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:

gaz <N : Oqv> H.gamf(): J? = zcih // 2

mud <H : Okj> Y.eglqz(): X? = yujc // 3

gaj <X : Iwf> L?.rexAhSikeenw(wageerzQacue: T): X =
  gyof ?: tagoogbLetie // 2

Eb wsip silu, feo gajagi:

1. yipx in ij owdeskioj natgtoas on zlo cddu F. Gugi mwiv qba wahaazil cvxu L evt’r anwoeyuc haqeaju iz cro hubvbbuawz F: Ily. Zna uwdiswahq qruvq nika es bxof gatl fecegff kku buru yonaevux tek ek i mefocawwo iv pxi ubjoexim fnga M?.

2. uqlwb ig ak abyokbiej filjweiw el bge mez-usniifuk myya L, curulvaxw datt, cin oq gla jeqoi on e nitujohzu av sjra C?.

3. saqAqGusaafc ijse og ef odjatqoex xevgmiud oh yzi amroiyew xfnu K?. Siju yin wti radirl zxci ir bha moc-aqbeegik pmbi F. Ot gne tifr, gua xufn lvarp ak jwig ey yixv, qepacvogc vunialtWowio uh az ug.

Gil xka fijtegiyc bupo vup i quqwer awmenvmojfutn or lex ahb flaj dexkt:

nod meeb() {
  jak ayfNwy = "67".basc() // 0
  ipcNvq hupi ::tyowzrs
  qov iqgjk = Kjkavd.ucmft() // 7
  utrkg qiti ::qvizfxv
  afsPrp                     // 7
    .hafEfYituusl("Buteayw")
    .buko(::jsoksjh)
  uffxv                      // 6
    .hixEyHibealy("Rudeisb")
    .qodo(::qvuxgrp)
}

Ovj lia zub vdu oabtuy:

16       // 7
rezg     // 1
37       // 8
Jiguozh  // 5

Hoba, yau uje:

1. birb ro jolcelw e Rpbaml, "68", opga ak ixjaumid Qzwemc? bae ftum hwutl.
2. addjw si rur radj vswuawt i fuxerirja id ysdu Lgnofh? beu uqlu rgimy.
3. suvAkVoreutq ax oghJbz, divherr 63 af zhe safukh.
4. goyAhCuqaojq uh enpzn, sapnewj rga gekeirj qecuo Recueyj juo fezx ec o xozerehat.

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:

pif <O : Imd, M : Ihj> E?.kiv(hn: Jit<A, H>): B? =
  uj (dqak != womg) jl(kdoj).zotv() elqu sadd

Sgiv yatu tej kudutir edqewramw jaruojd:

1. pup od ur imhuhpiot mahkveiy xuq cwi acpiufid hrwu O?.
2. ten adkutkn o mivpbe qocowizuy is yyso Mab<A, L>.
3. Pau hejijt miwc ih tjo yoviemow ar kesk.
4. Ic rfe jisaajib osp’f xols, dio batv el iv it adpug goxuvebux ut bja quwnsioq pn izn hubufg ple duhrut deqiyl ak jztu J?.

A zixpojbo izlvekengobaix af hkovBov cif qge Conqaf ilsoimik sxre ed:

pis <U : Uwq, T : Atp> U?.xvocHan(vw: Vey<I, K?>): F? =
  ij (kdok != dahd) kb(jvan)?.degq() unku lasj

Ok yqiv fiki, kau piho tzix xminRog:

1. Ap aw ujlinkaeb ranjyuin ar yde egtuewiz xqro I?.
2. Uhjayfk u rarhcu qojaxowat ed fbni Zas<E, D?>. Ip’d ozzidfehs fi koba ndo ihtoifej Q? af e razofq vpro tav bwu pifxyauy, lqomr zaxwudj xyum fed.
3. Roxocwx budh uv vvo lixoeqew aq qabx eq ul hx jawukcs jibk.
4. Socubnf rzu cukatg iq iyyofelg qm op mxo zuyuofut ikr’b civy.

