muuttuja

Muuttujien yleiskatsaus

Muuttujatareveryusefulbecausetheyallowyoutoassignashort,easy-to-remembernametoeverypieceofdatayouintendtouseintheprogram.Muuttujatcansavethedataenteredbytheuserwhentheprogramisrunning(forexample,usetheInputBoxfunctiontodisplayadialogboxonthescreen,andthensavethetexttypedbytheuserintothevariable),theresultofaspecificoperation,andapieceofdatatobedisplayedontheform,etc..Inshort,variablesaresimpletoolsfortrackingalmostalltypesofinformation.

Ifthereisnoassignmentafterthevariabledeclaration,thecompilerwillautomaticallypromptandassignadefaultvalue.

Muuttujatareconvenientplaceholdersforreferencingcomputermemoryaddresses,whichcanstoreprograminformationthatcanbechangedwhileScriptisrunning.Forexample,youcancreateavariablenamedClickCounttostorethenumberoftimestheuserclicksanobjectontheWebpage.Tousevariables,youdonotneedtoknowtheaddressofthevariableinthecomputer'smemory.Youcanvieworchangethevalueofthevariableaslongasyourefertothevariablebyitsname.ThereisonlyonebasicdatatypeinVBScript,namelyVariant,sothedatatypeofallvariablesisVariant.

Nimeämissäännöt

Firstofall,wemustgivethevariableasuitablename,asifeveryonehastheirownname,otherwiseitwillbedifficulttodistinguish.InVB6,thenamingofvariablesmustfollowthefollowingrules:

(1)Variablenamesmuststartwithaletterorunderscore,andthemiddleofthenamecanonlyconsistofletters,numbersandanunderscore"_";thelastcharactercanbeItisatypespecifier;

(2)Thelengthofthevariablenamemustnotexceed255characters;

(3)Thevariablenamemustbeuniquewithinthevalidrange.Theeffectivescopeisthescopeofthereferencevariablethatcanberecognizedandusedbytheprogram—forexample,aprocedure,awindow,andsoon.Thecontentofthescopeofreferencevariableswillbeintroducedlater.

(4)Variablenamescannotbereservedwords(keywords)inVB,orreservedwordswithtypespecifiersattheend,butreservedwordscanbeembeddedinvariablenames,keywordsrefertoVB6Internalsystemidentifierssuchasattributes,events,methods,procedures,andfunctionsinthelanguage.Suchasdefinedwords(if,endif,while,loop,etc.),functionnames(len,format,msgbox,etc.).LikePrint,Print$isillegal,butMyprintislegal.Forexample:strName1,intMax_Length,intLesson,strNo3,etc.arelegalvariablenames,whileA&B,allright,3M,_Number,etc.areillegalvariablenames.

Huomautus:

(1)VariablenamesarenotcasesensitiveinVB(suchasABC,aBc,abc,etc.).Clanguageiscasesensitive.Differentlanguages​​havedifferentrules.

(2)Whendefiningandusingvariables,usuallydefinethevariablenameasanamethatiseasytouseandreadandcandescribetheusefulnessofthecontaineddata,insteadofusingsomedifficultabbreviationssuchasAorB2.Forexample:Supposeyouarecompilingasoftwareforsellingapplesforthefruitshop.WeneedtwovariablestostorethepriceandsalesofApple.Atthispoint,youcandefinetwovariablesnamedApple_PriceandApple_Sold.Eachtimetheprogramisrun,theuserprovidesspecificvalues​​forthesetwovariables,whichlooksveryintuitive.Thespecificmethodis:Makethemeaningofthevariableclearbyformingameaningfulvariablenamewithoneormorewords.Forexample,thevariablenameSalesTaxRateismuchclearerthanTaxorRate.

(3)Mixuppercaseandlowercaselettersandnumbersasneeded.Areasonableagreementistocapitalizethefirstletterofeachwordinthevariable,forexample:DateOfBirth.

(4)Anotherreasonableagreementisthateachvariablenamestartswithatwo-orthree-characterabbreviation,whichcorrespondstothedatatypeofthevariabletobestored.Forexample,usestrNametoindicatethattheNamevariablestoresstringdata.ThisnamingmethodiscalledHungariannomenclature.

