variabilní

Přehled proměnných

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

Ifthereisnoassignmentafterthevariabledeclaration,thecompilerwillautomaticallypromptandassignadefaultvalue.

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

Pravidla pro pojmenování

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.

Poznámka:

(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#metoda pojmenování

stringstrName

Metoda VB pojmenování

DimstrNameasString

Althoughyoudon’tneedtopaytoomuchattentiontothedetailsofcharacterabbreviations,youstillneedtolookattheconventionsinthisregardinthefuture.ThedetailsofthisagreementcanbefoundintheVisualBasiconlinehelpandmanyVisualBasicadvancedprogrammingbooks.

Období přežití

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

Rozsah

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

Život

referstothewholeprocessofvariableallocationandrecovery,

Typ

AttributevariableandProměnnécreatedbyusersthemselves.

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

Userscanalsocreatetheirownvariablestostoretemporarydataorresultdataduringprogramexecution,etc.Intheprogram,suchvariablesareveryneeded.Thefollowingdescribeshowtocreateandusesuchvariables.

Deklarované proměnné

Před použitím proměnné musíte deklarovat v kódu, tedy vytvořit proměnnou.

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

Therearetwowaystodeclarevariables:implicitdeclarationandexplicitdeclaration.

Implicitní prohlášení:

Proměnnécanbeuseddirectlywithoutdeclaration.Atthistime,VBassignsdefaulttypesandvalues​​tothevariables.Thismethodisrelativelysimpleandconvenient.Proměnnécanbenamedandusedintheprogramcodeatanytime,butitisnoteasytocheck.

Explicitní prohlášení:

Použijte prohlášení deklarace k vytvoření proměnných.

Vynuceno explicitně deklarovat proměnné:

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.

Rozsahofthephysicalvariable

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

Typ úložiště

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.Rozsahreferstothevisiblerangeofthevariable,thatis,duringthelifecycleofthevariable,whichpartsoftheprogramcanusethevariable.

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

Forlocalvariables,whetheritisastaticlocalvariableoranautomaticvariable,thescopeislimitedtothefunctionormodulethatdefinesthevariable.

Aslongasthedynamicmemoryisnotreleased,itcanbeusedanywhereintheprogram,providedthattheaddressofthedynamicmemoryisknown.

Poznámka:Addingstaticbeforeglobalvariablesaffectsthescope,andaddingbeforelocalvariablesaffectsthelifecycle.

Typ proměnné

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

IntheClanguage,ifitisdividedaccordingtothestoragespace,itincludes:

Celočíselné proměnné(codeisint),celočíselnéproměnnézahrnujídlouhécelkovéproměnné(kód jedlouhý)akrátkočíselnéproměnné(kód je krátký),charakterové proměnné(kód je znak),proměnné s plovoucí desetinnou čárkou(kód plovoucí),atd.

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

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

Therearetwotypesofvariables:AttributevariablesandUser-createdvariables.

JavaScript proměnné

Proměnné

Justlikealgebra,JavaScript proměnnéareusedtostorevalues​​orexpressions.

Youcangivethevariableashortname,suchasx,oramoredescriptivename,suchaslength.

JavaScript proměnnécanalsostoretextvalues,suchascarname="Volvo".

Pravidla pro názvy proměnných

Proměnnéarecasesensitive(yandYaretwoDifferentvariables)

Proměnnémuststartwithlettersorunderscores

Poznámka:SinceJavaScriptiscase-sensitive,variablenamesarealsocase-sensitive.

Příklad

Duringtheexecutionofthescript,thevalueofthevariablecanbechanged.Youcanrefertoavariablebyitsnametodisplayorchangeitsvalue.

Tento příklad vám ukazuje princip.

DeclaringProměnné

CreatingvariablesinJavaScriptisoftencalled"declaring"variables.

YoucandeclareJavaScript proměnnéthroughthevarstatement:

varx;varcarname;

varx;varcarname;

Aftertheabovedeclaration,thevariableshavenovalues,butyoucanassignvalues​​tothevariableswhenyoudeclarethem:

varx=5;varcarname="Volvo";

varx=5;varcarname="Volvo";

Poznámka:Whenassigningatextvaluetoavariable,pleasequotethevalue.

VariableAssignment

AssignavaluetoJavaScriptvariablethroughassignmentstatement:

x=5;carname="Volvo";

x=5;carname="Volvo";

Thevariablenameisontheleftsideofthe=symbol,andthevaluetobeassignedtothevariableisontherightsideofthe=.

Aftertheabovestatementisexecuted,thevaluestoredinthevariablexis5,andthevalueofcarnameisVolvo.

Přiřadit hodnotu nedeklarované proměnné

Ifthevariableyouassignhasnotbeendeclared,thevariablewillbedeclaredautomatically.

Tato prohlášení:

x=5;carname="Volvo";

x=5;carname="Volvo";

Stejný účinek jako tato prohlášení:

varx=5;varcarname="Volvo";

varx=5;varcarname="Volvo";

Znovu deklarujte proměnnou

IfyoudeclareaJavaScriptvariableagain,thevariablewillnotloseitsoriginalvalue.

varx=5;varx;

varx=5;varx;

Aftertheabovestatementisexecuted,thevalueofvariablexisstill5.Whenthevariableisredeclared,thevalueofxwillnotberesetorcleared.

JavaScriptaritmetika

Justlikealgebra,youcanuseJavaScript proměnnétodoarithmetic:

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

y=x-5;z=y+5

phpproměnné

Proměnné v PHP: proměnné se používají k ukládání hodnot, jako jsou čísla, textové řetězce nebo pole.

Onceavariableisset,wecanuseitrepeatedlyinthescript.

Všechny proměnné v PHP začněte symbolem $.

Správný způsob nastavení proměnných v PHP:

$var_name=value;Začátečníci PHP zapomenou na symbol $ před proměnnou. Pokud to uděláte, proměnná bude neplatná.

Wecreateavariablewithastringandavariablewithavalue:

ItisnotnecessarytodeclarethedatatypeofthevariabletoPHP.

Accordingtothewaythevariableisset,PHPwillautomaticallyconvertthevariabletothecorrectdatatype.

Inastronglytypedprogramminglanguage,youmustdeclarethetypeandnameofthevariablebeforeusingit.

InPHP,variablesareautomaticallydeclaredwhentheyareused.

Proměnnéinphparerepresentedbyadollarsignfollowedbythevariablename.Variablenamesarecasesensitive.

Názvy proměnných se řídí stejnými pravidly jako jiné značky v php. Platný název proměnné začíná upozorněním nebo podtržítkem, za nímž následuje libovolný počet písmen, čísel nebo podtržítek. Podle běžného regulárního výrazu bude vyjádřen jako:'[a-zA-Z_/x7f-/-xff]7'[a-9/xff]]*.

Poznámka: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.

Pravidla pro pojmenovávání proměnných

Názvy proměnných musí začínat se znaménkem nebo podtržítkem „_“.

Variablenamescanonlycontainalphanumericcharactersandunderscores.

Názvy proměnných nesmějí obsahovat mezery. Pokud se název proměnné skládá z více slov, měl by být oddělen podtržítky (například $ můj_řetězec) nebo začít velkým písmenem (například $ můj_řetězec).