c language

Introduction

Clanguageisaprocess-orientedcomputerprogramminglanguage,whichisdifferentfromobject-orientedprogramminglanguages​​suchasC++,C#,andJava.ThedesigngoalofClanguageistoprovideaprogramminglanguagethatcancompileandprocesslow-levelmemoryinasimplemanner,generateonlyasmallamountofmachinecode,andrunwithoutanyoperatingenvironmentsupport.Clanguagedescribesproblemsfasterthanassemblylanguage,withlessworkload,betterreadability,easytodebug,modifyandtransplant,andthecodequalityisequivalenttoassemblylanguage.Clanguageisgenerallyonly10%-20%lessefficientthanthetargetprogramgeneratedbyassemblylanguagecode.Therefore,Clanguagecanwritesystemsoftware.

Atthecurrentstage,inthefieldofprogramming,Clanguageisusedverymuch.Ittakesintoaccounttheadvantagesofhigh-levellanguageandassemblylanguage,andhasgreateradvantagescomparedtootherprogramminglanguages.ComputersystemdesignandapplicationprogrammingaretwomajorareasofClanguageapplication.Atthesametime,theClanguageismoreuniversalandcanbeappliedinmanycomputeroperatingsystems,anditsefficiencyissignificant.

TheClanguagehasacompletetheoreticalsystemthathasgonethroughalonghistoryofdevelopment,andhasapivotalpositioninprogramminglanguages.

DevelopmentHistory

TheClanguagewasborninBellLabsintheUnitedStates.ItwasdesignedbyDennisMacAlistairRitchiebasedontheBlanguagedesignedbyKennethLaneThompson.Thefoundationdeveloped.Afteritsmaindesignwascompleted,ThompsonandRichusedittocompletelyrewriteUNIX.WiththedevelopmentofUNIX,theclanguagehasalsobeencontinuouslyimproved.InordertofacilitatetheoverallpromotionoftheClanguage,manyexperts,scholarsandhardwaremanufacturersjointlyformedtheCLanguageStandardsCommittee,andin1989,thefirstcompleteCstandardwasborn,referredtoas"C89",thatis,"ANSIC"Asof2020,thelatestClanguagestandardis"C18"releasedinJune2018.

TheClanguageisnamedCbecausetheClanguageisderivedfromtheBlanguageinventedbyKenThompson,andtheBlanguageisderivedfromtheBCPLlanguage.

In1967,MartinRichardsofCambridgeUniversitysimplifiedtheCPLlanguage,whichresultedintheBCPL(BasicCombinedProgrammingLanguage)language.

Inthe1960s,KennethLaneThompson(KennethLaneThompson),aresearcheratAT&TBellLaboratoriesintheUnitedStates,hadnothingtodowithhishands.Hewantedtoplaywithhim.Myownvideogamethatsimulatessailinginthesolarsystem-SpaceTravel.Withhisbossonhisback,hefoundanidlesmallcomputer-PDP-7.Butthiscomputerdoesnothaveanoperatingsystem,andthegamemustusesomeofthefunctionsoftheoperatingsystem,sohesetaboutdevelopinganoperatingsystemforthePDP-7.Later,thisoperatingsystemwasnamed-UNICS(UniplexedInformationandComputingService).

In1969,KenThompsonofBellLaboratories,USA,basedontheBCPLlanguage,designedaverysimpleandclosetothehardwareBlanguage(takethefirstletterofBCPL),andwrotethefirstversioninBlanguageTheUNIXoperatingsystem(calledUNICS).

In1971,DennisRich,whoalsolovesSpaceTravel,joinedThompson'sdevelopmentprojecttodevelopUNIXinordertoplaythegameearlier.HismainjobistotransformtheBlanguagetomakeitmoremature.

In1972,DennisRichofBellLabsintheUnitedStatesfinallydesignedanewlanguagebasedontheBlanguage.HetookthesecondletterofBCPLasthenameofthelanguage,ThisistheClanguage.

