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Lecture Set 04B - Variables, Constants, Expressions, Conversion

Course: CIS 3309, Fall 2011
School: Temple
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Declareandusevariables n LectureSet4 DataTypesandVariables PartBVariables,Constants,Expressions ConversionRules OptionsStrict,OptionExplicit ScopeofDefinition Slide1 Objectives Createuserdefinedconstantsanduse expressions n UnderstandtypeconversioninVB n Howtoavoidtroubleandgetmaximum protection(fromyourself) n UnderstandOptionExplicitandStrictand rulesforimplicitconversion Slide2 IntroductiontoVariables n Avariablestoresdatawhileanapplication runs n n Theprocessofcreatingavariableiscalled declaringavariable Avariablehasaname(identifier) n n Slide3 Thenamingrulesforvariablesandprocedures arethesame Avariablehasadatatype VariableCharacteristics n Allvariableshavethreecharacteristics n n n n Slide4 Avariablehasalifetime Avariablehasascope Avariablehasaccessibility Considerasimpleexamplefromaprevious Chapter4lecture VariableCharacteristicsIllustration PublicClasscounterType Thisisaclassvariable(akaaglobalvariableorclassdatastore orproperty) PrivatevalueasintegerScope?Lifetime?Accessibility? PublicSubNew(ByValinitialValueAsinteger) ***Constructorinitializecounter(value)toargumentvalue EndSubNew PublicSubIncrement() value=value+1 EndSubIncrement EndClasscounterType Codeappearingelsewhere Thismaybeassumedtoappearinsomefunctionorprocedure. Itisalocalvariable.Scope?Lifetime?Accessibility? DimmyCounterAscounterTypeWhatdoesthisdo? myCounter=NewcounterTypeWhatdoesthisdo?Drawpicture! Slide5 VariableLifetime n Variablelifetimereferstotheperiodoftime thatavariableexists n n Avariabledeclaredlocallywithinaprocedure existsonlywhileaprocedureexecutes Avariabledeclaredwithinaclass(ormodule) existswhileaclassinstanceexistsoramodule isloaded n Slide6 Youmayremembersuchvariablesasclass membersorclassdatastoresorvariablesglobal toaclass VariableScopeandAccessibility n n n Slide7 Variablescopereferstothestatementsthat canuseavariable Variablesdeclaredinsideaprocedurewith theDimstatementhavelocalscope(no relevanceoraccessibilityoutsidethe procedure) Variablesdeclaredoutsideaprocedurewith thePrivatestatementhavemodulelevel scope DeclaringaLocalVariable Local means local to a function or sub [Dim | Static] varname As type [=initexpr] n TheDimkeyworddeclaresalocalvariable n n n TheStatickeyworddeclaresalocalvariable n n n n Slide8 Thevariableiscreatedwhenthecontainingprocedure(subor function)isinvoked Thevariableandanyvalueitmighthaveisdestroyedwhen executionofthecontainingprocedureends Thevariableanditsvaluepersistfromoneprocedure invocationtothenext varnamecontainsthename(identifier)ofthevariable typedefinesthevariablesdatatype initexprisusedtoinitializethevariablesvalue InitializingVariables n n Variablescanbedeclaredandinitializedin thesamestatement Exampletodeclareavariablenamed CurrentYearandinitializeitsvalueto2006: Dim CurrentYear As Integer = 2006 Slide9 CommonInitializationErrors n n Numericinitializationvaluescannotcontain formattingcharacters Thefollowingstatementsareillegal: Dim Value1 As Double = 100,000.52 Dim Value2 As Double = $100,000.52 Slide10 DeclaringMultipleVariablesinOne Statement n n Multiplevariablescanbedeclaredinthesame statement Separateeachdeclarationwithacommaas follows: Dim IntegerValue As Integer, DoubleValue As Double n Ifvariableshavethesamedatatype,the followingshorthandnotationcanbeused: Dim IntegerValue1, IntegerValue2 As Integer GeneralsuggestionDontDoThis! Slide11 VariableNamingConventions n Aschemeofnamingvariablesiscalleda namingconvention n n Commonnamingconventions n n n Slide12 Namingconventionsimproveaprograms readability Hungariannotation Pascalcase Camelcase NamingConventions(Description) n n n Hungariannotation n Useaprefixtodenotethedatatypefollowedbya descriptivename Pascalcase n Usewholewords n Capitalizethefirstletterofeachword Camelcase n Usewholewords n Uselowercaseforthefirstletterofthefirstword n Capitalizethefirstletterofeachremainingword n CapitalizeallwordsinaConst IgenerallyuseacombinationofHungarianandCamel notations Slide13 Userdefined(named)Constants n n Constantsaresimilartovariables Thevalueofaconstantdoesnotchange whileanapplicationruns n n n n Slide14 Tryingtoassignavaluetoaconstantwillcause anerror UsetheConststatementtodeclarea constant Constantsaretypicallydeclaredatthe modulelevel Whybotherwiththis?CityWageTax?Y2K? UserdefinedConstant(Example) n DeclareaconstanttostorevalueofPI Private Const PI_VALUE As Single _ = 3.14159 n Declareaconstanttostorethevalueof