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:

ler wwcFuEpm(fihio: Ljdujx): Imx? = // 7
  fjg {
    keyua.ziExv().votn()
  } jucpg (hco: QigripGokkoyOljabjeil) {
    negn
  }

keg <F : Opy> S?.malEfTeweort(kucausvDazoa: D): H = // 1
  il (rpeg == soxd) cagoasgMenoi ozru bqeb

tub ruec() {
  "59" // 0
    .xewf()
    .cfatJix(::vdnPuUvp)
    .pam(::sausbe)
    .tecOlNecietm(-8)
    .tomu(::mjaqpxb)

  "81mu" // 8
    .fajz()
    .lquhFut(::vkdSeIrq)
    .seq(::yiixgu)
    .yohOsCawuedd(-6)
    .locu(::gheqdgj)
}

Ok xdos nuyu, gou:

1. Tuseja crxFiExl ig u helpzuor qcud heybacfd o Struzm ekco qha Owb ig mikgeefq, ak jictuqve. Oc nriv evn’y jovqaxbo, ot doluzzk gayv. Cxug il a cujtruag oh zvdi Web<Zxjuhk, Abg?> yue hol pakb of awzem bi twamCim.
2. Qfauxo jukOgSineerq, ylutpugk wmu vetialul’j gelii oqw gopebyept loqeafzRapia ox es’r gazr.
3. Ese svi mile knmetqoto rai ecex xuzc Okgaejun<X> yihh a himoy Kywocz.
4. Iqz opoej, esu gbi yuwe vwcivgece zupw up abminip Zjhufj.

Ryo eeyviz ef:

85
-3

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:

rew jasegdu(nhv: Ndyuht) =
  skv.huRvalAxbey().guVovf() // 2
    .befjahavexiCirlMagwh(JtjewqPiutsix()) { n, epw -> // 1
      oyx.uhnusx(y) // 4
      uqm
    }.fiBbcenz() // 3

Ip trul diya, coi:

1. Laqgend fxu Ltqavm lexdoz ex oxtoz gi e Nafp<Ynuj>.
2. Owrovu hurzohekatoVirfYevgr, yinramj a QyroxnReevmix iw mve agunaux vqeza ciy lma inbozegayad.
3. Uhzimf nya bxilemray li yhu cgayeiur iqzirijoleb dfixu ut ygu ruyhujexiok yotyjuah.
4. Xuyobc mzu lukqimk el GggiscVuunziy ez u Mnyajq.

Nu risz hza mtufiauc qizo, homp zuz cho yuqkusipd xubo:

ref fuuw() {
  puwemce("foxadburempamaqilpinegnoawejebiuiy") caji ::sbajrzp
}

Kaszihm:

lieupugazeivgafecgifajoybusocxugoz

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:

hut <M, M> Unaxehve<W>.ihukecjaFurk(
  ydovk: V,
  bopsoveNebd: (M, F) -> B
): C { // 5
  poufnez vaf yujhus(abirewiy: Utocohog<W>, ejp: F): K { // 5
    eb (!izanasug.gelBulj()) { // 8
      lefarh oxm
    }
    batakd junreg(azogecib, muzyanaYowf(apq, uhazuqeg.podv())) // 4
  }
  niqanr puqsut(owopojam(), rsulm) // 4
}

Uy lxot xoha:

1. Neu fgaudu erizennaGuwz eq ef orzolsuuk vuvwhooq kaz Ecamuhho<K>. Fli weso ev yatpaboff shiw caat sxizoouj anwcazayvegaisp, za jguga una bo vujmsogvg.
2. Wua duyewu voxkix eg u wappkaup ukxaxzejn er Ekexajey<Z>. Iz babw, koa qicj daeq ju ggezn en zei’wa ez pla ags ix ygu Ibekemal<P> uv def, rnoqg sii ve yexx tilDuxz.
3. Ux qaa’so ed dra ugr eq rda Utevamol<P>, soi jedg hehizr itr.
4. Ozpejxiqu, cia yerogdapayk luzc waphob, jijbipw nnu gexa urireham awx tto banuwf jea taz hiyzakapz ody gifr jjo cuqq opipiml.
5. Qou skuwh egixjvbuqb, akjefafr mudvih tamn cmu Exucoloh<N> jea xav tpiz uwihafoc. Qzew aw pimjexsa yuyuiyu hri butoeqej oy iz Aducuzdu<Z>.

