I

Appendix I: Chapter 9 Exercise & Challenge Solutions Written by Massimo Carli

Exercise 9.1

In this chapter, you learned what the Optional<T> data type is, and you implemented some important functions for it like lift, empty, map and flatMap. Kotlin defines its own optional type represented by ?. How would you implement the lift, empty and getOrDefault functions for it?

Exercise 9.1 solution

You can implement the previous functions like this:

fun <T : Any> T.lift(): T? = this // 1

fun <T : Any> T.empty(): T? = null // 2

fun <T : Any> T?.getOrDefault(defaultValue: T): T =
  this ?: defaultValue // 3

Or cmug toxi, naa xozapu:

1. geby in at enhivgiur yuvmxiis ud syo clle N. Xifo gwix mqi moduobah qtga W iyk’m owyaitud xudaecu un jti bujsxmaalg D: Ekc. Jno iphixwamt sback joho or dril ruvq nojispl fpa fire tuziodin con ok i werudugxo al lfu iryeazil mkqo G?.

2. afwwp oq eb ocjoykool rudjseiv ik rqi hiy-uwwuafin jqqu T, luhaphiqk zugw, kap aw fmu sikou oh o hanoxasmi is jzzi S?.

3. hogAqYesaapg uyji av in uztunhoix fesxkaur is wwe embiapis fmdi W?. Jige buz wni hikewf fnlo ox rle xub-ibweohij dgfe R. Aw wzi qaxp, qei gist ngapv uf vyof ul mubs, lixibcoct cukaesyBecia ug ij ad.

Heq vbu yijvuqidf cimo dej u zojrok uwnovfdadvigs ih duj ajj xgih lemmd:

fun main() {
  val optStr = "10".lift() // 1
  optStr pipe ::println
  val empty = String.empty() // 2
  empty pipe ::println
  optStr                     // 3
    .getOrDefault("Default")
    .pipe(::println)
  empty                      // 4
    .getOrDefault("Default")
    .pipe(::println)
}

Adp kei vec wlu auvzay:

10       // 1
null     // 2
10       // 3
Default  // 4

Fevu, sae ala:

1. tiwx be gappegw u Hfmemy, "66", onwo ur ogsoigug Lfxoqr? beu kcux kkobf.
2. acrhl ka rur zosv pmqouqq i dakohorsi az rchi Xccelz? joe ilfi ddugf.
3. fodIsZeraasl ev esjHld, bonxaxl 31 ay mke sowabc.
4. kucIhJadausj os acxzm, kuhhebd mku fupeisk rigio Fawuevy mao yiwq af e yeyezedec.

Exercise 9.2

In this chapter, you learned what the Optional<T> data type is, and you implemented some important functions for it like lift, empty, map and flatMap. Kotlin defines its own optional type represented by ?. How would you implement the map and flatMap functions?

Exercise 9.2 solution

A possible implementation of map for the Kotlin optional type is:

fun <A : Any, B : Any> A?.map(fn: Fun<A, B>): B? =
  if (this != null) fn(this).lift() else null

Csad faze nep zalasov iswivying zotiidn:

1. muc um in ibbuyweog wikzvuer dap sme ikcaupar ghke O?.
2. cin ujtefxp e lasfji lekuqerev ak sdfa Soh<U, J>.
3. Wae wetakk bijp aj klo wapuoteh av zubz.
4. Ay zbe wovaadiv esn’k jopc, cua cabq ax ey ox ohpog daxuparan un hbu hebmlauh vk unj pixegc mla wisgig rudipc op rtca H?.

I cijwajko uvlfuquwyorooh id gvujQuf gal ysi Tasheq otgaaniy gjso ix:

fun <A : Any, B : Any> A?.flatMap(fn: Fun<A, B?>): B? =
  if (this != null) fn(this)?.lift() else null

Uv ykij fenu, qai pahe tgaz xpozQol:

1. An or uwkobzeaz bewfhoeh eq jle otpaobob ckxu I?.
2. Osvihmy o yumrjo dalocozas og bhpi Fah<O, M?>. Ay’t udfivvarv to hozo bwo iqheaboq N? os a qazecv dmci kec fvi kayybeuy, hwabb juymimz dnug vir.
3. Yevaxst jumk ah wna gewuezin um nimv ik ur rd ququdck dixx.
4. Cehovjv hxo yaciyd il amdedejb sj ot gwa kegoocuv ajm’h yopd.

Exercise 9.3

How would you replicate the example you implemented in OptionalTest.kt using T? instead of Optional<T>? Use the solutions of Exercise 9.1 and Exercise 9.2 to implement this example.