PImultipliedby2 Private Const PI_VALUE As Single _ = PI_VALUE * 2 Slide15 IntroductiontoExpressions n n Variablescanbeusedinassignment statementsalongwithobjectproperties Examples: Dim As Result Integer Dim Example As Integer = 4 Result = 3 Result = txtExample.Height Result = Example Slide16 CommonExpressionErrors n n Valuesontherightandleftsidesofan assignmentstatementmusthavethesame datatype TheFontandIntegerdatatypesarenot compatiblesothefollowingstatementswill causeanerror: Dim SomeInteger As Integer SomeInteger = txtExample.Font Slide17 PartsofanExpression n Operators n n n Operands n n n n Slide18 Arithmeticoperatorssuchas(+,,*,/,^,\) Comparisonandlogicaloperators Literalvalues Objectproperties Constants Variables OperatorPrecedence n Operatorshaveanevaluationordercalled precedence n n n TheVisualBasicprecedencerulesarethesame asthealgebraicprecedencerules Useparenthesestooverridethedefault precedencerules Useparenthesestoclarifyevaluationorder n Slide19 Extraparenthesesdonoharm OperatorsandPrecedence Slide20 ArithmeticOperators(Example) n n Divide10by5andmultiplytheintermediate resultby100 Thevalue200isstoredinResult Dim Numerator As Double = 10 Dim Denominator As Double = 5 Dim Result As Double Result = Numerator / Denominator * 100 Slide21 IntegerDivision n n n Theresultofanintegerdivisionoperationis anintegralvalue Theresultistruncatedratherthanrounded Examples: Result = 4 \ 3 Result = 5 \ 2 '2 Result = 10 \ 6 Slide22 '1 '1 TheModOperator n TheModoperatorcalculatestheInteger remainderofadivisionoperation Theoperationisthesameastheremainder resultingfromalongdivisionoperation Examples: n n Result = 5 Mod 2 ' 1 Result = 4 Mod 2 ' 0 Slide23 OperatorPrecedence n Arithmeticoperatorsareappliedinadefault orderknownasprecedence n n Slide24 Exponentiation,multiplicationanddivision, additionandsubtraction Useparenthesestochangethedefault precedenceorder EvaluationOrderwithandwithoutParenthesis Slide25 DataTypesandTypeConversion n n Typeconversionisusedtoconvertdatafrom onedatatypetoanother Therearetwotypesofconversions n n Narrowingconversions:thereceivingvariableis notwideenoughtoreceivealltheinformation associatedwiththecoursevariable Wideningconversions:thereceivingvariableis wideenough n Slide26 Example:Convertingtypedoubletoanintegerisa narrowingconversion.Theopposite(integerto double)isawideningconversion n n n WideningandNarrowing Conversions Howtypeconversionisperformeddependson whetherstricttypecheckingisenabled ImplicitcastingisoftendonebyVBwhere wideningisconcerned.Itwillalsodoimplicit narrowingconversions.Dontletthishappen! Usestricttypechecking.Then n n n Slide27 Agiventypewillnotbeimplicitlyconvertedtoa morerestrictivetype(inotherwords,narrowingwill notbeallowedunlessyoudosoexplicitly) Stringswillnotbeimplicitlyconvertedtonumeric datatypes Lotsofotherconversionsareprohibited EnablingStrictTypeChecking n n n n n Slide28 Bydefault,VBusespermissivetypesemantics TheOption Strictstatementenablesor disablesstricttypechecking TheOption Strictstatementmustappear atthebeginningofamodule Enablestricttypecheckingtopreventhardto findtypeconversionerrors Example: Option Strict On Option Strict Off EnforcingExplicitVariableDeclaration n Explicitvariabledeclarationrequiresvariables tobedeclaredbeforetheyareused n n n Explicitvariabledeclarationisenabled,by default UsetheOptionExplicitstatementto enforceexplicitvariabledeclaration Example: Option Explicit On Option Explicit Off Slide29 StrictTypeCheckingRules n NarrowingversusWideningtypecoercionis thekeywideningallowsmorerestrictive typestobeimplicitlyconvertedtoless restrictivetypes n n n n Slide30 Integraldatatypeswillbeconvertedtofloating pointtypes IntegertoSingleislegal,forexample SingletoDoubleislegal DoubletoIntegerisnotlegal ConversionPrimitiveTypes(Pattison) Slide31 PerformingExplicitTypeConversion(brief) n n n n AlmostALWAYSagoodidea ThemembersoftheSystem.Convertclass explicitlyconvertdatafromonetypetoanother Methodsexistforeachprimarydatatype Example: Dim DoubleValue1 As Double = 123.44 Dim IntegerResult As Integer IntegerResult = _ System.Convert.ToInt32(DoubleValue1) Slide32 ExplicitTypeConversionMethods n n n n ToInt16,ToInt32,andToInt64convert theargumenttoanintegralvalue ToDecimalconvertstheargumenttothe Decimaldatatype ToDoubleandToSingleconvertthe argumenttofloatingpointdatatypes ToStringconvertstheargumenttoastring n n Slide33 AswewillseelaterinLectureSet4notes,thiscanbereally useful Notethatthesemethodsdonoroundingandhencecanlead toFormatExceptions(discussedlater)

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