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![2/12/22, 1:16 PM Dart function - Recursive Functions
https://softwarezay.com/notes/396_dart-function-recursive-functions 3/14
void repeat(String word, [int repetitions]) { // repetitions is optional
if (repetitions != null) { // check if repetitions was supplied
for (int i = 0; i < repetitions; i++) {
print(word);
}
} else { // repetitions was not supplied, so just print once
print(word);
}
}
The!=operatorisusedheretocheckwhethertheoptionalparameterrepetitionswassupplied.
Ifanoptionalparameterisnotsuppliedavalue,anditdoesnothaveadefaultvalue,thenitwillhavethevaluenull.In
thecaseofrepeat(),itwillprintwordrepetitionstimes,ifrepetitionsissupplied.Ifrepetitionsisnotsupplied,itwill
printwordjustonetime.Thisfunctioncanbesimplifiedbyusingadefaultvalueforrepetitions.
void repeat(String word, [int repetitions = 1]) {
for (int i = 0; i < repetitions; i++) {
print(word);
}
}
The=operatorisusedwithinthesquarebracketstosetthedefaultvalueofanoptionalpositionalparameter.
Commasseparatemultipleoptionalpositionalparameters.Whenafunctionwithoptionalpositionalparametersis
called,itsoptionalparametersmustbesuppliedinthesameorderthattheyaredefined.
void repeat(String word, [int repetitions = 1, String exclamation = ""]) { for (int i = 0; i
< repetitions; i++) {
print(word + exclamation); // the + operator can concatenate strings
}
}
void main() {
repeat("Dog"); // legal
repeat("Dog", 2, "!"); // legal
repeat("Dog", 2); // legal
repeat("Dog", "!"); // ILLEGAL
repeat("Dog", "!", 2); // ILLEGAL
}
Bothoptionalparametersdonotneedtobesupplied,butwhetheroneortwoissupplied,theymustbesuppliedinthe
rightorder.
Note
4 Dartfunction-Single-LineFunctions
Single-LineFunctions
Darthassyntacticsugarthatmakesitpossibletodeclareshortfunctionsmorecompactly.](https://image.slidesharecdn.com/dartfunction-recursivefunctions-220212065101/85/Dart-function-Recursive-functions-3-320.jpg)
![2/12/22, 1:16 PM Dart function - Recursive Functions
https://softwarezay.com/notes/396_dart-function-recursive-functions 9/14
NestingFunctions
Typically,youdefinefunctionseitheroutsideotherfunctionsorinsideDartclasses.However,youcanalsonestDart
functions.Forexample,youcannest isBanana inside main.
void main() {
bool isBanana(String fruit) {
return fruit == 'banana';
}
var fruit = 'apple';
print(isBanana(fruit)); // false
}
Youcanalsochangetheargumentyoutoafunction,thencallitagainwiththenewargument:
fruit = 'banana';
print(isBanana(fruit)); // true
Theresultofcallingthefunctiondependsentirelyontheargumentsyoupassin.
OptionalParameters
Ifaparametertoafunctionisoptional,youcansurrounditwithsquarebracketsandmakethetypenullable:
String fullName(String first, String last, [String? title]) {
if (title == null) {
return '$first $last';
} else {
return '$title $first $last';
}
}
Inthisfunction, title isoptional.Itwilldefaultto null ifyoudon’tspecifyit.
Now,youcancallthefunctionwithorwithouttheoptionalparameter:
print(fullName('Joe', 'Howard'));
// Joe Howard
print(fullName('Albert', 'Einstein', 'Professor'));
// Professor Albert Einstein
NamedParametersandDefaultValues
Whenyouhavemultipleparameters,itcanbeconfusingtorememberwhichiswhich.Dartsolvesthisproblem
with namedparameters,whichyougetbysurroundingtheparameterlistwithcurlybrackets: { }.
Theseparametersareoptionalbydefault,butyoucangivethemdefaultvaluesormakethemrequiredbyusingthe
required keyword:
bool withinTolerance({required int value, int min = 0, int max = 10}) {
return min <= value && value <= max;
}
value isrequired,while min and max areoptionalwithdefaultvalues.
Withnamedparameters,youcanpassinargumentsinadifferentorderbysupplyingtheparameternameswitha
colon:
print(withinTolerance(min: 1, max: 5, value: 11)); // false](https://image.slidesharecdn.com/dartfunction-recursivefunctions-220212065101/85/Dart-function-Recursive-functions-9-320.jpg)
![2/12/22, 1:16 PM Dart function - Recursive Functions
https://softwarezay.com/notes/396_dart-function-recursive-functions 11/14
Comments
Thereisnocomment.