Inearly1973,themainbodyoftheClanguagewascompleted.ThompsonandRichcouldn'twaittouseittocompletelyrewriteUNIX.Atthistime,thejoyofprogrammingmadethemcompletelyforgetthe"SpaceTravel"anddevotedthemselvestothedevelopmentofUNIXandClanguage.WiththedevelopmentofUNIX,theClanguageitselfisconstantlyimproving.Until2020,variousversionsoftheUNIXkernelandperipheraltoolsstillusetheClanguageasthemaindevelopmentlanguage,andtherearemanycodesinheritedfromthehandsofThompsonandRich.

Duringdevelopment,theyarealsoconsideringportingUNIXtoothertypesofcomputers.ThestrongportabilityoftheClanguageappearshere.Machinelanguageandassemblylanguagearenotportable.Programsdevelopedforx86cannotrunonAlpha,SPARC,ARMandothermachines.TheClanguageprogramcanbeusedonprocessorsofanyarchitecture,aslongastheprocessorofthatarchitecturehasacorrespondingClanguagecompilerandlibrary,andthentheCsourcecodeiscompiledandlinkedintoatargetbinaryfile.Thearchitectureoftheprocessorruns.

In1977,DennisRichpublishedtheClanguagecompiledtext"PortableCLanguageCompiler"thatdoesnotdependonspecificmachinesystems.

TheClanguagecontinuestodevelop.In1982,manypeopleofinsightandtheAmericanNationalStandardsInstitute(ANSI)decidedtoestablishtheCStandardsCommitteetoestablishtheClanguagestandardinordertomaketheClanguagehealthy.Thecommitteeiscomposedofhardwaremanufacturers,compilerandothersoftwaretoolmanufacturers,softwaredesigners,consultants,academics,Clanguageauthorsandapplicationprogrammers.In1989,ANSIpublishedthefirstcompleteClanguagestandard-ANSIX3.159-1989,referredtoas"C89",butpeoplearealsoaccustomedtocallit"ANSIC".C89wasadoptedbytheInternationalStandardOrganization(ISO)unchangedin1990.TheofficialnamegivenbyISOis:ISO/IEC9899,​​soISO/IEC9899:1990isoftenreferredtoas"C90".In1999,aftermakingsomenecessaryamendmentsandimprovements,ISOissuedanewClanguagestandardnamedISO/IEC9899:1999,or"C99"forshort.OnDecember8,2011,ISOofficiallyreleasedanewstandard,calledISO/IEC9899:2011,or"C11"forshort.

Languagefeatures

Mainfeatures

Clanguageisastructuredlanguagewithclearlevelsandcanbeprogrammedinamodulemanner.Itisveryconducivetothedebuggingoftheprogram,andtheprocessingandperformancecapabilitiesoftheClanguageareverypowerful.Relyingonaverycomprehensiveoperatorandavarietyofdatatypes,theconstructionofvariousdatastructurescanbeeasilycompleted,andthememorycanbedirectlyaccessedthroughthepointertype.Addressinganddirectmanipulationofhardware,soitcanbeusedtodevelopsystemprogramsaswellasapplicationsoftware.ThroughtheresearchandanalysisoftheClanguage,themainfeaturesaresummarizedasfollows:

(1)Conciselanguage

Clanguagecontainsonly9kindsofcontrolstatements,keywordsThereareonly32.Theprogrammingrequirementsarenotstrictandmainlyinlowercaseletters,andmanyunnecessarypartshavebeenstreamlined.Infact,thesentencecompositionislessrelatedtothehardware,andtheClanguageitselfdoesnotprovidehardware-relatedinputandoutput,filemanagementandotherfunctions.Ifsuchfunctionsareneeded,theyneedtobeprogrammedwithvariouslibrariessupportedbythecompilationsystem.,Sotheclanguagehasaveryconcisecompilationsystem.

(2)Structuredcontrolstatements

Clanguageisastructuredlanguage,andthecontrolstatementsprovidedhavestructuredfeatures,suchasforstatements,if...Elsestatementandswitchstatement,etc.Itcanbeusedtorealizethelogiccontrolofthefunctionandfacilitatetheprocess-orientedprogramming.

