IntroductiontoPerl BasedLooselyonLearningPerl,2ndEd byRandalSchwartz&TomChristiansen Cogs,FlywheelsandRubberbands (version1.2) ITComputingServices February1998 February2002 PartI expression,decisionsandflowcharting if while 3 Housecleaningitem: print(…);vs.printf…; • Upuntilnow,Ihavebeenusingprintinthefollowingway: print(”blah”); butyoucanalsoomittheparentheses: print”blah”; whichisniceandsimple. IwouldprefertoincludetheparenthesesbecauseitismoreC-likeand lessambiguous(inmymind)aboutwhattheparametersare. • Iwouldalsoliketobeginusingtheprintf()functioninsteadofthe print()function.Asyouwillseelater,printf()ismore flexiblethanprint(). 4 HouseCleaning:OtherOperatorsI “Neglected”toMention • Ihadtalkedaboutoperatorslasttimesuchas“+”,“-”,“.” and“=”.ThereisaclassofoperatorsthatIhaveleftout: therelationaloperators: NumericalRelationalOperators == equal != notequal < lessthan > morethan <= lessthanorequalto >= greaterthanorequalto 5 …Operators(cont’d) StringRelationalOperators eq equal ne notequal lt lessthan gt morethan le lessthanorequalto ge greaterthanorequalto andtherewillbemore. • Arelationaloperatormakesacomparisonbetweentwo valuesandreturnsaTRUEorFALSE. 6 ExamplesofUsing RelationalOperators • Suppose$a=3,$b=4,$i=1and $first=“Bill”,$last=“Clinton”,$full=“BillClinton” expression result expression result $a==3 TRUE $b>=$a TRUE $a>$b FALSE $b-$a==$i TRUE expression result $firsteq“Bill” TRUE $first.““.“$lasteq“BillClinton” TRUE $last<$first FALSE 7 Whatistruth? • Inotherwords,whatisconsideredTRUE? TRUEisdefinedinPerl(aswellasmanyotherlanguages) asanon-zerovalue: expression result 1==1 TRUE 1==0 FALSE 1 TRUE 3 TRUE 0 FALSE 8 ASlightbutImportant Diversion:Flowcharts • Giveusavisualrepresentationormappingofhowa programflows. • Flowchartingisjustaninterestingwayofthinkinghowto program. • It’sausefultoolespeciallyforlearning,butasyouwill soonsee,itcan’tbescaledupintofull-blownprograms. Youcantry... 9 AProcessStatement m a r g o r p e h t dosomething f o w o l f • Withprocessstatements,wedosomething. Agoodexampleofoneistheprintf statement:printf(“Hellothere.\n”); 10 Grouping: BracesaroundBodiesofBoxes • Therewillbetimeswhen wewanttogroupmany m a processstatements.We r pullout g o usuallydothisbecausewe cigarette r p aregroupingsmaller e h t light processestoformalarger f o cigarette process.Andtodothis,we w o l usuallyusecurly-braces: puff f {and}. cigarette 11 TheDecisionBox 1 2 • Upuntilnoweverystatement y y t t i thatwe’veseenandusedhas i l l i i b b i beenonepath.So,the i s s s s o programswe’vewrittenhave o p p beenlikealongriver:very w w o o l linear.Prettyboring. l f f Test • AndtheDecisionBoxchanges False theflowofaprogram.Thisis alotlikeaforkedriver. True 12 Example:ASimpleDecison withOutcome • DecisionsShouldHave Outcomes!Adecision withoutanoutcomeisnota $nameeq F usefuldecision! “Barry” T • Noticetheelementsofthe this:DecisionandOutcome whenTRUE. printf(“Hello Barry.\n”); • Whathappensifthedecision isnotTRUE? 13 ADecisionw/OutcomeinPerl • Noticetheextrasyntaxremindersaddedhere! Noticethecurly-bracesforthebodiesof processstatements.Noticealsothe $nameeq F parenthesesthatsurroundthetest.Always “Barry” surroundthetestwithparentheses! ( ) T • AdecisionwithanoutcomeinPerltakesthe form: if(expr){body} printf(“Hello Barry.