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6/23/2015 ThePartsofaC++Program

http://aelinik.online.fr/cpp/ch02.htm#Heading2 1/10

Day2ThePartsofaC++Program

ASimpleProgramListing2.1.HELLO.CPPdemonstratesthepartsofaC++program.ABriefLookatcoutListing2.2.Usingcout.Comments

TypesofCommentsUsingComments

Listing2.3.HELP.CPPdemonstratescomments.CommentsattheTopofEachFileAFinalWordofCautionAboutComments

FunctionsListing2.4.Demonstratingacalltoafunction.

UsingFunctionsListing2.5.FUNC.CPPdemonstratesasimplefunction.SummaryQ&AWorkshop

QuizExercises

Day2

ThePartsofaC++Program

C++programsconsistofobjects,functions,variables,andothercomponentparts.Mostofthisbookisdevotedtoexplainingthesepartsindepth,buttogetasenseofhowaprogramfitstogetheryoumustseeacompleteworkingprogram.Todayyoulearn

ThepartsofaC++program.

Howthepartsworktogether.

Whatafunctionisandwhatitdoes.

ASimpleProgram

EventhesimpleprogramHELLO.CPPfromDay1,"GettingStarted,"hadmanyinterestingparts.Thissectionwillreviewthisprograminmoredetail.Listing2.1reproducestheoriginalversionofHELLO.CPPforyourconvenience.



Listing2.1.HELLO.CPPdemonstratesthepartsofaC++program.

1:#include2:3:intmain()4:{5:cout



AllANSIcompliantprogramsdeclaremain()toreturnanint.Thisvalueis"returned"totheoperatingsystemwhenyourprogramcompletes.Someprogrammerssignalanerrorbyreturningthevalue1.Inthisbook,main()willalwaysreturn0.

Themain()functionendsonline7withtheclosingbrace.

ABriefLookatcout

OnDay16,"Streams,"youwillseehowtousecouttoprintdatatothescreen.Fornow,youcanusecoutwithoutfullyunderstandinghowitworks.Toprintavaluetothescreen,writethewordcout,followedbytheinsertionoperator(



Threevaluesarepassedtocoutonline7,andeachvalueisseparatedbytheinsertionoperator.Thefirstvalueisthestring"Hereis5:".Notethespaceafterthecolon.Thespaceispartofthestring.Next,thevalue5ispassedtotheinsertionoperatorandthenewlinecharacter(alwaysindoublequotesorsinglequotes).Thiscausestheline

Hereis5:5

tobeprintedtothescreen.Becausethereisnonewlinecharacterafterthefirststring,thenextvalueisprintedimmediatelyafterwards.Thisiscalledconcatenatingthetwovalues.

Online8,aninformativemessageisprinted,andthenthemanipulatorendlisused.Thepurposeofendlistowriteanewlinetothescreen.(OtherusesforendlarediscussedonDay16.)

Online9,anewformattingcharacter,\t,isintroduced.Thisinsertsatabcharacterandisusedonlines812tolineuptheoutput.Line9showsthatnotonlyintegers,butlongintegersaswellcanbeprinted.Line10demonstratesthatcoutwilldosimpleaddition.Thevalueof8+5ispassedtocout,but13isprinted.

Online11,thevalue5/8isinsertedintocout.Theterm(float)tellscoutthatyouwantthisvalueevaluatedasadecimalequivalent,andsoafractionisprinted.Online12thevalue7000*7000isgiventocout,andtheterm(double)isusedtotellcoutthatyouwantthistobeprintedusingscientificnotation.AllofthiswillbeexplainedonDay3,"VariablesandConstants,"whendatatypesarediscussed.

Online14,yousubstitutedyourname,andtheoutputconfirmedthatyouareindeedaC++programmer.Itmustbetrue,becausethecomputersaidso!

Comments

Whenyouarewritingaprogram,itisalwaysclearandselfevidentwhatyouaretryingtodo.Funnything,thoughamonthlater,whenyoureturntotheprogram,itcanbequiteconfusingandunclear.I'mnotsurehowthatconfusioncreepsintoyourprogram,butitalwaysdoes.

Tofighttheonsetofconfusion,andtohelpothersunderstandyourcode,you'llwanttousecomments.Commentsaresimplytextthatisignoredbythecompiler,butthatmayinformthereaderofwhatyouaredoingatanyparticularpointinyourprogram.

TypesofComments

C++commentscomeintwoflavors:thedoubleslash(//)comment,andtheslashstar(/*)comment.Thedoubleslashcomment,whichwillbereferredtoasaC++stylecomment,tellsthecompilertoignoreeverythingthatfollowsthiscomment,untiltheendoftheline.