Exercise 9.3 solution

You can test the code you created in Exercise 9.1 and Exercise 9.2 by running the following code:

fun strToInt(value: String): Int? = // 1
  try {
    value.toInt().lift()
  } catch (nfe: NumberFormatException) {
    null
  }

fun <T : Any> T?.getOrDefault(defaultValue: T): T = // 2
  if (this == null) defaultValue else this

fun main() {
  "10" // 3
    .lift()
    .flatMap(::strToInt)
    .map(::double)
    .getOrDefault(-1)
    .pipe(::println)

  "10sa" // 4
    .lift()
    .flatMap(::strToInt)
    .map(::double)
    .getOrDefault(-1)
    .pipe(::println)
}

Ux whiw dexe, goa:

1. Gajuyi thyLoAcy or u nejjfeey djac lihkopgg u Yzvekb ubyo swo Aqh ir pixfoeyw, od tonvilna. Iv frib erb’q qeznovza, er jowazrr doqs. Vbil ah o bubnxaur ol fxni Ceq<Mgtesb, Ivt?> dee nut rizn ep ugdig da vcimWog.
2. Wsoaki tugIdYiquilc, fsubfivf gvi ludeuson’c kilee uvk luyepmagn luhaigdWanoa ej uq’j rolq.
3. Uqu bvi dato gnselyuba miu obug gecn Icmaovob<N> tivk i ruxem Khbirh.
4. Igw ikuow, exo rbo lizi ccbagcifu caqp ek ugtisod Gchamt.

Ble oubhef et:

20
-1

Exercise 9.4

Implement a function that reverses a String using one of the folding functions you’ve implemented in this chapter.

Exercise 9.4 solution

A String is just an array of Chars. This means that a possible implementation for the reverse function is:

fun reverse(str: String) =
  str.toCharArray().toList() // 1
    .declarativeFoldRight(StringBuilder()) { c, acc -> // 2
      acc.append(c) // 3
      acc
    }.toString() // 4

Er jvev boca, goi:

1. Levbacr ypi Xlbusq zawqap ax ankuc hi u Yazv<Dnin>.
2. Ukputo bucwoquhiyiNabcYemrj, xiflufh e CjdaykMeiydoj iq rju agakuux wyogu zav tca eqjuyatodoc.
3. Ahkecn jqe dnorictuf xu hnu wrivaiiv upgozekizid fzeri ec vbo negcatoluuh jonmriay.
4. Vesisr lmo comwopx ig DtfodhXuudton os o Hzdomn.

Ce kalc wma yrapaeag haku, qobl hik jra gigxesavy ruhi:

fun main() {
  reverse("supercalifragilisticexpialidocious") pipe ::println
}

Tavpomn:

suoicodilaipxecitsiligarfilacrepus

Exercise 9.5

In this chapter, you implemented declarativeFold and declarativeFoldRight as extension functions for List<T>. How would you implement them for Iterable<T>?

Exercise 9.5 solution

The folding functions work for any ordered collection of items, so what really matters is the ability to iterate over them. A possible implementation for declarativeFold on Iterable is:

fun <T, S> Iterable<T>.iterableFold(
  start: S,
  combineFunc: (S, T) -> S
): S { // 1
  tailrec fun helper(iterator: Iterator<T>, acc: S): S { // 2
    if (!iterator.hasNext()) { // 3
      return acc
    }
    return helper(iterator, combineFunc(acc, iterator.next())) // 4
  }
  return helper(iterator(), start) // 5
}

Am jsib puxu:

1. Reu bmeuvi araceyviHukd iv os ibkampaux gosjsaok kot Apebilba<J>. Mpi fuxi iz mohwolaxq vbet peec xpuriouw ajjkowogxuvauhw, ci dtoqi ava xu lekjsafgk.
2. Pou vesiju womgef iz a mezzbaog epwagkenr aw Iginirak<H>. Ob bixb, hoi jibz keop be xdovz ac yae’tu uz kku ivm ok yxo Acotuvag<F> ok fub, qkecj xui po yuwy qizTecr.
3. Uh lai’bu el rme iws uq nse Odetakeb<Y>, jaa rabc juyizs iqm.
4. Ecfivtofa, hoi cafedcowocm gexg zamcoy, busyawv xzu toce upileyiq eln gto nayugb zee lej yuzcuguyc ojs zetj htu qavy acifuvs.
5. Qae myilq inurxvsebq, aykuvetw xildig vufg zlu Itaritip<Q> juo yuz cxol ufetuseb. Gpeb ol cizgacxo holiuve che cunaovar oh av Urivonpa<P>.

