variable

OverviewofVariables

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

Ifthereisnoassignmentafterthevariabledeclaration,thecompilerwillautomaticallypromptandassignadefaultvalue.

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

Namingrules

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.

Note:

(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.

Formatvariabletype+variablename

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

Notethatvariabletypesarealllowercaseandvariablenamesarecapitalizedatthebeginning

C#namingmethod

stringstrName

VBnamingmethod

DimstrNameasString

Althoughyoudon’tneedtopaytoomuchattentiontothedetailsofcharacterabbreviations,youstillneedtolookattheconventionsinthisregardinthefuture.ThedetailsofthisagreementcanbefoundintheVisualBasiconlinehelpandmanyVisualBasicadvancedprogrammingbooks.

Survivalperiod

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

Scope

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

Lifetime

referstothewholeprocessofvariableallocationandrecovery,

Type

AttributevariableandVariablescreatedbyusersthemselves.

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

Userscanalsocreatetheirownvariablestostoretemporarydataorresultdataduringprogramexecution,etc.Intheprogram,suchvariablesareveryneeded.Thefollowingdescribeshowtocreateandusesuchvariables.

Declarevariables

Beforeusingthevariable,youmustdeclareitinthecode,thatis,createthevariable.

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

Therearetwowaystodeclarevariables:implicitdeclarationandexplicitdeclaration.

Implicitdeclaration:

Variablescanbeuseddirectlywithoutdeclaration.Atthistime,VBassignsdefaulttypesandvalues​​tothevariables.Thismethodisrelativelysimpleandconvenient.Variablescanbenamedandusedintheprogramcodeatanytime,butitisnoteasytocheck.

Explicitdeclaration:

Usedeclarationstatementstocreatevariables.

Forcedtodeclarevariablesexplicitly:

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.

Scopeofthephysicalvariable

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

Storagetype

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.Scopereferstothevisiblerangeofthevariable,thatis,duringthelifecycleofthevariable,whichpartsoftheprogramcanusethevariable.

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

Forlocalvariables,whetheritisastaticlocalvariableoranautomaticvariable,thescopeislimitedtothefunctionormodulethatdefinesthevariable.

Aslongasthedynamicmemoryisnotreleased,itcanbeusedanywhereintheprogram,providedthattheaddressofthedynamicmemoryisknown.

Note:Addingstaticbeforeglobalvariablesaffectsthescope,andaddingbeforelocalvariablesaffectsthelifecycle.

Variabletype

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

IntheClanguage,ifitisdividedaccordingtothestoragespace,itincludes:

Integervariables(codeisint),integervariablesincludelongintegervariables(codeislongint)andshortintegervariables(codeisshortint),charactervariables(codeischar),floating-pointvariables(codefloat),etc.Ofcoursetherearearrays,structurevariables,etc.Butnovicesneedtopayattentiontothateachvariabletypeultimatelycorrespondstotheformatcharacters.Forexample:%d.

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

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

Therearetwotypesofvariables:AttributevariablesandUser-createdvariables.

JavaScriptvariables

Variables

Justlikealgebra,JavaScriptvariablesareusedtostorevalues​​orexpressions.

Youcangivethevariableashortname,suchasx,oramoredescriptivename,suchaslength.

JavaScriptvariablescanalsostoretextvalues,suchascarname="Volvo".

Variablenamerules

Variablesarecasesensitive(yandYaretwoDifferentvariables)

Variablesmuststartwithlettersorunderscores

Note:SinceJavaScriptiscase-sensitive,variablenamesarealsocase-sensitive.

Example

Duringtheexecutionofthescript,thevalueofthevariablecanbechanged.Youcanrefertoavariablebyitsnametodisplayorchangeitsvalue.

Thisexampleshowsyoutheprinciple.

DeclaringVariables

CreatingvariablesinJavaScriptisoftencalled"declaring"variables.

YoucandeclareJavaScriptvariablesthroughthevarstatement:

varx;varcarname;

varx;varcarname;

Aftertheabovedeclaration,thevariableshavenovalues,butyoucanassignvalues​​tothevariableswhenyoudeclarethem:

varx=5;varcarname="Volvo";

varx=5;varcarname="Volvo";

Note:Whenassigningatextvaluetoavariable,pleasequotethevalue.

VariableAssignment

AssignavaluetoJavaScriptvariablethroughassignmentstatement:

x=5;carname="Volvo";

x=5;carname="Volvo";

Thevariablenameisontheleftsideofthe=symbol,andthevaluetobeassignedtothevariableisontherightsideofthe=.

Aftertheabovestatementisexecuted,thevaluestoredinthevariablexis5,andthevalueofcarnameisVolvo.

Assignavaluetoanundeclaredvariable

Ifthevariableyouassignhasnotbeendeclared,thevariablewillbedeclaredautomatically.

Thesestatements:

x=5;carname="Volvo";

x=5;carname="Volvo";

Sameeffectasthesestatements:

varx=5;varcarname="Volvo";

varx=5;varcarname="Volvo";

Re-declarethevariable

IfyoudeclareaJavaScriptvariableagain,thevariablewillnotloseitsoriginalvalue.

varx=5;varx;

varx=5;varx;

Aftertheabovestatementisexecuted,thevalueofvariablexisstill5.Whenthevariableisredeclared,thevalueofxwillnotberesetorcleared.

JavaScriptarithmetic

Justlikealgebra,youcanuseJavaScriptvariablestodoarithmetic:

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

y=x-5;z=y+5

phpvariables

VariablesinPHP:variablesareusedtostorevalues,Suchasnumbers,textstringsorarrays.

Onceavariableisset,wecanuseitrepeatedlyinthescript.

AllvariablesinPHPstartwiththe$symbol.

ThecorrectwaytosetvariablesinPHPis:

$var_name=value;PHPbeginnerswillforgetthe$symbolinfrontofthevariable.Ifyoudothat,thevariablewillbeinvalid.

Wecreateavariablewithastringandavariablewithavalue:

ItisnotnecessarytodeclarethedatatypeofthevariabletoPHP.

Accordingtothewaythevariableisset,PHPwillautomaticallyconvertthevariabletothecorrectdatatype.

Inastronglytypedprogramminglanguage,youmustdeclarethetypeandnameofthevariablebeforeusingit.

InPHP,variablesareautomaticallydeclaredwhentheyareused.

Variablesinphparerepresentedbyadollarsignfollowedbythevariablename.Variablenamesarecasesensitive.

Variablenamesfollowthesamerulesasothertagsinphp.Avalidvariablenamestartswithaletterorunderscore,followedbyanynumberofletters,numbers,orunderscores.Accordingtothenormalregularexpression,itwillbeexpressedas:'[a-zA-Z_/x7f-/xff][a-zA-Z0-9_/x7f-/xff]*'.

Note: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.

Variablenamingrules

Variablenamesmuststartwithaletterorunderscore"_".

Variablenamescanonlycontainalphanumericcharactersandunderscores.

Variablenamescannotcontainspaces.Ifthevariablenameconsistsofmultiplewords,itshouldbeseparatedbyunderscores(suchas$my_string),orstartwithacapitalletter(suchas$myString).