Theslashstarcommentmarktellsthecompilertoignoreeverythingthatfollowsuntilitfindsastarslash(*/)commentmark.ThesemarkswillbereferredtoasCstylecomments.Every/*mustbematchedwithaclosing*/.

Asyoumightguess,CstylecommentsareusedintheClanguageaswell,butC++stylecommentsarenotpartoftheofficialdefinitionofC.

ManyC++programmersusetheC++stylecommentmostofthetime,andreserveCstylecommentsforblockingoutlargeblocksofaprogram.YoucanincludeC++stylecommentswithinablock



"commentedout"byCstylecommentseverything,includingtheC++stylecomments,isignoredbetweentheCstylecommentmarks.

UsingComments

Asageneralrule,theoverallprogramshouldhavecommentsatthebeginning,tellingyouwhattheprogramdoes.Eachfunctionshouldalsohavecommentsexplainingwhatthefunctiondoesandwhatvaluesitreturns.Finally,anystatementinyourprogramthatisobscureorlessthanobviousshouldbecommentedaswell.

Listing2.3demonstratestheuseofcomments,showingthattheydonotaffecttheprocessingoftheprogramoritsoutput.

Listing2.3.HELP.CPPdemonstratescomments.

1:#include2:3:intmain()4:{5:/*thisisacomment6:anditextendsuntiltheclosing7:starslashcommentmark*/8:cout



Arevisionhistory(notesoneachchangemade).

Whatcompilers,linkers,andothertoolswereusedtomaketheprogram.

Additionalnotesasneeded.

Forexample,thefollowingblockofcommentsmightappearatthetopoftheHelloWorldprogram.

/************************************************************

Program:HelloWorld

File:Hello.cpp

Function:Main(completeprogramlistinginthisfile)

Description:Printsthewords"Helloworld"tothescreen

Author:JesseLiberty(jl)

Environment:TurboC++version4,486/6632mbRAM,Windows3.1DOS6.0.EasyWinmodule.

Notes:Thisisanintroductory,sampleprogram.

Revisions:1.0010/1/94(jl)Firstrelease1.0110/2/94(jl)Capitalized"World"

************************************************************/

Itisveryimportantthatyoukeepthenotesanddescriptionsuptodate.Acommonproblemwithheaderslikethisisthattheyareneglectedaftertheirinitialcreation,andovertimetheybecomeincreasinglymisleading.Whenproperlymaintained,however,theycanbeaninvaluableguidetotheoverallprogram.

Thelistingsintherestofthisbookwillleaveofftheheadingsinanattempttosaveroom.Thatdoesnotdiminishtheirimportance,however,sotheywillappearintheprogramsprovidedattheendofeachweek.

AFinalWordofCautionAboutComments

Commentsthatstatetheobviousarelessthanuseful.Infact,theycanbecounterproductive,becausethecodemaychangeandtheprogrammermayneglecttoupdatethecomment.Whatisobvioustoonepersonmaybeobscuretoanother,however,sojudgmentisrequired.

Thebottomlineisthatcommentsshouldnotsaywhatishappening,theyshouldsaywhyitishappening.

DOaddcommentstoyourcode.DOkeepcommentsuptodate.DOusecommentstotellwhatasectionofcodedoes.DON'Tusecommentsforselfexplanatorycode.

Functions

Whilemain()isafunction,itisanunusualone.Typicalfunctionsarecalled,orinvoked,duringthecourseofyourprogram.Aprogramisexecutedlinebylineintheorderitappearsinyoursourcecode,



untilafunctionisreached.Thentheprogrambranchesofftoexecutethefunction.Whenthefunctionfinishes,itreturnscontroltothelineofcodeimmediatelyfollowingthecalltothefunction.

Agoodanalogyforthisissharpeningyourpencil.Ifyouaredrawingapicture,andyourpencilbreaks,youmightstopdrawing,gosharpenthepencil,andthenreturntowhatyouweredoing.Whenaprogramneedsaserviceperformed,itcancallafunctiontoperformtheserviceandthenpickupwhereitleftoffwhenthefunctionisfinishedrunning.Listing2.4demonstratesthisidea.

Listing2.4.Demonstratingacalltoafunction.

1:#include2:3://functionDemonstrationFunction4://printsoutausefulmessage5:voidDemonstrationFunction()6:{7:cout



Aparameterisadeclarationofwhattypeofvaluewillbepassedintheactualvaluepassedinbythecallingfunctioniscalledtheargument.Manyprogrammersusethesetwoterms,parametersandarguments,assynonyms.Othersarecarefulaboutthetechnicaldistinction.Thisbookwillusethetermsinterchangeably.