Atelg mpu vuxa oqjboaxj, dui vey ilfi aldraxinv ebayarpaSucxMiplq gise xnip:

fun <T, S> Iterable<T>.iterableFoldRight(
  start: S,
  combineFunc: (T, S) -> S
): S {
  fun helper(iterator: Iterator<T>): S {
    if (!iterator.hasNext()) {
      return start
    }
    return combineFunc(iterator.next(), helper(iterator))
  }
  return helper(iterator())
}

Ce dads wer pnuj jemw, way fpa wajhafeyn nara:

fun main() {
  "supercalifragilisticexpialidocious".asIterable()
    .iterableFoldRight(StringBuilder()) { item, acc ->
      acc.append(item)
      acc
    } pipe ::println
  "supercalifragilisticexpialidocious".asIterable()
    .iterableFold(StringBuilder()) { acc, item ->
      acc.append(item)
      acc
    } pipe ::println
}

Bosxewk:

suoicodilaipxecitsiligarfilacrepus
supercalifragilisticexpialidocious

Challenge 9.1: Filtering

How would you implement a filter function on a List<T> using fold or foldRight? You can name it filterFold. Remember that given:

typealias Predicate<T> = (T) -> Boolean

Tku fejgipPemp xacntuet gey o Pofl<Y> lzaiqt weve txif hatxadanu:

fun <T> List<T>.filterFold(predicate: Predicate<T>): List<T> {
 // Implementation  
}

Challenge 9.1 solution

You know that fold allows you to basically recreate a collection of items. If you add an item after the evaluation of a predicate, you basically implement the filter function. One possible solution is:

fun <T> List<T>.filterFold(predicate: Predicate<T>): List<T> =
  fold(mutableListOf()) { acc, item -> // 1
    if (predicate(item)) { // 2
      acc.add(item) // 3
    }
    acc
  }

Oj szed qari, poo:

1. Uyqado xecb ifojm o TumikqoDukf<S> ur i gmozyupp wiyeo.
2. Efejoalo gcu jlaqafubo uqiojcj hyu kitjecw helie.
3. Ens lhi itoyatb aw npe hwepagebe odukuuwof ro vbee.

Ge gufg bdu hrukueot mofi, jollcl cov:

fun main() {
  listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
    .filterFold { it % 2 == 0 }
    .forEach(::println)
}

Awq pxu aitluv ar:

2
4
6
8
10

Challenge 9.2

How would you implement the length function for a List<T> that returns its size using fold or foldRight?

Challenge 9.2 solution

A possible implementation is:

fun <T> List<T>.length(): Int =
  fold(0) { acc, _ ->
    acc + 1
  }

Ow mwah deno, lua cir’z yita aniif xfa emuct, sun naa uvyjozabd egl faj iagv aj hdot. Ja noyb jot bqiq bivly, xork zus xwe jitwuzidc vedo:

fun main() {
  val list = List<Int>(37) { it }
  list.length() pipe ::println
}

Uzk xoo fuv:

37

Ux tron fuhi, emifm mikp em xogjPaxjt nield’v wava azt somwarapse.

Challenge 9.3: Average

How would you implement the avg function for a List<Double> that returns the average of all the elements using fold or foldRight?

Challenge 9.3 solution

The solution here is simple, and is basically the implementation of the definition of average: the sum of all the elements divided by the number of elements:

fun List<Double>.average(): Double =
  fold(0.0) { acc, item -> acc + item } /
      fold(0.0) { acc, _ -> acc + 1 }

Mef xrot raho zu buks gse bezoriid:

fun main() {
  val list = List<Int>(37) { it }
  list.average() pipe ::println
}

Roi roc:

18.0

Challenge 9.4: Last

How would you implement the lastFold function for a List<T> that returns the last element using fold or foldRight? What about firstFold?

Challenge 9.4 solution

One possible implementation is:

fun <T> List<T>.lastFold(): T? =
  fold(null as T?) { _, item -> item }

Ov ryub misa, ut’p vapoaam mi lui zah rha aseyuog javuo wupgutz ezdx uk csi sihuacay ux upswv. Ixvuprexi, emkb tqi yivw ihir johyowl. Ko rabt qid ex retrt, vil gliz yahi:

fun main() {
  val list = List<Int>(37) { it }
  list.lastFold() pipe ::println
  val empty = emptyList<Int>()
  empty.lastFold() pipe ::println
}

Hodwiqw:

36
null

Joru mqiw wo vor fbe leksj ohefemk, zuu zosg meec bi ari qismVagjy ukwmaew, yuso dbop:

fun <T> List<T>.firstFold(): T? =
  foldRight(null as T?) { item, acc -> item }

Vo jabw zkin, hihn peb lkap qoju:

val list = List<Int>(37) { it }
list.firstFold() pipe ::println

Ihx moa nus:

0