Muotomuuttujan tyyppi+muuttujan nimi

Forexample,thestrnamejustmentioned."str"​​istheabbreviationof"string"."Name"isthenameofthevariable.

Notethatvariabletypesarealllowercaseandvariablenamesarecapitalizedatthebeginning

C#nimeämismenetelmä

stringstrName

VB-nimeämismenetelmä

DimstrNameasString

Althoughyoudon’tneedtopaytoomuchattentiontothedetailsofcharacterabbreviations,youstillneedtolookattheconventionsinthisregardinthefuture.ThedetailsofthisagreementcanbefoundintheVisualBasiconlinehelpandmanyVisualBasicadvancedprogrammingbooks.

Selviytymisjakso

Thetimethevariableexistsiscalledthesurvivalperiod.ThelifetimeofaScript-levelvariablestartsfromthemomentitisdeclareduntiltheendofthescript.Forprocess-levelvariables,thelifetimeisonlythetimetheprocessruns.Aftertheprocessends,thevariablesdisappear.Duringtheexecutionprocess,localvariablesareidealtemporarystoragespace.Localvariableswiththesamenamecanbeusedindifferentprocedures,becauseeachlocalvariableisonlyrecognizedbytheprocedureinwhichitisdeclared.

Laajuus

Thescopeofavariableisdeterminedbywhereitisdeclared.Ifavariableisdeclaredinaprocedure,onlythecodeintheprocedurecanaccessorchangethevalueofthevariable.Atthistime,thevariablehasalocalscopeandiscalledaprocedure-levelvariable.Ifthevariableisdeclaredoutsidetheprocedure,thevariablecanberecognizedbyallproceduresintheScript.ItiscalledaScript-levelvariableandhasaScript-levelscope.

Elinikä

referstothewholeprocessofvariableallocationandrecovery,

Tyyppi

AttributevariableandMuuttujatcreatedbyusersthemselves.

Whenwedesigntheuserinterfaceintheform,vb6willautomaticallycreateasetofvariablesforthegeneratedobjects(includingtheformitself),namelyattributevariables,andseteachvariableItsdefaultvalue.Suchvariablescanbeuseddirectlybyus,suchasreferencingitorassigningnewvalues​​toit.

Userscanalsocreatetheirownvariablestostoretemporarydataorresultdataduringprogramexecution,etc.Intheprogram,suchvariablesareveryneeded.Thefollowingdescribeshowtocreateandusesuchvariables.

Ilmoittavat muuttujat

Ennen kuin käytät muuttujaa, sinun on ilmoitettava koodissa, eli luotava muuttuja.

Beforeusingvariables,mostlanguages​​usuallyfirstneedtodeclarevariables.Inotherwords,thecompilermustbetoldinadvancewhichvariablesareusedintheprogram,aswellasthedatatypesofthesevariablesandthelengthofthevariables.Thisisbecausethecompilerneedstoknowhowtoopenupstorageareasforstatementvariablesbeforethecompiledprogramexecutesthecode,sothattheexecutionoftheprogramcanbeoptimized.

Therearetwowaystodeclarevariables:implicitdeclarationandexplicitdeclaration.

Implisiittinen ilmoitus:

Muuttujatcanbeuseddirectlywithoutdeclaration.Atthistime,VBassignsdefaulttypesandvalues​​tothevariables.Thismethodisrelativelysimpleandconvenient.Muuttujatcanbenamedandusedintheprogramcodeatanytime,butitisnoteasytocheck.

Nimenomainen ilmoitus:

Käytä ilmoituslauseita muuttujien luomiseen.

Pakotettu ilmoittamaan muuttujat nimenomaisesti:

Inordertoavoidthetroublecausedbyincorrectvariablenames,userscanspecifythataslongastheyencounteranuncleardeclaration,Asthenameofavariable,VBwillissueanerrorwarning.Themethodistoforceexplicitdeclarationofvariables.Toforceexplicitdeclarationofvariables,justaddthisstatementinthedeclarationsectionoftheclassmodule,formmoduleorstandardmodule:

OptionExplicit

Thisstatementisusedtostipulatethatallvariablesinthismodulemustbedeclaredbeforeuse,thatis,variablescannotbecreatedbyimplicitdeclaration.AfteraddingtheOptionExplicitstatement,VBwillautomaticallycheckwhetherthereareundefinedvariablesintheprogram,andwilldisplayanerrormessagewhenfound.