Thebodyofafunctionconsistsofanopeningbrace,zeroormorestatements,andaclosingbrace.Thestatementsconstitutetheworkofthefunction.Afunctionmayreturnavalue,usingareturnstatement.Thisstatementwillalsocausethefunctiontoexit.Ifyoudon'tputareturnstatementintoyourfunction,itwillautomaticallyreturnvoidattheendofthefunction.Thevaluereturnedmustbeofthetypedeclaredinthefunctionheader.

NOTE:FunctionsarecoveredinmoredetailonDay5,"Functions."Thetypesthatcanbereturnedfromafunctionarecoveredinmoredet+[radical][Delta][infinity]onDay3.Theinformationprovidedtodayistopresentyouwithanoverview,becausefunctionswillbeusedinalmostallofyourC++programs.

Listing2.5demonstratesafunctionthattakestwointegerparametersandreturnsanintegervalue.Don'tworryaboutthesyntaxorthespecificsofhowtoworkwithintegervalues(forexample,intx)fornowthatiscoveredindetailonDay3.

Listing2.5.FUNC.CPPdemonstratesasimplefunction.

1:#include2:intAdd(intx,inty)3:{4:5:cout



ThefunctionAdd()isdefinedonline2.Ittakestwointegerparametersandreturnsanintegervalue.Theprogramitselfbeginsonline9andonline11,whereitprintsamessage.Theprogrampromptstheuserfortwonumbers(lines13to15).Theusertypeseachnumber,separatedbyaspace,andthenpressestheEnterkey.main()passesthetwonumberstypedinbytheuserasargumentstotheAdd()functiononline17.

ProcessingbranchestotheAdd()function,whichstartsonline2.Theparametersaandbareprintedandthenaddedtogether.Theresultisreturnedonline6,andthefunctionreturns.

Inlines14and15,thecinobjectisusedtoobtainanumberforthevariablesaandb,andcoutisusedtowritethevaluestothescreen.Variablesandotheraspectsofthisprogramareexploredindepthinthenextfewdays.

Summary

Thedifficultyinlearningacomplexsubject,suchasprogramming,isthatsomuchofwhatyoulearndependsoneverythingelsethereistolearn.ThischapterintroducedthebasicpartsofasimpleC++program.Italsointroducedthedevelopmentcycleandanumberofimportantnewterms.

Q&A

Q.Whatdoes#includedo?

A.Thisisadirectivetothepreprocessor,whichrunswhenyoucallyourcompiler.Thisspecificdirectivecausesthefilenamedafterthewordincludetobereadin,asifitweretypedinatthatlocationinyoursourcecode.

Q.Whatisthedifferencebetween//commentsand/*stylecomments?

A.Thedoubleslashcomments(//)"expire"attheendoftheline.Slashstar(/*)commentsareineffectuntilaclosingcomment(*/).Remember,noteventheendofthefunctionterminatesaslashstarcommentyoumustputintheclosingcommentmark,oryouwillgetacompiletimeerror.

Q.Whatdifferentiatesagoodcommentfromabadcomment?

A.Agoodcommenttellsthereaderwhythisparticularcodeisdoingwhateveritisdoingorexplainswhatasectionofcodeisabouttodo.Abadcommentrestateswhataparticularlineofcodeisdoing.Linesofcodeshouldbewrittensothattheyspeakforthemselves.Readingthelineofcodeshouldtellyouwhatitisdoingwithoutneedingacomment.

Workshop

TheWorkshopprovidesquizquestionstohelpyousolidifyyourunderstandingofthematerialcoveredandexercisestoprovideyouwithexperienceinusingwhatyou'velearned.TrytoanswerthequizandexercisequestionsbeforecheckingtheanswersinAppendixD,andmakesureyouunderstandtheanswersbeforecontinuingtothenextchapter.

Quiz



1.Whatisthedifferencebetweenthecompilerandthepreprocessor?

2.Whyisthefunctionmain()special?

3.Whatarethetwotypesofcomments,andhowdotheydiffer?

4.Cancommentsbenested?

5.Cancommentsbelongerthanoneline?

Exercises

1.WriteaprogramthatwritesIloveC++tothescreen.

2.Writethesmallestprogramthatcanbecompiled,linked,andrun.

3.BUGBUSTERS:Enterthisprogramandcompileit.Whydoesitfail?Howcanyoufixit?

1:#include2:voidmain()3:{4:cout