(3)Richdatatypes

Clanguagecontainsawiderangeofdatatypes,includingnotonlytraditionalcharacter,integer,floating-point,arraytypesandotherdatatypes,Italsohasdatatypesthatotherprogramminglanguages​​donothave.Thepointertypedataisthemostflexibletouse,andvariousdatastructurescanbecalculatedthroughprogramming.

(4)Richoperators

TheClanguagecontains34operators.Ittreatsassignments,parentheses,etc.asoperatorstooperate,makingtheexpressiontypeoftheCprogramAndoperatortypesareveryrich.

(5)Thephysicaladdresscanbedirectlymanipulated

TheClanguageallowsdirectreadingandwritingofthehardwarememoryaddress,whichcanrealizethemainfunctionsofassemblylanguageandcanbedirectlymanipulatedhardware.Clanguagenotonlyhasthegoodcharacteristicsofhigh-levellanguages,butalsocontainsmanyadvantagesoflow-levellanguages,soithasawiderangeofapplicationsinthefieldofsystemsoftwareprogramming.

(6)Thecodehasgoodportability

Clanguageisaprocess-orientedprogramminglanguage,usersonlyneedtopayattentiontotheproblemtobesolved,withouthavingtospendtoomuchtimeMoreenergytounderstandtherelevanthardware,andfordifferenthardwareenvironments,thecodeisbasicallythesamewhenusingtheClanguagetoachievethesamefunction,andthetransplantationcanbecompletedwithoutoronlyafewchanges,whichmeansthatforacomputerThewrittenCprogramcanbeeasilyrunonanothercomputer,whichgreatlyreducestheworkintensityofprogramtransplantation.

(7)Programsthatcangeneratehigh-qualityandhigh-efficiencytargetcodeexecution

Comparedwithotherhigh-levellanguages,Clanguagecangeneratehigh-qualityandhigh-efficiencytargetcodes,soItisusuallyusedtowriteembeddedsystemprogramsthatrequirehighcodequalityandexecutionefficiency.

Uniquefeatures

Clanguageisthemostuniversallyapplicablecomputerprogrameditinglanguage.Itcannotonlyexertthefunctionsofhigh-levelprogramminglanguage,butalsohastheadvantagesofassemblylanguage.Therefore,ithasitsownuniquecharacteristicscomparedtootherprogramminglanguages.Itisembodiedinthefollowingthreeaspects:

First,extensiveness.ThesizeoftheoperationrangeoftheClanguagedirectlydeterminesitsprosandcons.Clanguagecontains34kindsofoperators,sothescopeofoperationisbeyondmanyotherlanguages,andtheexpressionoftheresultofitsoperationisalsoveryrich.Inaddition,theClanguagecontainsavarietyofdatastructureformssuchascharactertypeandpointertype,soitcanalsohandlelargerdatastructureoperations.

Second,simplicity.Ninetypesofcontrolstatementsand32keywordsarethebasicfeaturesoftheClanguage,whichmakesitwidelyapplicableincomputerapplicationprogramming.Itcannotonlyapplytotheoperationofthemajorityofprogrammers,improvetheirworkefficiency,butalsosupportAdvancedprogrammingavoidsthecumbersomelanguageswitching.

Thirdly,thestructureisperfect.Clanguageisastructuredlanguage.Itcanrealizemodularizedapplicationsintheformofmodularunits.Ithassignificantadvantagesinsystemdescription.Atthesametime,thisfeatureenablesittoadapttoavarietyofdifferentprogrammingrequirementsandexecuteefficient.

Disadvantages

1.ThedisadvantagesoftheClanguagearemainlytheweakdataencapsulation,whichmakesChaveabigflawindatasecurity,whichisalsoCandC++Onebigdifference.

2.ThegrammaticalrestrictionofClanguageisnottoostrict,andthetyperestrictionofvariablesisnotstrict,whichaffectsthesafetyoftheprogram,anddoesnotcheckthearraysubscriptbeyondtheboundary.Fromtheapplicationpointofview,Clanguageismoredifficulttomasterthanotherhigh-levellanguages.Inotherwords,peoplewhouseClanguagearerequiredtobemoreproficientinprogramming.