Addcomment
// 1
final drinks = ['water', 'juice', 'milk'];
// 2
final bigDrinks = drinks.map(
// 3
(drink) => drink.toUpperCase()
);
// 4
print(bigDrinks); // (WATER, JUICE, MILK)
Let’slookateachstep:
1.Definealistof drinks thathavelowercaseletters.
2..map takesallthelistvaluesandreturnsanewcollectionwiththem.
3.Ananonymousfunctionispassedasaparameter.Inthatanonymousfunction,youhavea drink argumentthat
representseachelementofthelist.
4.Thebodyoftheanonymousfunctionconvertseachelementtouppercaseandreturnsthevalue.Sincetheoriginal
listisalistofstrings, drink alsohastype String.
Usingananonymousfunctionandcombiningitwith .map isaconvenientwaytotransformonecollectionintoanother.
Note:Don’tconfusethe .map methodwiththe Map type.
source:raywenderlich.com
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Dartfunction-FunctionsWithinFunctions
11Feb2022,22:26pm
Dartfunction-NamedOptionalParameters
11Feb2022,22:18pm
Dartfunction-PositionalOptionalParameters
11Feb2022,22:07pm](https://image.slidesharecdn.com/dartfunction-recursivefunctions-220212065101/85/Dart-function-Recursive-functions-11-320.jpg)
![2/12/22, 1:16 PM Dart function - Recursive Functions
https://softwarezay.com/notes/396_dart-function-recursive-functions 3/14
void repeat(String word, [int repetitions]) { // repetitions is optional
if (repetitions != null) { // check if repetitions was supplied
for (int i = 0; i < repetitions; i++) {
print(word);
}
} else { // repetitions was not supplied, so just print once
print(word);
}
}
The!=operatorisusedheretocheckwhethertheoptionalparameterrepetitionswassupplied.
Ifanoptionalparameterisnotsuppliedavalue,anditdoesnothaveadefaultvalue,thenitwillhavethevaluenull.In
thecaseofrepeat(),itwillprintwordrepetitionstimes,ifrepetitionsissupplied.Ifrepetitionsisnotsupplied,itwill
printwordjustonetime.Thisfunctioncanbesimplifiedbyusingadefaultvalueforrepetitions.
void repeat(String word, [int repetitions = 1]) {
for (int i = 0; i < repetitions; i++) {
print(word);
}
}
The=operatorisusedwithinthesquarebracketstosetthedefaultvalueofanoptionalpositionalparameter.
Commasseparatemultipleoptionalpositionalparameters.Whenafunctionwithoptionalpositionalparametersis
called,itsoptionalparametersmustbesuppliedinthesameorderthattheyaredefined.
void repeat(String word, [int repetitions = 1, String exclamation = ""]) { for (int i = 0; i
< repetitions; i++) {
print(word + exclamation); // the + operator can concatenate strings
}
}
void main() {
repeat("Dog"); // legal
repeat("Dog", 2, "!"); // legal
repeat("Dog", 2); // legal
repeat("Dog", "!"); // ILLEGAL
repeat("Dog", "!", 2); // ILLEGAL
}
Bothoptionalparametersdonotneedtobesupplied,butwhetheroneortwoissupplied,theymustbesuppliedinthe
rightorder.
Note
4 Dartfunction-Single-LineFunctions
Single-LineFunctions
Darthassyntacticsugarthatmakesitpossibletodeclareshortfunctionsmorecompactly.](https://image.slidesharecdn.com/dartfunction-recursivefunctions-220212065101/75/Dart-function-Recursive-functions-3-2048.jpg)
![2/12/22, 1:16 PM Dart function - Recursive Functions
https://softwarezay.com/notes/396_dart-function-recursive-functions 9/14
NestingFunctions
Typically,youdefinefunctionseitheroutsideotherfunctionsorinsideDartclasses.However,youcanalsonestDart
functions.Forexample,youcannest isBanana inside main.
void main() {
bool isBanana(String fruit) {
return fruit == 'banana';
}
var fruit = 'apple';
print(isBanana(fruit)); // false
}
Youcanalsochangetheargumentyoutoafunction,thencallitagainwiththenewargument:
fruit = 'banana';
print(isBanana(fruit)); // true
Theresultofcallingthefunctiondependsentirelyontheargumentsyoupassin.
