmStatic
    fun <T> of(vararg items: T): IFList<T> {
      val tail = items.sliceArray(1 until items.size)
      return if (items.isEmpty()) {
        empty()
      } else {
        ICons(items[0], of(*tail))
      }
    }

    @JvmStatic
    fun <T> empty(): IFList<T> = INil
  }
}

private object INil : IFList<Nothing>() { // 2
  override fun iterator(): Iterator<Nothing> =
    object : Iterator<Nothing> {
      override fun hasNext(): Boolean = false
      override fun next(): Nothing =
        throw NoSuchElementException()
    }
}

private data class ICons<T>(
  val head: T,
  val tail: IFList<T> = INil
) : IFList<T>() {

  override fun iterator(): Iterator<T> =
    object : Iterator<T> { // 3
      var current: IFList<T> = this@ICons // 4
      override fun hasNext(): Boolean = current is ICons<T> // 5

      override fun next(): T {
        val asICons = current as? ICons<T> ?:
          throw NoSuchElementException() // 6
        current = asICons.tail // 7
        return asICons.head // 8
      }
    }
}
fun main() {
  IFList.of(1, 2, 3).forEach {
    println(it)
  }
}
1
2
3

Exercise 7.5

Implement addHead, which adds a new element at the head of an existing FList<T>.

Exercise 7.5 solution

The addHead function is very simple:

fun <T> FList<T>.addHead(newItem: T): FList<T> =
  FCons(newItem, this)
fun main() {
  val initialList = FList.of(1, 2)
  val addedList = initialList.addHead(0)
  initialList.forEach {
    print("$it ")
  }
  println()
  addedList.forEach {
    print("$it ")
  }
}
1 2
0 1 2

Exercise 7.6

Kotlin defines the take function on Iterable<T> that allows you to keep a given number of elements.

 fun main() {
   listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
     .take(3)
     .forEach { print("$it ") }
1 2 3

Exercise 7.6 solution

A possible implementation of the take function for FList<T> is the following:

fun <T> FList<T>.take(n: Int): FList<T> = match( // 1
  whenNil = { FList.empty() }, // 2
  whenCons = { head, tail ->
    if (n > 0) {
      FCons(head, tail.take(n - 1)) // 3
    } else {
      FList.empty() // 4
    }
  }
)
fun main() {
  FList.of(1, 2, 3, 4, 5)
    .take(0) // 1
    .forEach { print("$it ") }
  println()
  FList.of(1, 2, 3, 4, 5)
    .take(1) // 2
    .forEach { print("$it ") }
  println()
  FList.of(1, 2, 3, 4, 5)
    .take(5) // 3
    .forEach { print("$it ") }
  println()
  FList.of(1, 2, 3, 4, 5)
    .take(6) // 4
    .forEach { print("$it ") }
}
             // 1
1            // 2
1 2 3 4 5    // 3
1 2 3 4 5    // 4

Exercise 7.7

Kotlin defines the takeLast function on Iterable<T> that allows you to keep a given number of elements at the end of the collection. For instance, running the following code:

 fun main() {
   listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
     .takeLast(3)
     .forEach { print("$it ") }
8 9 10

Exercise 7.7 solution

This exercise is a little more complicated than the previous one. A first implementation could be the following:

fun <T> FList<T>.takeLast(n: Int): FList<T> = match( // 1
  whenNil = { FList.empty() }, // 2
  whenCons = { head, tail ->
    if (tail.size() >= n) {
      tail.takeLast(n) // 3
    } else {
      FCons(head, tail) // 4
    }
  }
)
fun main() {
  (0..6).forEach {
    println("takeLast $it")
    FList.of(1, 2, 3, 4, 5)
      .takeLast(it)
      .forEach { print("$it ") }
    println()
  }
}
takeLast 0

takeLast 1
5
takeLast 2
4 5
takeLast 3
3 4 5
takeLast 4
2 3 4 5
takeLast 5
1 2 3 4 5
takeLast 6
1 2 3 4 5
fun <T> FList<T>.skip(n: Int): FList<T> = match( // 1
  whenNil = { FList.empty() }, // 2
  whenCons = { head, tail ->
    if (n > 0) {
      tail.skip(n - 1) // 3
    } else {
      FCons(head, tail) // 4
    }
  }
)
fun main() {
  // ...
  (0..6).forEach {
    println("Skipping $it")
    FList.of(1, 2, 3, 4, 5)
      .skip(it)
      .forEach { print("$it ") }
    println()
  }
}
Skipping 0
1 2 3 4 5
Skipping 1
2 3 4 5
Skipping 2
3 4 5
Skipping 3
4 5
Skipping 4
5
Skipping 5