Languagecomposition

Basiccomposition

Datatype

ThedatatypeofCincludes:integer(short,int,long,longlong),charactertype(char),realtypeorfloatingpointtype(single-precisionfloatanddouble-precisiondouble),enumerationtype(enum),arraytype,structuretype(struct),sharedBodytype(union),pointertypeandvoidtype(void).

Constantsandvariables

Thevalueofaconstantcannotbechanged,andthenameofasymbolicconstantisusuallycapitalized.

Avariableistheamountwhosevaluecanbechangedwithanidentifierasitsname.Anidentifierisasequenceofletters,numbersorunderscoresstartingwithaletterorunderscore.Pleasenotethatthefirstcharactermustbealetterorunderscore,otherwiseitisanillegalvariablename.Variablesareallocatedcorrespondingstorageunitsatcompiletime.

Array

Ifavariablenameisfollowedbyabracketwithanumber,thedeclarationisanarraydeclaration.Stringisalsoanarray.TheyendwithASCIINULLastheendofthearray.Itisimportanttonotethattheindexvalueinthebracketsiscountedfrom0.

Pointer

Ifavariableisdeclaredwith*infrontofit,itmeansthatitisapointervariable.Inotherwords,thevariablestoresanaddress,and*(herespecificallyreferstotheunaryoperator*,thesamebelow.Thereisalsoabinaryoperator*intheClanguage)isacontentoperator,whichmeanstotakethismemoryaddressThecontentstoredin.PointerisoneofthemainfeaturesthatdistinguishesClanguagefromothercontemporaryhigh-levellanguages.

Pointerscanbenotonlytheaddressesofvariables,butalsotheaddressesofarrays,arrayelements,andfunctions.Usingpointersasformalparameterscangetmorethanonereturnvalueinthecallingprocessofthefunction,unlikereturn(z),whichcanonlygetonereturnvalue.

Pointersareadouble-edgedsword.Manyoperationscanbeexpressednaturallythroughpointers,butincorrectorexcessiveuseofpointerswillbringalotofpotentialerrorstotheprogram.

String

Clanguagestringsareactuallychararraysendingwith'\0'characters,andtheuseofcharactertypesdoesnotrequirereferencetothelibrary,ButtheuseofstringsrequiressomefunctionsintheCstandardlibraryformanipulatingstrings.Theyaredifferentfromcharacterarrays.Tousethesefunctions,youneedtoquotetheheaderfile.

Fileinput/output

InClanguage,inputandoutputareimplementedthroughasetoffunctionsinthestandardlibrary.InANSIC,thesefunctionsaredefinedintheheaderfile;.

Standardinput/output

Therearethreestandardinput/outputthatarepredefinedbythestandardI/Olibrary:

stdinstandardinput

stdoutstandardoutput

stderrinputandoutputerror

Calculation

TheoperationofClanguageisveryflexible,thefunctionisveryrich,andtheoperationtypeFarmorethanotherprogramminglanguages.Intermsofexpressions,itismoreconcisethanotherprogramminglanguages.Forexample,addition,subtraction,commaoperationsandtrinocularoperationsmakeexpressionseasier,butbeginnersoftenfindsuchexpressionsdifficulttoread.ThekeyreasonisthatTheoperatorsandtheorderofoperationsarenotfullyunderstood.Whenmultipledifferentoperationsformanoperationexpression,thatis,whenmultipleoperatorsappearinanexpression,theorderofprecedenceandcombinationrulesofoperationswillbecomeveryimportant.

Keywords

Keywordsarealsocalledreservedwords,whicharewordsthathavebeenusedbytheClanguageitselfandcannotbeusedforotherpurposes.Forexample,keywordscannotbeusedasidentifierssuchasvariablenamesandfunctionnames.