OptionalParameters
Ifaparametertoafunctionisoptional,youcansurrounditwithsquarebracketsandmakethetypenullable:
String fullName(String first, String last, [String? title]) {
if (title == null) {
return '$first $last';
} else {
return '$title $first $last';
}
}
Inthisfunction, title isoptional.Itwilldefaultto null ifyoudon’tspecifyit.
Now,youcancallthefunctionwithorwithouttheoptionalparameter:
print(fullName('Joe', 'Howard'));
// Joe Howard
print(fullName('Albert', 'Einstein', 'Professor'));
// Professor Albert Einstein
NamedParametersandDefaultValues
Whenyouhavemultipleparameters,itcanbeconfusingtorememberwhichiswhich.Dartsolvesthisproblem
with namedparameters,whichyougetbysurroundingtheparameterlistwithcurlybrackets: { }.
Theseparametersareoptionalbydefault,butyoucangivethemdefaultvaluesormakethemrequiredbyusingthe
required keyword:
bool withinTolerance({required int value, int min = 0, int max = 10}) {
return min <= value && value <= max;
}
value isrequired,while min and max areoptionalwithdefaultvalues.
Withnamedparameters,youcanpassinargumentsinadifferentorderbysupplyingtheparameternameswitha
colon:
print(withinTolerance(min: 1, max: 5, value: 11)); // false](https://image.slidesharecdn.com/dartfunction-recursivefunctions-220212065101/75/Dart-function-Recursive-functions-9-2048.jpg)
![2/12/22, 1:16 PM Dart function - Recursive Functions
https://softwarezay.com/notes/396_dart-function-recursive-functions 11/14
Comments
Thereisnocomment.
Addcomment
// 1
final drinks = ['water', 'juice', 'milk'];
// 2
final bigDrinks = drinks.map(
// 3
(drink) => drink.toUpperCase()
);
// 4
print(bigDrinks); // (WATER, JUICE, MILK)
Let’slookateachstep:
1.Definealistof drinks thathavelowercaseletters.
2..map takesallthelistvaluesandreturnsanewcollectionwiththem.
3.Ananonymousfunctionispassedasaparameter.Inthatanonymousfunction,youhavea drink argumentthat
representseachelementofthelist.
4.Thebodyoftheanonymousfunctionconvertseachelementtouppercaseandreturnsthevalue.Sincetheoriginal
listisalistofstrings, drink alsohastype String.
Usingananonymousfunctionandcombiningitwith .map isaconvenientwaytotransformonecollectionintoanother.
Note:Don’tconfusethe .map methodwiththe Map type.
source:raywenderlich.com
Be the first of your friends to like this.
Like Share
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Dartfunction-FunctionsWithinFunctions
11Feb2022,22:26pm
Dartfunction-NamedOptionalParameters
11Feb2022,22:18pm
Dartfunction-PositionalOptionalParameters
11Feb2022,22:07pm](https://image.slidesharecdn.com/dartfunction-recursivefunctions-220212065101/75/Dart-function-Recursive-functions-11-2048.jpg)
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Dart function - Recursive functions
- 1. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 1/14 DEVELOPER'SNOTE Dartfunction-RecursiveFunctions 11Feb2022,22:34pm Dart Arecursivefunctionisafunctionthatcallsitself.Arecursivefunctionisusedforrepeatingcode.Ifthestateofthefunctiondoesnot change,andithasnopresetexitconditionthatisreached,arecursivecallcanturnintoaninfiniteloop. void forever() { forever(); } forever()isindeedaninfiniteloop.HereisarecursivefunctionthatwillrepeatedlyaddoneStringto anotherString. String addOn(String original, String additional, int times) { if (times <= 0) { // exit condition to end "recursive loop" return original; } return addOn(original + additional, additional, times - 1); // recursive call } Note 1 Dartfunction-FunctionsWithinFunctions Functionscanbedefinedwithinotherfunctions.These,too,canbereferencedbyvariablesandpassedtoother functions. void talkAbout(String toShout, shoutFunc) { print(shoutFunc(toShout)); } void main() { String exclame(String toExclame) => toExclame + "!"; String manyTalk(String toMany) { String allTogether = ""; for (int i = 0; i < 10; i++) { allTogether = allTogether + toMany; // keep concatenating toMany onto the end of itself } return allTogether; } talkAbout("Hello", exclame); talkAbout("TicToc", manyTalk); talkAbout("Wow", (String s) => s.toUpperCase()); // a function with no title!? } Theanonymousfunctioninthelastlineofmain()?That’sright,afunctionwithnotitletoidentifyitisknownasan anonymousfunction. Anonymousfunctionsareusefulinsituationswhereafunctiondoesnotneedtobecalledfrom multiplepartsofaprogrambyitsidentifier,orwheretheysimplymakesensestylistically.
