Chapter2

BeginningtoPrograminC++2.1Constants(常量)DatainC++：Constants；variablesConstants：doesnotchangeitsvalueinaprogram.整型：int浮點型：float字符型：char字符串型：String2.2Variables(變量)Variables：canvaryitsvaluesinaprogramavariablemustbedefinedbeforeitcanbeused.Avariableisdefinedbygivingitadatatypeandanamemain()marksthepointwhereaC++programstartstoexecuteandmustappearonceTheprogramstatementsarecontainedwithinthebraces{and}Eachstatementendswith’;’definesvariables’nameandtypeAssignvaluetovariables2.2VariablesAvariablecanbegivenanyname,calledanidentifier

inC++,provideditiswithinthefollowingrulesAnidentifiercanonlybeconstructedusingletters,numeralsorunderscores(_).Anidentifiermuststartwithaletteroranunderscore.AnidentifiercannotbeaC++keyword.Akeywordisawordthathasaspecialmeaning.

Anidentifiercancontainanynumberofcharacters,butonlythefirstthirty-onecharactersaresignificanttotheC++compiler.Avariableislikeaboxinthememoryofthecomputer.2.3Simpleoutputtothescreencout#include<iostream.h>directivecausesthefiletomakeavailabletotheprogram；<iostream.h>containssomeC++statementstomakeiteasytoperforminputandoutputcout：sendsthedatatocoutthestreaminsertionoperator<<isusedtoinsertastringofcharactersintotheoutputstreamobject

coutendl：gothestartofthenextlineonthescreenastringofcharacters:lettersornumbersenclosedin“”Thevariablenameisnotinquotationmarks2.4Comments(注釋)CommentsareaddedtoaC++programtomakeitmorereadablefortheprogrammercompletelyignoredbythecompiler.commentsstartwiththecharacters//andendattheendoftheline.2.4CommentsC++commentscannotspanmorethanoneline.Typically,commentsareplacedatthestartoftheprogramtodescribethepurposeoftheprogram,theauthor,datewrittenandanyotherrelevantinformationC-stylecommentsbeginwiththecharacters/*andendwiththecharacters*/isalsousedinC++Getintothehabitofusingcommentsbadcommentscanleadtoconfusion2.5Datatypes（數(shù)據(jù)類型）Differentdatatypesrequiredifferentamountsofmemoryandthereforevaryintherangeofvaluestheycanstore.shortinteger：短整型LongIntegerdatatypes:長整型Booleandatatypes：布爾數(shù)據(jù)類型canstoreonlyoneoftwovalues:true（1）orfalse（0）DoubleFloating-pointdatatypes：increasetherangeandprecision(oraccuracy)ofafloating-pointnumber.2.6Datatypesizessizeof

operatortodisplaythenumberofbytesofmemoryrequiredbysomecommondatatypes.runningresults2.7Operators(運算符)2.7.1Theassignmentoperator(賦值運算符)“=”assignvaluestovariables.2.7.2Arithmeticoperators(代數(shù)運算符)%:

取余運算符,獲取除法運算的余數(shù)2.7OperatorsProgramExampleP2Erunningresults2.7Operators2.7.3Increment（自加）、decrement（自減）operators++：adds1tothevalueofavariable--：subtracts1fromthevariablevar1Theincrementoperator++hastwoforms,prefixandpostfix2.7OperatorsProgramExampleP2FLine9ofthisprogramdefinesthevariablesvar1andvar2asintegersandinitialisesthemto1and2,respectively.Line15adds1tovar1,andline16subtracts1fromvar2.Lines19and20displaythefinalvalueofvar1andvar2.2.7OperatorsProgramExampleP2G運行結果prefix----Ifthe++isbeforeavariable,thevariableisincrementedbeforeitisused.postfix----Ifthe++isafterthevariable,thevariableisusedandthenincremented.Thedecrementoperator--alsohasprefixandpostfixforms.2.7Operators2.7.4CombinedassignmentoperatorsThe+=operatoraddsthevalueonitsrighttothevariableonitsleft.Therearefivecombinedassignmentoperators:+=,-=,*=,/=,and%=,correspondingtothefivearithmeticoperators+,-,*,/,and%.2.8Operatorprecedence（運算符的優(yōu)先級）Considerthefollowingstatement:2+7*8=72var2+7*8=58varOperatorprecedence×√1）*，/haveahigherprioritythan+、-2）Expressionscontainingoperatorsofthesameprecedenceareevaluatedaccordingtotheirassociativity3）use（）tochangetheorderofevaluation2.8OperatorprecedenceUsingparentheseswillremoveanyconfusionaboutwhichoperationsaredonefirst.unaryminus：“-”appearsbeforeanoperandhighestprioritybinaryminus：appearsbetweentwooperands2.9Typeconversionsandcasts（類型轉換和強轉)Automaticconversion-----ProgramExampleP2HrunningresultsForcalculationsinvolvingmixeddatatypes,C++automaticallyconvertsthevalue.2.9TypeconversionsandcastsWhendoingcalculationsinvolvingmixeddatatypes,C++ranksthedatatypesinthisorder:promotionorwidening

ofthedataForcalculationsinvolvingmixeddatatypes,C++automaticallyconvertsthevalueinthelowerdatatypetoahighertype.Promotionwillcausenolossofdata,becausethehigherdatatypesoccupymorememorythanthelowertypesandcanthereforeholdthedataprecisely.2.9TypeconversionsandcastsdemotionornarrowingofthedatawhendataisassignedtoavariableofalowertypeDemotionmayresultinalossofdata,becausethelowerdatatypemaynothaveenoughbytesofmemorytostorethehigherdatatype.2.9TypeconversionsandcastsManualconversionwithastaticcastUsingastaticcastisequivalenttoassigningtheexpressiontoavariableofthespecifiedtypeandusingthatvariableinplaceoftheexpression.Theexpressionvar1/4.0ismixed,andthevalueofvar1isthereforepromotedtoafloatingpointvalueautomatically.Programmingpitfalls1.Donottypeasemicolonaftereither2.EndeachC++statementwithasemicolon.3.Asemicolonisnotalwaysattheendofaline.Thiswillcauseacompilererror,becausethesemicolonispartofthecomment.Programmingpitfalls4.Atypingerrormayresultinastatementthatdoesnothingbutthatisvalidnonetheless.Programmingpitfalls5.Youmusti