Datatypekeyword

short:Modifiedint,shortintegerdata,modifiedintcanbeomitted.(IntroducedintheK&Rperiod)

long:Modifiedint,longintegerdata,modifiedintcanbeomitted.(IntroducedintheK&Rperiod)

longlong:Modifiedint,superlongintegerdata,modifiedintcanbeomitted.(NewtoC99standard)

signed:Modifiedintegerdata,signeddatatype.(NewtoC89standard)

unsigned:Modifiedintegerdata,unsigneddatatype.(IntroducedduringtheK&Rperiod)

restrict:Usedtorestrictandconstrainthepointer,andindicatethatthepointeristheinitialandonlyonetoaccessadataobjectWay.(NewtoC99standard)

Complextypekeywords

struct:structuredeclaration.(IntroducedduringtheK&Rperiod)

union:Unionstatement.(IntroducedduringtheK&Rperiod)

enum:Enumerationdeclaration.(NewtoC89standard)

typedef:Declaretypealias.(IntroducedintheK&Rperiod)

sizeof:Getthesizeofaspecifictypeoraspecifictypeofvariable.(IntroducedduringtheK&Rperiod)

inline:Inlinefunctionsareusedtoreplacemacrodefinitionsandwillbeexpandedwhereveritiscalled.(NewtoC99standard)

Storagelevelkeywords

auto:designatedasautomaticvariables,automaticallyassignedbythecompilerAndrelease.Usuallyallocatedonthestack.Contrarytostatic.Whenthevariableisnotspecified,itdefaultstoauto.(IntroducedintheK&Rperiod)

static:Specifyasastaticvariableandallocateitinthestaticvariablearea.Whenmodifyingafunction,specifythefunctionscopetobeinsidethefile.(IntroducedduringtheK&Rperiod)

register:Specifyasaregistervariable,itisrecommendedthatthecompilerstorethevariableintheregisterforuse,ormodifythefunctionparameters,itisrecommendedthatthecompilerpassParametersarepassedinregistersinsteadofthestack.(IntroducedintheK&Rperiod)

extern:Specifythecorrespondingvariableasanexternalvariable,thatis,markthedefinitionofthevariableorfunctioninanotherfile,promptingthecompilertoencounterthisvariableLookfordefinitionsinothermoduleswhensumfunctions.(IntroducedduringtheK&Rperiod)

const:Thespecifiedvariablecannotbechangedbythecurrentthread(butitmaybechangedbythesystemorotherthreads).(NewtoC89standard)

volatile:Thevalueofthespecifiedvariablemaybechangedbythesystemorotherthreads,forcingthecompilertoobtainthevalueofthevariablefrommemoryeverytime.Preventthecompilerfromoptimizingthisvariableintoaregistervariable.(NewtoC89standard)

Processcontrolkeywords

Jumpstructure

return:Usedinthebodyofafunctiontoreturnaspecificvalue(ifitisofvoidtype,thefunctionvalueisnotreturned).(IntroducedduringK&Rperiod)

continue:Endthecurrentcycleandstartthenextcycle.(IntroducedduringtheK&Rperiod)

break:Jumpoutofthecurrentlooporswitchstructure.(IntroducedduringtheK&Rperiod)

goto:Unconditionaljumpstatement.(IntroducedduringtheK&Rperiod)

Branchstructure

if:Conditionalstatement,noneedtoputasemicolonafterit.(IntroducedduringK&Rperiod)

else:Negativebranchofconditionalstatement(usedinconjunctionwithif).(IntroducedduringtheK&Rperiod)

switch:switchstatement(multiplebranchstatement).(IntroducedduringtheK&Rperiod)

case:Thebranchmarkintheswitchstatement,usedinconjunctionwithswitch.(IntroducedduringtheK&Rperiod)

default:The"other"branchintheswitchstatement,optional.(IntroducedduringtheK&Rperiod)

Grammaticalstructure

Sequentialstructure

Theprogramdesignofsequencestructureisthesimplest,aslongasJustwritethecorrespondingstatementsintheorderofsolvingtheproblem,andtheorderofexecutionisfromtoptobottom,inturn.