- 2. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 2/14 Forexample,perhapsafunctiontakesanotherfunctionasaparameter.Ifthefunctionthatisgoingtobepassedtoit isparticularlysmall,itmaylookmore“clean”toimplementitasananonymousfunctionthantocreateaseparatefull- fledgedfunction.WhentalkAbout()runsfromthiscall,theidentifiershoutFuncwillrefertothisanonymousfunction, anditwillbecallablethroughshoutFunc(). Note 2 Dartfunction-NamedOptionalParameters NamedOptionalParameters Namedoptionalparametersareverysimilartopositionaloptionalparameters,buttheyaredefinedandcalled differently.Whenafunctionwiththemiscalled,theordertheyaresuppliedwithdoesnotmatter.Theyaredefined withcurlybraces{}andcolonstoseparatetheirnamefromtheirdefaultvalue.Likepositionaloptionalparameters, commasseparatethem.Alsolikepositionaloptionalparameters,theydonotneedtohavedefaultvalues.Incases whennamedoptionalparametersarenotsuppliedwithavalue,theywillalsohaveadefaultvalueofnull. void repeat(String word, {int repetitions: 1, String exclamation: ""}) { for (int i = 0; i < repetitions; i++) { print(word + exclamation); // the + operator can concatenate strings } } void main() { repeat("Dog"); // legal repeat("Dog", repetitions: 2, exclamation: "!"); // legal repeat("Dog", repetitions: 2); // legal repeat("Dog", exclamation: "!"); // legal, even without repetitions repeat("Dog", exclamation: "!", repetitions: 2); // legal, even out of order } Namedoptionalparametersarealsoslightlymoreflexible,becauseoftheirindifferencetowardorder. Note 3 Dartfunction-PositionalOptionalParameters PositionalOptionalParameters Positionaloptionalparametersaredefinedwithsquarebracketswithinafunctiondefinition. Forexample,thefollowingfunctionrepeat()hasanoptionalparametercalledrepetitions.
- 3. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 3/14 void repeat(String word, [int repetitions]) { // repetitions is optional if (repetitions != null) { // check if repetitions was supplied for (int i = 0; i < repetitions; i++) { print(word); } } else { // repetitions was not supplied, so just print once print(word); } } The!=operatorisusedheretocheckwhethertheoptionalparameterrepetitionswassupplied. Ifanoptionalparameterisnotsuppliedavalue,anditdoesnothaveadefaultvalue,thenitwillhavethevaluenull.In thecaseofrepeat(),itwillprintwordrepetitionstimes,ifrepetitionsissupplied.Ifrepetitionsisnotsupplied,itwill printwordjustonetime.Thisfunctioncanbesimplifiedbyusingadefaultvalueforrepetitions. void repeat(String word, [int repetitions = 1]) { for (int i = 0; i < repetitions; i++) { print(word); } } The=operatorisusedwithinthesquarebracketstosetthedefaultvalueofanoptionalpositionalparameter. Commasseparatemultipleoptionalpositionalparameters.Whenafunctionwithoptionalpositionalparametersis called,itsoptionalparametersmustbesuppliedinthesameorderthattheyaredefined. void repeat(String word, [int repetitions = 1, String exclamation = ""]) { for (int i = 0; i < repetitions; i++) { print(word + exclamation); // the + operator can concatenate strings } } void main() { repeat("Dog"); // legal repeat("Dog", 2, "!"); // legal repeat("Dog", 2); // legal repeat("Dog", "!"); // ILLEGAL repeat("Dog", "!", 2); // ILLEGAL } Bothoptionalparametersdonotneedtobesupplied,butwhetheroneortwoissupplied,theymustbesuppliedinthe rightorder. Note 4 Dartfunction-Single-LineFunctions Single-LineFunctions Darthassyntacticsugarthatmakesitpossibletodeclareshortfunctionsmorecompactly.
