return is the equivalent of return. next is the equivalent of continue. last is the equivalent of break. 'if' statements are followed by blocks.
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WhatarethePerlequivalentsofreturnandcontinuekeywordsinC?
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IamgettingthiserrorwhenIusethiscode
subsearch{
my($a,@a_list)=@_;
foreach(@a_list){
if($_==$a)returnTRUE;
#continue;
}
returnFALSE;
}
syntaxerroratcode.plline26,near")return"
WhatistherightwaytoreturnTRUE?
Also,whatistherightwaytocontinue?
IknowIshouldbethinkingmoreinPerltermsthantryingtoconvertCcodetoPerlthisway.
perl
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editedAug26,2010at15:40
Lazer
askedAug26,2010at14:14
LazerLazer
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Yoursearchfunctioncanbecollapsedintoacalltoany:useList::MoreUtils'any';my$matched=any{$_eq$a}@a_list;
– Ether
Aug26,2010at16:37
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58
Equivalentsofreturn,continue,break
returnistheequivalentofreturn.
nextistheequivalentofcontinue.
lastistheequivalentofbreak.
'if'statementsarefollowedbyblocks
Yourproblemisthatanifstatementisalwaysfollowedbyablockinsidebraces.
if($_==$a){returnTRUE;}
elsif(...){...}
else{...}
Forallitsometimesseemsabitverbose,IagreewithysththatthisissomethingthatPerlgotright.Foraninterestingalternativetake,seetheGoprogramminglanguage,whichtreatstheparenthesesasunnecessaryandmandatesthebraces.
'if'canbeusedasaqualifier
Oryoucanusetheifasastatementmodifier:
returnTRUEif$_==$a;
Notethatinthiscase,youdon'thavetouseparenthesesaroundtheconditionalexpression,thoughthere'dbenoharminaddingthem.
Using'unless'
Youcanalsouseunlessinsteadofiftoinvertthecondition:
returnTRUEunless$_!=$a;
(And,asPhillipPotterpointedout,mixingunlesswithanegatedconditionmakescomprehensionharder;theexampleisdirectlydoingthesameasthequestion,butisbetterwrittenasanifwithequality.)
Using'next'and'last'
Youcanusenextandlastsimilarly:
subsearch{
my($a,@a_list)=@_;
foreach(@a_list){
returnTRUEif$_==$a;
lastif$_>$a;
nextif$ptom!=LAST_QUARTER;
...
}
returnFALSE;
}
Notethefixupintheforeachloop.(Questionamendedtoincludethefixup.)Makesureyoualwayshave'usestrict;'and'usewarnings;'atthetopofyourscript.Expertsalwaysusethemtomakesuretheyhaven'tmademistakes.Beginnersshouldusethemforexactlythesamereason.
TRUEandFALSE
Also,aspointedoutfirstinotheranswers,Perldoesnothavepre-definedconstantsTRUEandFALSE,anymorethanCorC++do(C++hasabuilt-intrueandfalse;C99hastrueandfalseconditionallyavailableifyou#include).YoucanprovidethedefinitionsforPerlas:
useconstantTRUE=>1;
useconstantFALSE=>0;
Bewary,though.Somethingswillbe'true'evenwhennotequaltoTRUE;otherthingswillbe'false'evenwhennotequaltoFALSE.
Discussionaboutuseof'$a'and'$b'
Thecommentscontainadiscussionaboutnotusing$aand$basvariables.Insequence,therelevantcommentswere:
Pleaseavoidusing$aunlessit'sinasortblock.–Zaid
@Zaid:Goodpointthat$aand$barespecialinthecontextofasortblock.I'mnotsurewhetherthereareedictsthattheyshouldneverbeusedotherwise-itwouldbeatrocioustousethemwhenthereisalsoasortblocklurkingaround,butintheabsenceofsortblocks,Idon'tseeanyreasontotreat$aanddifferentthan$z.–JonathanLeffler
$aand$bareglobals,andassuchbehavedifferentthanlexicals.–phaylon
@phaylon:well,strictlytheyare'packageglobals'(seePerlsort).Yes,whenyouaresorting,theyaredifferentfromlexicals(my)variables.Whenyouaren'tdoingsorting,thentheycanbetreatedaslexicalsifyoudeclarethemexplicitly.–JonathanLeffler
@JonathanLeffler,theyarealsoexemptfromusestrictqw(vars);soyoumightnotnoticethatyouaretramplingonthemfromanotherscope.–Ven'Tatsu
Pointingouttheobvious:Ionlyused$abecausethequestiondid-andratherthaninundatetheoriginalposterwithlotsofdetails,Ikeptmostlytothemainpoints.Forexample,thediscussionoflastandnextdoesnotmentionlooplabels.
Thatsaid,theadvice"avoidusing$aand$basvariables"issound;theyarespecialnamesforthereasonspointedout,andusingthemleavesopenthepossibilityofmistakesthatmayormaynotbebedetectable.
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editedMay23,2017at11:33
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answeredAug26,2010at14:18
JonathanLefflerJonathanLeffler
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...butdon'teveruseunlesswithanegativeconditionsuchasonewhichuses!or!=.It'sthecodeequivalentofadoublenegative,anditjustconfusespeople.
– PhilipPotter
Aug26,2010at14:25
4
Pleaseavoidusing$aunlessit'sinasortblock.
– Zaid
Aug26,2010at14:27
2
@Zaid:Goodpointthat$aand$barespecialinthecontextofasortblock.I'mnotsurewhetherthereareedictsthattheyshouldneverbeusedotherwise-itwouldbeatrocioustousethemwhenthereisalsoasortblocklurkingaround,butintheabsenceofsortblocks,Idon'tseeanyreasontotreat$aanddifferentthan$z.
– JonathanLeffler
Aug26,2010at14:31
1
@JonathanLeffler:$aand$bareglobals,andassuchbehavedifferentthanlexicals.
– phaylon
Aug26,2010at17:10
1
@JonathanLeffler,theyarealsoexemptfromusestrictqw(vars);soyoumightnotnoticethatyouaretramplingonthemfromanotherscope.
– Ven'Tatsu
Aug26,2010at21:55
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7
Yourquestionhasbeen(very!)comprehensivelyansweredbyJonathan,butIwantedtopointoutafewother,moreperlishwaystoreplaceyour"search"function.
usestrict;
usewarnings;
use5.010;
my@a_list=(1,2,3);
my$a=2;
#Ifyou'reusing5.10orhigher,youcanusesmartmatching.
#Notethatthisisonlyequivalentif$aisanumber.
#Thatis,"2.0"~~["1","2","3"]isfalse.
say$a~~@a_list;
#NeedtoinstallList::MoreUtils,notastandardmodule
useList::MoreUtilsqw(any);
sayany{$a==$_}@a_list;
#List::Utilisacoremodule
useList::Utilqw(first);
saydefinedfirst{$a==$_}@a_list;
#Nomodulesused,butlessefficient
saygrep{$a==$_}@a_list>0;
Formoreinfo,seethedocumentationforsmartmatching,List::MoreUtils,
List::Utilandgrep.
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editedAug26,2010at15:30
answeredAug26,2010at15:23
mschamscha
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Perldoesnothavebuiltinconstantsfortrueandfalse.Thecanonicalvaluefortrueis1andforfalseis()(whichisanemptylistinlistcontext,andundefinscalarcontext).
Amoreidomaticwaytowriteyourcodewouldbe:
subsearch{
my$key=shift;
foreach(@_){
return1if$_==$key;
}
return();
}
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editedAug26,2010at14:31
answeredAug26,2010at14:26
EricStromEricStrom
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