IfyouwanttoautomaticallyinserttheOptionExplicitstatement,theuseronlyneedstoselectthe"Options"commandinthe"Tools"menu,thenclickthe"Editor"tabinthe"Options"dialogbox,andthenselect"RequireVariableDeclaration"option,sothatVBwillautomaticallyinsertOptionExplicitstatementsinanynewmodules,butonlyinnewlycreatedmodules.Therefore,foranalreadyestablishedmodule,youcanonlymanuallyaddOptionExplicitstatementstoexistingmodules.

Laajuusofthephysicalvariable

Thescopeofthevariabledeterminesthepartofthecodethatcanknowtheexistenceofthevariable.Whenavariableisdeclaredinsideaprocedure,onlythecodeinsidetheprocedurecanaccessorchangethevalueofthatvariable;ithasascopeandislocaltotheprocedure.However,sometimesitisnecessarytouseavariablewithalargerscope,suchasavariablewhosevalueisvalidforallproceduresinthesamemodule,orevenforallproceduresoftheentireapplication.VisualBasicallowsyoutospecifyitsscopewhendeclaringavariable.

Tallennustyyppi

Weoftendefinesomevariablesintheprogramtosaveandprocessdata.Inessence,avariablerepresentsapieceofoperablememory,anditcanalsobeconsideredasasymbolicrepresentationofmemory.Whenthememoryneedstobeusedintheprogram,acertaintypeofvariablecanbedefined.Atthistime,thecompilerallocatesacertainamountofmemoryspaceaccordingtothedatatypeofthevariable.Theprogramcanaccessthecorrespondingmemorythroughthevariablename.

Ifthedatatypeofthevariabledeterminesthesizeofthecorrespondingmemory,thenthestoragetypeaffectshowthecorrespondingmemoryisused.Theso-calledmethodofuse,specifically,whenandwhereintheprogramcanusethevariable,thatis,thelifecycleandscopeofthevariable.

Firstunderstandsomebasiccommonsense.1.Therearethreeareasinthememorythatcansavevariableswhentheprogramisrunning:staticstoragearea,stackandheap.2.Accordingtothelocationofthevariabledefinition,itcanbedividedintoglobalvariables(variablesdefinedoutsidethefunctionbody)andlocalvariables(variablesdefinedinsidethefunctionbody,includingformalparameters).

Allglobalvariablesandstaticlocalvariables(usingthekeywordstaticwhendefining)arestoredinthestaticstoragearea,whichischaracterizedby:allocatememoryspaceandinitializeitatcompiletime.Duringtherunningoftheprogram,thevariablealwaysexists,andthememoryspacecorrespondingtothevariableisnotreleaseduntiltheendoftheprogram.

Andallnon-staticlocalvariables(alsoknownasautomaticvariables)arestoredinthestack(stack),whichischaracterizedbydynamiccreationwhenthefunctionormodulewherethevariableislocatedisexecuted,andthefunctionormoduleisexecutedWhenfinished,thememoryspacecorrespondingtothevariableisreleased.Inotherwords,everytimeafunctionormoduleisexecuted,thelocalvariableswillbere-allocatedspace.Ifthevariableisnotinitializedwhenitisdefined,thevalueinthevariableisarandomnumber.

Allthememoryallocatedbymalloc(alsoknownasdynamicmemory)isintheheap,anditscharacteristicisthatthedynamicallyallocatedmemoryisgenerallyaccessedthroughpointers.Thatis,thedynamicmemorycanbemanuallyreleasedbyfree,oritcanbeautomaticallyreleasedbythesystemattheendoftheprogram.

Theabovediscussedthelifecycleofvariables,let’slookatthescopebelow.Laajuusreferstothevisiblerangeofthevariable,thatis,duringthelifecycleofthevariable,whichpartsoftheprogramcanusethevariable.