- 4. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 4/14 Thesyntax=>isusedforseparatingthereturntype,name,andparametersofasingle-linefunctionfromits implementation. Aswithregularfunctions,specifyingthereturntypeofasingle-linefunctionisoptional.Single-linefunctionsdonot usereturnstatements. Instead,thereturnvalueisimplied.Theevaluatedresultoftheexpressionontherightofthe=>isthereturnvalue. Thefollowingareexamplesofsomefunctionswesawearlierinthechapter,implementedassingle-linefunctions: sayHello(String name) => print("Hello, $name"); // just as valid as next version void sayHello(String name) => print("Hello, $name"); // same as previous but with void int multiply(int num1, int num2) => num1 * num2; // think of as return num1 * num 2; int luckyNumber() => 7; // return is implied Single-linefunctionsarenotinherentlyanymoreorlessusefulthantheirlongercounterparts. Theyaresimplyanotherwayofdefiningthesamething.Theyaresuppliedbythelanguageforconvenience. Note 5 Dartfunction-FunctionReturnValues FunctionReturnValues Notonlycanfunctionsreceiveinformationbywayofparameters,theycanalsoreturninformationtotheircallerby wayofareturnvalue.Thereturnvalueisprecededbythekeywordreturn.Thetypeofthereturnvaluecanbe indicatedbeforethenameofthefunction.Forexample,perhapsafunctionmultiply()returnstheproductoftwo numbers. int multiply(int num1, int num2) { int product = num1 * num2; return product; } Webetterlookatanexamplewithmultiply(). main() { int result = multiply(5, 5); // result will be equal to 25, the product returned by multiply print("2 multipled by 2 is ${multiply(2, 2)}"); // 4 is inserted for multiply(2, 2) } Ifnoreturntypeisspecifiedinthedefinitionofthefunction,thenitcaneitherreturnanythingorreturnnothing. Thatsoundsveryambiguous.Itisgoodpracticetospecifyareturntypewhenafunctionwillreturnavalue.
- 5. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 5/14 multiply(int num1, int num2) { // VALID code, but wouldn't it be less ambiguous to specify int? int product = num1 * num2; return product; } afunctionneednothaveanyreturntypeatall.Inthatcase,thefunction’sreturntypecansimplybeleftblank. Alternatively,itcanbespecifiedexplicitlythatafunctiondoesnotreturnavaluewiththekeywordvoid. void sayHello(String name) { // void specifies this function does not return a value print("Hello, $name!"); } Ifavoidfunctiondidreturnavalue,thatwouldbeanerror.Whyisthekeywordvoidusefulifitisneverrequired? Perhaps,liketypesinthemselves,itisahelpfulsanitycheckbothfortheprogrammerandthecompiler.Inother words,ifweknowwedon’twantafunctiontoreturnavalue,andwedeclareitvoid,itisgoodthatwelatergetanerror whenweforgetitisvoidandaccidentallytrytoreturnavalue. Whenafunctionreachesareturnstatement,itwillimmediatelyexit.Nocodeinthefunctionwillbeexecutedaftera returnstatementisexecuted. int luckyNumber() { return 7; // function exits here print("I'm thinking of a number..."); // DEAD CODE, never executed } Thiscanbeusefulwhenafunctionshouldexitpriortosomelargeoperationtakingplace. Forexample,perhapsafunctionalreadyhasenoughinformationtoproduceitsreturnvalue,soitdoesnotneedto keepexecutingalongloop.Itcanalsobeconfusingifafunctionhastoomanydifferentpossiblepointsofexit,andit cannoteasilybedeterminedwhichonewillbefollowed.Inotherwords,usereturnstatementswhentheymakesense, butnotexcessively. Note 6 Dartfunction-MultipleParameters Functionsneednotbelimitedtozerooroneparameter.Theycancontainanynumberofparameters.Ourbeep() function,mayneedtoknowwhichsoundtoplay.Anotherparametercanprovidethatinformation. beep(int milliseconds, String soundName) {...} //... is used here to represent more code