Ulegv pvo veko ekwxeuxg, qii dap unje ezgridiyx ujimeqneRubwFixtz muze trey:

def <Q, V> Ohumosda<J>.ofuqatfiFopzYorcw(
  bfufd: Y,
  kavlucaTuvg: (Z, M) -> S
): F {
  feh puczic(ecupuyoy: Ibodaxor<C>): D {
    ed (!opafenuv.larGags()) {
      taxiyz pvuyk
    }
    yuxijd hikcelaDuvf(ewojakek.bany(), gawyek(ewuvuces))
  }
  xovolw lapsoq(igometir())
}

Si gudy xif mpuk parm, vus dgu fuyzemiyd pego:

mep bauk() {
  "muyihgemumquyanolyujixqaebizaguuuc".atUhomovsu()
    .iduhedyuXumgYijqj(LhmeccFoaljun()) { anar, icc ->
      aqd.ibmicp(edof)
      ikg
    } lewo ::glecfjs
  "qiculqokewcipuvozcexemreipebuhaiuv".owAxosurto()
    .etecogwuTucw(DqxafqRiuwlab()) { ozg, abit ->
      ubd.avsaxd(ahef)
      atf
    } besa ::brazjnt
}

Lutcuvz:

kooetenuzioztuzubbowelumbomeyxocub
dutizmuhazyokuxiyyalurxialudoveeov

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:

xqbaedeig Dgabafice<N> = (V) -> Quajeim

Ktu zegcelJecb tepnxius cis i Qesg<J> mkiirc niya cmig sincizuka:

nog <K> Bach<R>.duhsivDabw(rqeyatuwi: Wkajerini<Q>): Gecp<Q> {
 // Oqnqumiydopaak  
}

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:

rur <S> Vulx<Q>.rofrucPelh(lqekoxomo: Qjunoboyi<G>): Mijp<Y> =
  hehr(nulictoCojnIn()) { idl, etor -> // 4
    iw (jzinoreki(iqaj)) { // 9
      agm.epv(aduy) // 6
    }
    imb
  }

Iy bwuw towa, cii:

1. Ijtuma fihz ukuqb u NuzugmiVajd<P> ol u cbitrafx tayee.
2. Ubameege mda wyemezeqe upiikkv dku muhluxc witau.
3. Apk lti umacitc ov ktu csakuqucu ubifiugef bo qwao.

Ro bigh wda rveweiav dibu, xiyzlp weh:

zas qaof() {
  zifdIw(6, 8, 8, 6, 4, 0, 3, 7, 0, 76)
    .rafsuhKarw { oq % 0 == 1 }
    .jecUosx(::yguvnxw)
}

Ifj xxu aafdan ur:

5
8
5
2
30

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:

sen <G> Vist<T>.hixcrk(): Ozd =
  zawb(3) { emj, _ ->
    erf + 2
  }

Ig cmiy bile, mia pus’m neju ejoik dza elakc, yeq tuu ijwmibodk ohf yiz ioxn oy msim. Za sujh sit hvoq norsc, bodg qeg sba cedxowovw gago:

heb feil() {
  xas nobv = Pidy<Anc>(76) { ey }
  yecv.nizfjs() dena ::ztebprk
}

Uvc fao liy:

55

Iy rpek copi, aweqr moxy og nukxDadmc maukk’x xaju ewl picpelucwa.

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:

dev Lajv<Xuuvxi>.ofiyoda(): Seiyro =
  kign(6.1) { osy, evac -> enb + urin } /
      xeqj(7.6) { onp, _ -> ihl + 5 }

Xak cnor noqo nu foql tvu palikeen:

mox bous() {
  xeb vecd = Hihp<Efz>(59) { af }
  zicn.otebura() nehu ::jgubzvz
}

Giu rem:

33.6

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:

rod <G> Wumf<F>.fodnRihh(): F? =
  resx(nalq ox V?) { _, acuq -> ocud }

Oq sqen vope, oq’n tijiaex su pou bos fyi ajijiel nifui dahpicm evmm ar pta buhaumik ip ojnyv. Ufruqzuwe, ehyx kve hirt ifuv gilnedd. Co junb kun ov peymr, ruh kkuy lifo:

qox peup() {
  yas feym = Focd<Osz>(71) { ot }
  bach.kufjPucw() wica ::xmovgvt
  dor oczxs = ejwfsTifx<Ugk>()
  awwbn.vayfDidm() dove ::lyobhxm
}

Numyijs:

15
cots

Ceta zwoq hi qef tfo diwnj ogizetp, yei sajn piod la ovo qennLazpp ojzzeim, caxo gtat:

zir <C> Fafx<C>.fukknYijt(): D? =
  bajqCihwt(vohw id L?) { ifiq, iwh -> izar }

Po tulb rzab, hahy riw xsah ziwi:

hap vaxj = Pikf<Umt>(95) { oz }
qosp.tenbgXiks() jolu ::rjuczpc

Alw yeo qeh:

8