\n”); • Andasinourexample: if($nameeq“Barry”){ printf(“HelloBarry.\n”); } AMoreComplicatedDecision: 14 TwoOutcomes Whatdoesthisdo? $nameeq F “Barry” T printf(“You’re notBarry.\n”); printf(“Hello Barry.\n”); ...AndnowinPerl 15 if(expr){body1}else{body2} $nameeq F ( “Barry” ) T printf(“You’re notBarry.\n”); printf(“Hello Barry.\n”); • andwewritethisinPerlas: if($nameeq“Barry”){ printf(“HelloBarry.\n”); }else{ printf(“You’renotBarry.\n”); } 16 Bewareofthe TwoTypesofEquality • This $a==3 Thistestsforequality. isdifferentfrom $a=3 Thisassignsthevalueof3to$a. • So,don’teverwrite: if($a=3){ printf(“aisnow3!\n”); } (Makethis$a==3instead!) (Imadethisdumbmistakewhenwritingthispresentation.) 17 BewareofAnother KindofEquality • This $name==“Barry” Thistestsforequalityfor numbersandiswrong!. isdifferentfrom $nameeq“Barry” Thisassignstheequalityin stringsandiscorrect. • So,don’teverwrite: if($name==“Barry”){ printf(“HelloBarry!\n”); } (Imadethismistaketoowhenwritingthispresentation.) 18 Excerciseswithif’s • 1)Writethecodefragmentforthisrather contrivedandmeaninglessexample: T $catch printf( !=22 “Rock.\n”) F printf( “Hard place.\n”); Andhowdoesitwork? Whathappensif$catchis11?22?0? 19 Excerciseswithif’s(cont’d) • 2)Whatifyoudidnothaveelseinyourlanguagelikesomeolder computerlanguages?Rewritethiscodefragment: if($nameeq“Barry”){ printf(“HiBarry.\n”); }else{ printf(“You’renotBarry.\n”); } • 3)Supposeyouhadthisvariablecalled$indian,andyouhadto printftheword“One”ifthevalueof$indianis1and“Two”ifthe valueof$indianis2andsoonupto5.Writethecodefragment forthat. 20 ANew&MoreInterestingKindofDecisionStructure $num_indian =0 T printf( $num_indian $num_indian “$num_indian !=10 ++ ( ){ littleindian.\n}”) •Whatwillthisdo? F printf( “Tenlittle indianboys.\n”) •Hereisanotherwaywhichwecan divergefromthelinearwayof programming.Theifstatementallowedustojustbranchoffto anotherpossibleflow.Thisnewwayallowsustoreturntothe decisionagainsometimelater.Thisisknownasaloop. 21 Thewhile(expr){body}Loop $num_indian =0 T printf( $num_indian $num_indian “$num_indian !=10 ++ ( ){ littleindian.\}n”) F printf( “Tenlittle • Andthistypeofloopisknown indianboys.\n”) asthewhileloop.HereitisinPerl: • $num_indian=0; while($num_indian!=10){ $num_indian++; printf(“$num_indianlittleindian.\n); } printf(“Tenlittleindianboys.”); FourAreasofInterestofthe 22 while(expr){body}Loop Area1 Initialization BodyofLoop T Area4 Area2 Area3 Incrementor Test ) Dosomething UpdateState ( { ofLoop } F Outsideof Loop • Area1:Beforetheloopstars,startoffwithsomeset-uporinitial conditions. Area2:Makeatest,gointoloop(again)orcontinueon? Area3:Dosomething(whateveryouwrotetheloopfor) Area4:Incrementorchangethestateoftheloop. OutsidetheLoop:Loopfinishes.Notveryinteresting.Sometimes neededtorepackagetheanswerforanotherpartofthecode. FourAreasofInterestofthe 23 while(expr){body}Loop(cont’d) Area1 Initialization BodyofLoop T Area4 Area2 Area3 Incrementor Test ) Dosomething UpdateState ( { ofLoop } F Outsideof Loop • SometimesArea4mightseemlikeitcanbeincludedinArea3,butit servesanimportantenoughpartthatitcanstandonitsown.Area4is likethehousekeepingpartoftheloop. YoucouldsaythatArea3doesallthe“important”workoftheloop andArea4hastheresponsibilityofkeepingtheloopgoing. 