- 6. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 6/14 Commasseparatemultiplefunctionparameters.Ingeneral,itshouldbeagoaltomakeafunctionasself-sufficientas possible.Inotherwords,top-levelvariablesaregenerallyfrowneduponwhentheyareusedinsituationswherethe designcouldbebetterservedbyfunctionparameters.Takealookatthefollowingprogramandtrytoimaginehowthe top-levelvariablenumberOfPrintscouldbeeliminatedinfavorofafunctionparameter. int numberOfPrints = 0; printMany(String forPrinting) { for (int i = 0; i < numberOfPrints; i++) { print(forPrinting); } } main() { numberOfPrints = 10; printMany("I will not overuse top-level variables."); } Youshouldtakenotethatthefor-loopinprintMany()willexecutenumberOfPrintstimes. Thisdesignmayseemsimpleenoughforasmallprogramofafewlinesofcode.Inamorecomplexprogram,things willquicklybecomeunwieldywhentop-levelvariablesareusedinappropriately.WhatifnumberOfPrintsischangedby anotherfunctionwhenwedonotexpectittobe?Iftherewere20top-levelvariables,woulditnotbehardtokeep trackofwhicharebeingusedwhere?Hereisabetter,cleaner,morecompactversionofthesameprogram: printMany(String forPrinting, int numberOfPrints) { for (int i = 0; i < numberOfPrints; i++) { print(forPrinting); } } main() { printMany("I will not overuse top-level variables.", 10); } Doyouseewhythisismorelogical?numberOfPrintsisdeclaredasaparameterofprintMany(),sinceitwillbe accessedexclusivelyinprintMany().Thegoalistoassociateavariableascloselyaspossiblewiththeplaceinthe programwhereitisutilized. Note 7 Dartfunction-TheLocalityofVariables TheLocalityofVariables
- 7. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 7/14 someFunction() { String name = "Thomas"; } main() { print(name); // ERROR, name does not exist in this context } TheLocalityofVariables Functionparametersbecomevariableswithinafunction.Variableswithinafunctionareatypeoflocalvariables.Local variableshaveascopethatislimitedtotheblockofcodewithinwhichtheyaredefined.Thismeanstheycanonlybe accessedfromwithinthatblock. Forexample,thevariableswehavebeendefininginthemain()functioncannotbeaccessedinanotherfunction. Similarly,inthefollowingcodesnippet,thevariablenamecannotbeaccessedinmain(): someFunction() { String name = "Thomas"; } main() { print(name); // ERROR, name does not exist in this context } Variablescanalsobelocaltoblocksofcodethatarewithinfunctions.Avariablemaybelocaltoafor-loop,for example,andnotaccessibleoutsideofit.Or,itcouldbelocaltoanif-statement,switch-statement,etc.Hereisamore complexexamplethatillustrateslocalityfurther: Variablescanalsobedeclaredoutsideoffunctions.Inthecasewheretheyare,theyarenolongerlocalvariables.A variabledeclaredattheso-calledtoplevelofaDartsourcefileisaccessibleanywherewithinit.Forexample,the followingprogramiscompletelylegal.Youwillnoticethatname2isusedinmain(),whichisdeclaredbeforename2. main() { for (int i = 0; i < 10; i++) { bool dog = true; if (dog) { int howMany = 100; } if (howMany == 100) { // ERROR, howMany is only accessible within the if-statement above dog = false; } print("howMany is $howMany"); // ERROR, howMany is not accessible here either } if (dog) { // ERROR dog is not accessible here, since it is local to the for-loop print("dog is true"); } dog = true; // ERROR dog is not accessible here either }
- 8. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 8/14 String name1 = "Tom"; sayHello(String name) { print("Hello, $name!"); } main() { sayHello(name1); sayHello(name2); } String name2 = "John"; Note 8 Dartbasic-Function Functions Functionsletyoupackagemultiplerelatedlinesofcodeintoasinglebody.Youthensummonthefunctiontoavoid repeatingthoselinesofcodethroughoutyourDartapp. Afunctionconsistsofthefollowingelements: Returntype Functionname Parameterlistinparentheses Functionbodyenclosedinbrackets DefiningFunctions Thecodeyou’returningintoafunctiongoesinsidethecurlybrackets.Whenyoucallthefunction,youpass in arguments thatmatchthetypesoftheparametersofthefunction. Next,you’llwriteanewfunctioninDartPadthatwillchecktoseeifagivenstringis banana: bool isBanana(String fruit) { return fruit == 'banana'; } Thefunctionuses return toreturna bool.Theargumentyoupasstothefunctiondeterminesthe bool. Thisfunctionwillalwaysreturnthesamevaluetypeforanygiveninput.Ifafunctiondoesn’tneedtoreturnavalue, youcansetthereturntypeto void. main doesthis,forexample. WorkingWithFunctions Youcancallthefunctionbypassinginastring.Youmightthenpasstheresultofthatcallto print: void main() { var fruit = 'apple'; print(isBanana(fruit)); // false }