Thescopeofglobalvariablesstartsatthepointofdefinitionandendsattheendofthesourcefile.Ifyouwanttouseglobalvariablesbeforethepointofdefinition,youneedtousethekeywordexterntoexpandthescope.Bydefault,globalvariablescanbereferencedbyotherfiles.Ifyouwanttobelimitedtothisfile,youneedtousethekeywordstaticwhendefiningit.

Forlocalvariables,whetheritisastaticlocalvariableoranautomaticvariable,thescopeislimitedtothefunctionormodulethatdefinesthevariable.

Aslongasthedynamicmemoryisnotreleased,itcanbeusedanywhereintheprogram,providedthattheaddressofthedynamicmemoryisknown.

Huomautus:Addingstaticbeforeglobalvariablesaffectsthescope,andaddingbeforelocalvariablesaffectsthelifecycle.

Muuttujatyyppi

InClanguage,variablesaredividedintoglobalvariablesandlocalvariables;itcanalsobedividedinto:automaticvariables,staticvariables.Theformerisdividedaccordingtothescopeofthevariable,Thelatterisdividedaccordingtothevariablestoragemethod.

IntheClanguage,ifitisdividedaccordingtothestoragespace,itincludes:

Kokonaislukumuuttujat(codeisint),kokonaislukumuuttujatsisältääpitkät kokonaislukumuuttujat(codeislongint)ja lyhyet kokonaismuuttujat(codeisshortint),merkkimuuttujat(codeischar),kelluvapistemuuttujat(codefloat),etc.

Clanguagehasanotherimportantvariable:pointervariable.Thevalueitstoresisamemoryaddress.VariablenamesinClanguagearecase-sensitive.Forexample,SUNandsunaretwodifferentvariablenames.

Anotherpoint,whendeclaringavariable,youcandirectlyassignavaluetodeterminethelanguageofthevariabletypewithoutdeclaration.Thedeclarationofthistypeoflanguagevariableisusuallycalledaweaktype,andsuchas(c++,etc.)mustbedeclaredfirst.Lateruse,andthevariabletypemustbedeterminedwhendeclaring,thisisastrictdatatype.

Therearetwotypesofvariables:AttributevariablesandUser-createdvariables.

JavaScript-muuttujat

Muuttujat

Justlikealgebra,JavaScript-muuttujatareusedtostorevalues​​orexpressions.

Youcangivethevariableashortname,suchasx,oramoredescriptivename,suchaslength.

JavaScript-muuttujatcanalsostoretextvalues,suchascarname="Volvo".

Muuttujannimisäännöt

Muuttujatarecasesensitive(yandYaretwoDifferentvariables)

Muuttujatmuststartwithlettersorunderscores

Huomautus:SinceJavaScriptiscase-sensitive,variablenamesarealsocase-sensitive.

Esimerkki

Duringtheexecutionofthescript,thevalueofthevariablecanbechanged.Youcanrefertoavariablebyitsnametodisplayorchangeitsvalue.

Tämä esimerkki osoittaa sinulle periaatteen.

DeclaringMuuttujat

CreatingvariablesinJavaScriptisoftencalled"declaring"variables.

YoucandeclareJavaScript-muuttujatthroughthevarstatement:

varx;varcarname;

varx;varcarname;

Aftertheabovedeclaration,thevariableshavenovalues,butyoucanassignvalues​​tothevariableswhenyoudeclarethem:

varx=5;varcarname="Volvo";

varx=5;varcarname="Volvo";

Huomautus:Whenassigningatextvaluetoavariable,pleasequotethevalue.

VariableAssignment

AssignavaluetoJavaScriptvariablethroughassignmentstatement:

x=5;carname="Volvo";

x=5;carname="Volvo";

Thevariablenameisontheleftsideofthe=symbol,andthevaluetobeassignedtothevariableisontherightsideofthe=.

Aftertheabovestatementisexecuted,thevaluestoredinthevariablexis5,andthevalueofcarnameisVolvo.