Forexample:a=3,b=5,nowexchangethevalues​​ofaandb.Thisproblemislikeexchangingthewaterintwocups.Ofcourse,athirdcupisused.IfthethirdIfthecupisc,thenthecorrectprogramis:c=a;a=b;b=c,theexecutionresultisa=5,b=c=3,ifyouchangetheorder,write:a=b;c=a;b=c;Thentheexecutionresultbecomesa=b=c=5,whichcannotachievetheexpectedpurpose,andbeginnersaremostlikelytomakethiskindofmistake.Thesequencestructurecanbeusedindependentlytoformasimplecompleteprogram.Thecommoninput,calculation,andoutputthree-stepprogramisthesequencestructure.Forexample,tocalculatetheareaof​​acircle,thesequenceofstatementsintheprogramistheradiusroftheinputcircle,andthecalculations=3.14159*r*r,theareasoftheoutputcircle.However,inmostcases,thesequencestructureisusedasapartoftheprogramandformsacomplexprogramtogetherwithotherstructures,suchascompoundstatementsinbranchstructures,loopbodiesinloopstructures,andsoon.

Selectthestructure

Althoughtheprogramofthesequencestructurecansolvetheproblemsofcalculationandoutput,itcannotbejudgedandthenselected.Forproblemsthatneedtobejudgedfirstandthenselected,aselectionstructureshouldbeused.Theexecutionoftheselectionstructureisbasedoncertainconditionstoselecttheexecutionpath,ratherthanstrictlyinaccordancewiththephysicalorderofthestatements.Thekeytotheprogramdesignmethodofselectingthestructureistoconstructappropriatebranchconditionsandanalyzetheprogramflow,andselecttheappropriateselectionstatementaccordingtothedifferentprogramflow.Theselectedstructureissuitableforcalculationswithconditionaljudgmentssuchaslogicorrelationalcomparison.Whendesigningsuchprograms,itisoftennecessarytodrawtheprogramflowchartfirst,andthenwritethesourceprogramaccordingtotheprogramflow.Inthisway,theprogramdesignanalysisisseparatedfromthelanguage.Makestheproblemsimpleandeasytounderstand.Theprogramflowchartisbasedontheprogramexecutionflowchartdrawnbytheproblem-solvinganalysis.

Cyclicstructure

Thecyclicstructurecanreducetheworkloadofrepetitivewritingofthesourceprogram,andisusedtodescribetheproblemofrepeatedexecutionofacertainalgorithm.ThisisprogramdesignTheprogramstructurethatcangivefullplaytocomputerspecialtiesinClanguageprovidesfourkindsofloops,namelygotoloop,whileloop,dowhileloopandforloop.Thefourloopscanbeusedtodealwiththesameproblem.Undernormalcircumstances,theycanreplaceeachother.However,itisgenerallynotrecommendedtousegotoloops,becauseforciblychangingtheorderoftheprogramwilloftenbringunexpectederrorstotheoperationoftheprogram.

Specialattentionshouldbepaidtotheendingstatement(thatis,thechangeoftheloopvariablevalue)intheloopbody,otherwiseitmaybecomeanendlessloop,whichisacommonmistakeofbeginners.

Thesimilaritiesanddifferencesofthethreeloops:whenusingwhileanddowhileloops,theinitializationofloopvariablesshouldbebeforetheloopbody,andtheforloopisgenerallycarriedoutinstatement1;whileloopandforloopTheexpressionisjudgedfirst,andthentheloopbodyisexecuted.Thedowhileloopexecutestheloopbodyfirstandthenjudgestheexpression.Thatistosay,theloopbodyofdowhileisexecutedatleastonce,whilethewhileloopandformaynotbeexecutedonce..Alsonotethatthesethreetypesofloopscanallusethebreakstatementtojumpoutoftheloop,andusethecontinuestatementtoendtheloop.Theloopformedbythegotostatementandtheifcannotbecontrolledbythebreakandcontinuestatements.

Sequentialstructure,branchstructureandloopstructurearenotisolatedfromeachother.Therecanbebranchesandsequencestructuresinloops,andloopsandsequencestructuresinbranches.Infact,nomatterwhichstructure,itcanbegeneralizedThinkofthemasonesentence.Intheactualprogrammingprocess,thesethreestructuresareoftencombinedtoimplementvariousalgorithmsanddesigncorrespondingprograms.However,theprogrammingproblemsarerelativelylarge.Theprogramswrittenareoftenverylongandthestructureisrepeated,resultinginpoorreadabilityanddifficulty.UnderstandthatthewaytosolvethisproblemistodesigntheCprogramintoamodularstructure.