24 There’smorethanonewayto writealoop... • TheexampleoftheTen-little-indianprogramdoesn’t fitourexample.Noticethattheincrementisatthe beginningoftheloop;andmymodelofloopsputsit attheend. Let’srewritetheloopsothatitdoesfollowthatform. There’sagoodreasonforthis.I’llsaymore 25 AMatterofStyle #!/opt/local/bin/perl I(personally)liketowritemy $num_indian=0; perlscriptsthisway.This followsastyleinCcalled while($num_indian!=10) { K&Rstyle.Noticethe $num_indian++; printf(“$num_indianlittleindian.\n); beginningbracebeginsonthe endofthelineofthetestand } theclosingbracelinesupwith printf(“Tenlittleindianboys.”); thelineofthetest.It’sgreatat conservingscreenspace. #!/opt/local/bin/perl $num_indian=0; while($num_indian!=10) Thisisalsoaperfectlyvalidwayto indentyourprograms.Andthisis { morelogicalforbeginners.A $num_indian++; beginningbracebeginsonthenext printf(“$num_indianlittleindian.\n); lineofaloopandtheclosingbrace linesupwiththebeginningbrace. } Butyou’llseemeuseK&Rstyle. printf(“Tenlittleindianboys.”); Sorry. 26 AMatterofStyleII Styleisnotonlycurly-braces.Indentationishelpsyoukeeptrackof multipleloops.Wewon’tgetintothisnow,butIwanttosufficiently scareyouintogoodhabits.Noticethetwoloops(oneinsidetheother) andhoweasyitistoseethem: MainProgram $minute=10; while($minute!=0){ OuterLoop printf(“Beginningminute$minute.\n”); second=60; while($second!=0){ InnerLoop printf(“\tsecond$second.\n”); $second--; } print(“Endingminute$minute.\n”); $minute--; } Indentyourprogramstoo!!!Oryou’llbesorry... 27 while(expr){body}Benchpresses 1)Writethecodefragmentforthisflowchart: printf(“Theshuttle ispreparing fortakeoff.\n”); $time=30; T printf(“Tminus $time!=0 $timeand $time--; ( ){ counting.\n); } F Whatnewconceptisin printf(“Blast thisfragment/flowchart? off!\n); 28 while(expr){body}Benchpresses: MoreReps • 2)Havetheuserenterintennumbersandhavethe computercomputethesumandaverageofthoseten numbers. • 3)Dothesameexcepttheusergetstochoosehowmany numbersthecomputermustsumandaverage.I’mleaving thisonevague. 29 AllYouNeediswhileandif • Youcandojustabouteverythingyouwantwith onlywhile(expr){body}and if(expr){body}constructs! • Allotherloopconstructswewillbetalkingabout arevariantsoforaremadeofdifferent combinationsofwhileandif.Sometimesit takesmoreworkandyouhavetobecreativeabout howyoucodesomesituations…,butitcanbe done!Wewilltalkaboutthevariantssoon. 30 SuggestedReadings • Chapter4:ControlStructures(2ndEd) sections: StatementBlocks(pg.58) Theif/unlessStatement(pgs.59-60) Skipthepartaboutunless;we’llcomebacklater. Thewhile/untilStatement(pg.61) Skipthepartaboutuntil;we’llcomebacklater. Skipthesectiononforandforeach,that’snexttime&later. 31 SuggestedReadings • Chapter2:ScalarData(3rdEd) sections: ComparisonOperators(pg.33) ifControlStructure(pg.34) whileControlStructure(pg.37) BooleanValues(pg.34) GettingUserInput(pg.35) 32 SuggestedHomework Sendsolutions(codeandoutput)[email protected] • PROBLEM0:Typeineveryprograminthispresentation andtrythemout. 1littleindian • PROBLEMS1,2,3:DoinLearningPerl,Chapter4: 2littleindian 3littleindian pg.65(2nded.) 4littleindian