Skipping 6

fun <T> FList<T>.takeLast2(n: Int): FList<T> = // 1
  skip(size() - n) // 2
fun main() {
  (0..6).forEach {
    println("takeLast2 $it")
    FList.of(1, 2, 3, 4, 5)
      .takeLast2(it)
      .forEach { print("$it ") }
    println()
  }
}
takeLast2 0

takeLast2 1
5
takeLast2 2
4 5
takeLast2 3
3 4 5
takeLast2 4
2 3 4 5
takeLast2 5
1 2 3 4 5
takeLast2 6
1 2 3 4 5

Challenge 7.1

Kotlin provides the functions first and last as extension functions of List<T>, providing, if available, the first and last elements. Can you implement the same for FList<T>?

Challenge 7.1 solution

The implementation of first for FList<T> is simple because it’s exactly the same as head, which you implemented in the chapter.

fun <T> FList<T>.first() = head()
fun <T> FList<T>.last() = skip(size() - 1).head()
fun main() {
  println(FList.empty<Int>().first())
  println(FList.empty<Int>().last())
  println(FList.of(1).first())
  println(FList.of(1).last())
  println(FList.of(1, 2).first())
  println(FList.of(1, 2).last())
}
null
null
1
1
1
2

Challenge 7.2

Kotlin provides an overload of first for Iterable<T> that provides the first element that evaluates a given Predicate<T> as true. It also provides an overload of last for List<T> that provides the last element that evaluates a given Predicate<T> as true. Can you implement firstWhen and lastWhen for FList<T> with the same behavior?

Challenge 7.2 solution

A very simple first implementation for firstWhen is:

fun <T> FList<T>.firstWhen(predicate: Predicate<T>): T? =
  filter(predicate).first()
fun <T> FList<T>.fastFirstWhen(predicate: Predicate<T>): T? = match(
  whenNil = { null },
  whenCons = { head, tail ->
    if (predicate(head)) {
      head
    } else {
      tail.fastFirstWhen(predicate)
    }
  }
)
fun <T> FList<T>.lastWhen(predicate: Predicate<T>): T? =
  filter(predicate).last()
fun main() {
  val isEven: Predicate<Int> = { a: Int -> a % 2 == 0 }
  println(FList.of(1, 2, 3, 4, 5, 6).firstWhen(isEven))
  println(FList.of(1, 2, 3, 4, 5, 6).lastWhen(isEven))
  println(FList.of(1, 2, 3, 4, 5, 6).fastFirstWhen(isEven))
}
2
6
2

Challenge 7.3

Implement the function get that returns the element at a given position i in FList<T>. For instance, with this code:

fun main() {
  println(FList.of(1,2,3,4,5).get(2))
}
3

Challenge 7.3 solution

Creating a set of reusable functions is a very powerful tool and allows you to implement a complete library. A possible solution for the get function is the following:

fun <T> FList<T>.get(i: Int): T =
  skip(i).head() ?: throw ArrayIndexOutOfBoundsException()
fun main() {
  val list = FList.of(1, 2, 3)
  println(list.get(0))
  println(list.get(1))
  println(list.get(2))
  println(list.get(3))
}
1
2
3
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException
operator fun <T> FList<T>.get(i: Int): T =
  skip(i).head() ?: throw ArrayIndexOutOfBoundsException()
fun main() {
  val list = FList.of(1, 2, 3)
  println(list[0])
  println(list[1])
  println(list[2])
  println(list[3])
}

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