Specificcontent:

Forloop

TheforloopstructureisthemostdistinctiveinClanguageTheloopstatementisthemostflexibleandconvenienttouse.Itsgeneralformis:for(expression1;expression2;expression3)loopbodystatement.(Amongthem;cannotbeomitted)

Expression1isaninitialvalueexpression,whichisusedtoassigninitialvalues​​totheloopvariablesbeforetheloopstarts.

Expression2isaloopcontrollogicexpression,whichcontrolstheconditionofloopexecutionanddeterminesthenumberofloops.

Expression3modifiestheexpressionfortheloopcontrolvariable,whichmakestheforlooptendtoend.

Theloopbodystatementisastatementthatisexecutedrepeatedlywhentheloopcontrolconditionissatisfied.

Butintheentireforloopprocess,expression1isonlycalculatedonce,expression2andexpression3maybecalculatedmultipletimes,ormaynotbecalculatedonce.Thebodyoftheloopmaybeexecutedmultipletimes,oritmaynotbeexecutedonce.

Executeexpression2first,thenexecutetheloopstructure,andfinallyexpression3,andkeeploopinglikethis.

TheforloopstatementisthemostpowerfulstatementintheClanguage,anditcanevenreplaceotherloopstatementstoacertainextent.

do

doloopstructure,do1while(2);executionorderis1->2->1...loop,2isthecyclecondition.

while

whileloopstructure,theexecutionorderofwhile(1)2;is1->2->1...loop,1istheloopcondition

Theaboveloopstatement,whentheloopconditionexpressionistrue,theloopwillcontinue,andifitisfalse,itwilljumpoutoftheloop.

Function

Cprogramiscomposedofasetofvariablesorexternalobjectsoffunctions.Afunctionisaself-containedexecutioncodesegmentthatcompletescertainrelatedfunctions.Wecanthinkofafunctionasa"blackbox",youcangettheresultaslongasyousendthedatain,andtheexternalprogramdoesn'tknowhowthefunctionworksinside.Whattheexternalprogramknowsislimitedtowhatisinputtothefunctionandwhatthefunctionoutputs.Functionsprovideameansofprogrammingtomakeiteasytoread,write,understand,eliminateerrors,modify,andmaintain.

ThenumberoffunctionsinaCprogramisactuallyunlimited.Ifthereisanyrestriction,thatis,theremustbeatleastonefunctioninaCprogram,andtheremustbeoneandonlyoneofthemThefunctionnamedmain,thisfunctioniscalledthemainfunction,andtheentireprogramisexecutedfromthismainfunction.

Clanguageprogramsencourageandencouragepeopletodivideabigproblemintosub-problems,whichcorrespondstosolvingasub-problemandcompileafunction.Therefore,ClanguageprogramsgenerallyconsistofalargenumberofsmallfunctionsratherthanafewLargefunctionsconstitutetheso-called"smallfunctionsconstitutealargeprogram."Theadvantageofthisisthateachpartisfullyindependentofeachotherandthetaskissingle.Therefore,thesesufficientlyindependentsmallmodulescanalsobeusedasasmall"component"withafixedspecificationtoformanewlargeprogram.

ForsomanyyearsinthedevelopmentofClanguage,thereareavastarrayofsystemsandprogramsdevelopedinClanguage.Alotoflibraryfunctionsthatcanbeuseddirectlyhavebeenaccumulatedwhiledeveloping.

ANSICprovidesstandardClanguagelibraryfunctions.

TurboC2.0,whichispreferredbyClanguagebeginners,providesmorethan400runtimefunctions,eachofwhichperformsaspecificfunction,whichuserscancallatwill.Thesefunctionsaregenerallydividedintoinputandoutputfunctions,mathematicalfunctions,stringandmemoryfunctions,functionsrelatedtoBIOSandDOS,characterscreenandgraphicsfunctionfunctions,processcontrolfunctions,directoryfunctions,etc.