- 9. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 9/14 NestingFunctions Typically,youdefinefunctionseitheroutsideotherfunctionsorinsideDartclasses.However,youcanalsonestDart functions.Forexample,youcannest isBanana inside main. void main() { bool isBanana(String fruit) { return fruit == 'banana'; } var fruit = 'apple'; print(isBanana(fruit)); // false } Youcanalsochangetheargumentyoutoafunction,thencallitagainwiththenewargument: fruit = 'banana'; print(isBanana(fruit)); // true Theresultofcallingthefunctiondependsentirelyontheargumentsyoupassin. OptionalParameters Ifaparametertoafunctionisoptional,youcansurrounditwithsquarebracketsandmakethetypenullable: String fullName(String first, String last, [String? title]) { if (title == null) { return '$first $last'; } else { return '$title $first $last'; } } Inthisfunction, title isoptional.Itwilldefaultto null ifyoudon’tspecifyit. Now,youcancallthefunctionwithorwithouttheoptionalparameter: print(fullName('Joe', 'Howard')); // Joe Howard print(fullName('Albert', 'Einstein', 'Professor')); // Professor Albert Einstein NamedParametersandDefaultValues Whenyouhavemultipleparameters,itcanbeconfusingtorememberwhichiswhich.Dartsolvesthisproblem with namedparameters,whichyougetbysurroundingtheparameterlistwithcurlybrackets: { }. Theseparametersareoptionalbydefault,butyoucangivethemdefaultvaluesormakethemrequiredbyusingthe required keyword: bool withinTolerance({required int value, int min = 0, int max = 10}) { return min <= value && value <= max; } value isrequired,while min and max areoptionalwithdefaultvalues. Withnamedparameters,youcanpassinargumentsinadifferentorderbysupplyingtheparameternameswitha colon: print(withinTolerance(min: 1, max: 5, value: 11)); // false
- 10. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 10/14 Youcanleaveofftheparameterswithdefaultvalueswhencallingthefunction. print(withinTolerance(value: 5)); // true AnonymousFunctions Dartsupports first-classfunctions,meaningthatittreatsfunctionslikeanyotherdatatype.Youcanassignthemto variables,passthemasargumentsandreturnthemfromotherfunctions. Topassthesefunctionsaroundasvalues,omitthefunctionnameandreturntype.Sincethere’snoname,thistypeof functioniscalledan anonymousfunction. Youcanassignananonymousfunctiontoavariablenamed onPressed,likeso: final onPressed = () { print('button pressed'); }; onPressed hasavalueoftype Function.Theemptyparenthesesindicatethefunctionhasnoparameters.Likeregular functions,thecodeinsidethecurlybracketsisthefunctionbody. Toexecutethecodewithinthefunctionbody,callthevariablenameasifitwerethenameofthefunction: onPressed(); // button pressed Youcansimplifyfunctionswhosebodyonlycontainsasinglelinebyusing arrowsyntax.Todothis,removethecurly bracketsandaddafatarrow =>. Here’sacomparisonoftheanonymousfunctionaboveandarefactoredversion: // original anonymous function final onPressed = () { print('button pressed'); }; // refactored final onPressed = () => print('button pressed'); That’smuchnicertoread. UsingAnonymousFunctions You’lloftenseeanonymousfunctionsinFlutter,likethoseabove,passedaroundascallbacksforUIevents.Thislets youspecifythecodethatrunswhenauserdoessomething,likepressingabutton. Anothercommonplacewhereyou’llseeanonymousfunctionsiswithcollections.Youcangiveacollectionan anonymousfunctionthatwillperformsometaskoneachelementofthecollection.Forexample:
- 11. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 11/14 Comments Thereisnocomment. Addcomment // 1 final drinks = ['water', 'juice', 'milk']; // 2 final bigDrinks = drinks.map( // 3 (drink) => drink.toUpperCase() ); // 4 print(bigDrinks); // (WATER, JUICE, MILK) Let’slookateachstep: 1.Definealistof drinks thathavelowercaseletters. 2..map takesallthelistvaluesandreturnsanewcollectionwiththem. 3.Ananonymousfunctionispassedasaparameter.Inthatanonymousfunction,youhavea drink argumentthat representseachelementofthelist. 4.Thebodyoftheanonymousfunctionconvertseachelementtouppercaseandreturnsthevalue.Sincetheoriginal listisalistofstrings, drink alsohastype String. Usingananonymousfunctionandcombiningitwith .map isaconvenientwaytotransformonecollectionintoanother. Note:Don’tconfusethe .map methodwiththe Map type. source:raywenderlich.com Be the first of your friends to like this. Like Share Login&Comment Dartfunction-FunctionsWithinFunctions 11Feb2022,22:26pm Dartfunction-NamedOptionalParameters 11Feb2022,22:18pm Dartfunction-PositionalOptionalParameters 11Feb2022,22:07pm