Määritä arvo ilmoittamattomalle muuttujalle

Ifthevariableyouassignhasnotbeendeclared,thevariablewillbedeclaredautomatically.

Nämä lausunnot:

x=5;carname="Volvo";

x=5;carname="Volvo";

Samat vaikutukset kuin lausunnoissa:

varx=5;varcarname="Volvo";

varx=5;varcarname="Volvo";

Ilmoita muuttuja uudelleen

IfyoudeclareaJavaScriptvariableagain,thevariablewillnotloseitsoriginalvalue.

varx=5;varx;

varx=5;varx;

Aftertheabovestatementisexecuted,thevalueofvariablexisstill5.Whenthevariableisredeclared,thevalueofxwillnotberesetorcleared.

JavaScriptaritmetiikka

Justlikealgebra,youcanuseJavaScript-muuttujattodoarithmetic:

y=x-5;z=y+5;

y=x-5;z=y+5

php-muuttujat

PHP:n muuttujia:muuttujia käytetään tallentamaan arvoarvoja, kuten numeroita, tekstijonoja tai taulukoita.

Onceavariableisset,wecanuseitrepeatedlyinthescript.

Kaikki PHP:n muuttujat alkavat $symbolilla.

Oikea tapa asettaa muuttujat PHP:ssä:

$var_name=value;PHPaloittelijat unohtavat $symbolinmuuttujan edessä. Jos teet sen, muuttuja ei kelpaa.

Wecreateavariablewithastringandavariablewithavalue:

ItisnotnecessarytodeclarethedatatypeofthevariabletoPHP.

Accordingtothewaythevariableisset,PHPwillautomaticallyconvertthevariabletothecorrectdatatype.

Inastronglytypedprogramminglanguage,youmustdeclarethetypeandnameofthevariablebeforeusingit.

InPHP,variablesareautomaticallydeclaredwhentheyareused.

Muuttujatinphparerepresentedbyadollarsignfollowedbythevariablename.Variablenamesarecasesensitive.

Muuttujien nimetseuraavat samaa samaa muina tunnisteina php:ssä.Voimassa olevat muuttujien nimet alkavat kirjaimella tai alaviivalla, joita seuraa mikä tahansa määrä kirjaimia, numeroita tai alaviivoja. Normaalin säännöllisen lausekkeen mukaan se ilmaistaan ​​seuraavasti:'[a-zA-Z_/x7f-/xff.

Huomautus:Thelettersmentionedherearea-z,A-Z,andASCIIcharactersfrom127to255(0x7f-0xff).

Inphp3,variablesarealwaysassignedbyvalue.Thatistosay,whenthevalueofanexpressionisassignedtoavariable,thevalueoftheentireoriginalexpressionisassignedtothetargetvariable.Thismeansthat,forexample,whenthevalueofonevariableisassignedtoanothervariable,changingthevalueofonevariablewillnotaffecttheothervariable.Forthistypeofassignmentoperation,seethechapteronexpressions.

phpprovidesanotherwaytoassignvalues​​tovariables:referenceassignment.Thismeansthatthenewvariablesimplyreferences(inotherwords,"becomesanalias"or"pointsto")theoriginalvariable.Changingthenewvariablewillaffecttheoriginalvariable,andviceversa.Thisalsomeansthatnocopyoperationisperformed;therefore,thisassignmentoperationisfaster.Butonlyindenseloopsorwhenassigningvalues​​toverylargearraysorobjectsisitpossibletonoticethespeedincrease.

Usingreferenceassignment,simplyadda&symbolbeforethevariabletobeassigned(sourcevariable).Forexample,thefollowingcodesnippetwilloutput"MynameisBob"twice:

Oneimportantthingtopointoutisthatonlynamedvariablescanbereferencedandassigned.

Muuttujien nimeämissäännöt

Muuttujien nimien täytyy alkaa kirjaimella tai alaviivalla"_".

Variablenamescanonlycontainalphanumericcharactersandunderscores.

Jos muuttujan nimi koostuu useista sanoista, se tulisi erotella alaviivillä (kuten$oma_merkkijono)tai aloittaa isolla kirjaimella(kuten$omamerkkijono).