TheWindowsSDKprovidedbytheWindowssystemcontainsthousandsoffunctionsrelatedtoWindowsapplicationdevelopment.Otheroperatingsystems,suchasLinux,alsoprovidealargenumberoffunctionsforapplicationdeveloperstocall.

Asaprogrammer,youshouldtrytobefamiliarwiththefunctionsofthetargetplatformlibraryfunctions.Inthisway,youcandevelopapplicationsforaspecificplatformwithease.Forexample,asaWindowsapplicationdeveloper,youshouldbeasfamiliarwiththeWindowsSDKaspossible;asaLinuxapplicationdeveloper,youshouldbeasfamiliarwithLinuxsystemcallsandPOSIXfunctionspecificationsaspossible.

Operationsymbol

Whatismorespecialisthatthebitshiftright(>>)operatorcanbearithmetic(complementthemostsignificantbitontheleftend)orlogic(complement0ontheleftend)Displacement.Forexample,if11100011isshiftedtotherightby3bits,thearithmeticshifttotherightbecomes11111100,andthelogicalshifttotherightbecomes00011100.Sincetherightshiftofarithmeticbitsismoresuitableforprocessingnegativeintegers,almostallcompilersshiftrightarithmeticbits.

Theprecedenceofoperatorsfromhightolowisroughly:unaryoperators,arithmeticoperators,relationaloperators,logicaloperators,conditionaloperators,assignmentoperators(=)andcommaoperators.

Programmingdevelopment

Compiler

GCC,anopensourceandfreecompilerdevelopedbytheGNUorganization

MinGW,undertheWindowsoperatingsystemGCC

Clang,theopensourceBSDprotocol-basedLLVM-basedcompiler

VisualC++::cl.exe,thecompilerthatcomeswithMicrosoftVC++

integrationDevelopmentenvironment

Code::Blocks,opensourceandfreeC/C++IDE

CodeLite,opensource,cross-platformC/C++integrateddevelopmentenvironment

Dev-C++,portableC/C++IDE

C-Free

LightTable

VisualStudioseries

Sampleprogram

Thefollowingisasimpleprogramthatprintsthestring"Hello,world!"onthestandardoutputdevice(stdout).Asimilarprogramisusuallyusedasthefirstprogramwhenlearningaprogramminglanguage:

#includeintmain(){printf("Hello,World!\n");return0;}

1. Thefirstlineoftheprogram#includeisapreprocessordirective,tellingtheCcompilertoincludethestdio.hfilebeforetheactualcompilation.

2. Thenextlineintmain()isthemainfunction,andtheprogramexecutesfromhere.

3. Thenextlineprintf(...)isanotherfunctionavailableinCthatwilldisplaythemessage"Hello,World!"onthescreen.

4. Thenextlinereturn0;Terminatethemain()functionandreturnthevalue0.

#includeintmain{voidmove(int*xa,int*xb);inta,b;printf("Pleaseentertwonumbersa,b:");scanf("%d,%d",&a,&b);move(&a,&b);printf("a=%d,b=%d\n",a,b);return0;}voidmove(int*xa,int*xb){inta;a=*xa;*xa=*xb;*xb=a;}

1.Thethirdlineisthedeclarationofthefunction,whichdeclaresafunctionwithnoreturnvaluewithtwopointerparameters.Sincethedefinitionofthefunctionisunderthecallingfunction,itmustbedeclared.

2.Thefourthlineisthedefinitionofvariables,whichdefinestwointegervariables.

3.Entertwonumbersonthesixthline,whichmustbeenteredwithacommalike1,0.Puttwonumbersintovariablesaandb."&"istheaddressoperator.

4.Theseventhlinecallsthemovefunctiontoconvertthevalues​​ofthetwovariables.

5.Afterthemainfunctionisthedefinitionofthemovefunction.

Ascanbeseenfromthisexample,pointervariablescangetmorereturnvalues,whichisveryconvenient.