- 12. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 12/14 Dartfunction-Single-LineFunctions 11Feb2022,22:00pm Dartfunction-FunctionReturnValues 11Feb2022,21:50pm Dartfunction-MultipleParameters 11Feb2022,21:45pm Dartfunction-TheLocalityofVariables 11Feb2022,21:37pm Flutter-CreatingaFlutterplatformclient 11Feb2022,21:16pm DartMap/Object-returnsacopyoftheprovidedmap-dartMap.from() 09Feb2022,23:46pm DartMap/Object-Updatesthevalueforthegivenkey-dartupdate() 09Feb2022,23:43pm DartMap/Object-Removeseverykey/valuepairinthemap-dartclear() 09Feb2022,23:40pm DartMap/Object-Removesthekey/valuepairifthegivenconditionistrue-dartremoveWhere() 09Feb2022,23:38pm DartMap/Object-Removestheprovidedkeyanditsassociatedvalue-dartremove() 09Feb2022,23:37pm DartMap/Object-Addsakey/valuepairifnon-existent-dartputIfAbsent() 09Feb2022,23:36pm DartMap/Object-Runsthegivenfunctionovereachkey/valuepair-dartforEach() 09Feb2022,23:33pm DartMap/Object-Checkswhetherthegivenvalueexists-dartcontainsValue() 09Feb2022,23:31pm DartMap/Object-Checkswhetherthegivenkeyexists-dartcontainKey() 09Feb2022,23:29pm DartMap/Object-tomergeallkey/valuepairsoftheprovidedmaptothecurrentone-addAll() 09Feb2022,23:25pm Dartarray-Expandseachelementintozeroormoreelements 09Feb2022,23:21pm Dartarray-Createsanewlistfromthegivencollection 09Feb2022,23:19pm Dartarray-Returnsacollectionwhileincludingorskippingelements-take()-skip() 09Feb2022,23:17pm Dartarray-where(),firstWhere(),singleWhere() 09Feb2022,23:15pm Dartarray-every() 09Feb2022,23:12pm Dartarray-Compressestheelementstoasinglevalue 09Feb2022,23:04pm Dartarray-Ordertheelementsbasedontheprovidedorderingfunction 09Feb2022,23:03pm Dartarray-Checkstoconfirmthatthegivenelementisinthelist 09Feb2022,23:01pm Dartarray-Producesanewlistaftertransformingeachelementinagivenlist
- 13. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 13/14 09Feb2022,23:00pm Dartarray-Runsafunctiononeachelementinthelist 09Feb2022,22:57pm Dartstring-splitsthegivenstring,convertsitspartsperrequirement,andcombinesthemintoanewstring-dart splitMapJoin 09Feb2022,21:17pm Dartstring-tocompareoneinputwithotherandreturnsaninteger-dartcompareTo 09Feb2022,21:14pm Dartstring-tomatchethepatternagainstthegivenstring-dartallMatches 09Feb2022,21:12pm Dartstring-toreplacefirstfewcharactersfromagivenstring 09Feb2022,21:08pm Dartstring-toreplacepartofgivenstringthatmatchesthepatternwithnewstring-dartreplaceAll 09Feb2022,21:06pm Dartstring-tosplitgivenstringintosubstring-dartsplit 09Feb2022,21:04pm Dartstring-returnsaboolwhenthestringmatchesthepattern-dartstartsWith 09Feb2022,21:01pm Dartstring-tocapitalizethefirstletterofastring-substring 09Feb2022,20:58pm Dartbasic-Function 09Feb2022,20:36pm Dartbasic-Maps 09Feb2022,20:29pm Dartbasic-Collection-list 09Feb2022,20:25pm Dartbasic-Controlflow-ForLoops 09Feb2022,20:11pm Dartbasic-Controlflow-WhileLoops 09Feb2022,20:04pm Dartbasic-Controlflow-IfStatements 09Feb2022,19:56pm Dartbasic-Nullability 09Feb2022,19:53pm Dartbasic-Immutability 09Feb2022,19:51pm Dartbasic-Strings 09Feb2022,19:48pm Dartbasic-LogicalOperators 09Feb2022,19:45pm Dartreturnsremainingiterablefromgivenlist-dartskip() 09Feb2022,19:41pm Dartreturnsthecountprovidedfromgivenlist-darttake 09Feb2022,19:39pm Dartreturnsabooleandependinguponwhetherallelementssatisfiestheconditionornotinthegivenlist-dartevery 09Feb2022,19:38pm Dartreturnsabooleandependinguponwhetheranyelementsatisfiestheconditionornotinthegivenlist-dartany
- 14. 2/12/22, 1:16 PMDart function - Recursive Functions https://softwarezay.com/notes/396_dart-function-recursive-functions 14/14 Copyright© 2022 .Allrightsreserved. 09Feb2022,19:34pm