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Mi ch ae l Br yd on (m j br yd on @sfu. ca )Last updat e: 02-May-01 1 o f 14

Lesson 18: An introduction to VISUAL BASIC

18.1 Introduction: Learning the basics of

programming

Program mi ng can be enorm ously com plex anddif f i cul t , or i t can be easy and st ra ight forw ard.In many cases, t he dif f erence l i es in t he choiceof t ools used for w ri t ing the program. In othercases, it is a quest ion of scalelarge pr oj ect sare unavoidably complex.

In this proj ect , w e are going to f ocus on solvingsmall problems using an easy-to-useprogramm ing language. I t i s im port ant t orecognize, however, that basic programmingconcept s rem ain t he same r egardl ess oflanguage or pr oj ect scale. The pr ograms youcreate here are t r iv ial , but t hey int roduce a

handful of pr ogramm ing const ruct s t hat can bef ound in any t h ird generat ion language, not

j ust VISUAL BASIC.

Strict ly speaking, the language that isinc luded wi t h ACCESS is not VISUAL BASICi tis a subset of t he ful l, st and-alone VISUALBASIC language (whi ch MICROSOFT sellsseparately). In ACCESS version 2.0, t hesubset is called ACCESS BASIC . In version7.0 and above, it is sl i ghtl y enlargedsubset cal led VISUAL BASICFOR

APPLICATIONS (VBA). In t he context o f t hesimpl e programs we are wr i t ing here,

these terms are interchangeable.

ACCESS provides t wo w ays of int eract i ng wi t h theVBA language. The most useful of t he t w o issaved modules that contain VBA procedures.Procedur es are bat ches of programm ingcommands t hat can be executed t o do

int erest ing t hings l i ke process t ransact ionsagainst master tables, provide sophist icatederr or checking, and so on.

The second w ay to int eract wi t h VBA is d irect lyt hrough the int erpret er. When you type ast a tement in to t he in terpre t er, i t is executedimm ediately. Al t hough t here is l i t t le businessuse for t h is feat ure, i t does making learning thelanguage and t est ing new st at ement s easier.

In the f ir st part of t his lesson, you are going toinvoke ACCESS VBA interpret er and executesome very simpl e st at em ent s. In t he secondpart of t he tut or ia l , you are going to create VBA

modules t o explore generic programm ingconst ruct s such as looping, condit ionalbranching, and paramet er p assing.

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Learning object ives2 o f 14

An in t r oduct ion t o VISUAL BASIC

18.2 Learning objectives

! invoke and use the debug/ imm ediatewindow

! underst and t he di f f erence bet weenfundament al program const ruct s(st at ement s, vari ables, t he assignmentoperator, and predef ined funct ions)

! underst and t he di f f erence bet weensubroutines and functions

!creat e a modul e cont aining VBA code

! gain experience w it h looping andcondit ional branching const ruct s

! use t he VBA debugger i n ACCESS

! underst and t he di f f erence bet ween anin terpre t ed and compi led programming

language

18.3 Exercises

18.3.1 Invoking the interpreter

Click on the Modules t ab in t he databasewi ndow and press New.

This opens t he module window w hich we w i l lre tu rn to in Sect ion 18.3.3.

You have to have a modul e w indow openin order for t he debug window to be

avai lable f r om t he menu.

Select View Debug Window f r om themain menu. Note t hat Ctrl-G can be used inversion 7.0 and above as a short cut t o brin gup t he debug wi ndow.

In version 2.0 , t he debug window iscal led the imm ediat e window and youuse View Immediate Window t oact ivat e i t .

18.3.2 Basic programming constructsIn this section, you wil l use the VBA interpretert o explore some fundament al programm ingconst ruct s and VISUAL BASIC syntax.

18.3.2.1 Statements

St at ement s are built around special keyw ords ina programming language that do somethingwhen execut ed. For example, t he Printst atem ent in VBA print s an expression on t hescreen.

In the debug wi ndow, t ype the f o l lowi ng:

NL Print Hello world!

Th e symbol at t he end of a l i ne means press t he Return or Enter key . Fromt his point f orward, assume t hat each l ineis fo l l owed by an Enter.?

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In VBA (as in al l d ial ect s of BASIC), t he questi onmark (?) is t ypicall y used as short hand for t hePrint sta tement .

Type the f o l lowi ng int o the debug wi ndow:NL ? Hello world!

As show n in Figure 18.1, t he result is ident icalt o f i rst Print sta tement .

18.3.2.2 Action statements

Actions are special statements that allowprogrammers to use elements of ACCESS macro

language in VBA program s. Som e act ions arestand-alone (such as the MsgBox act ion below)and ot hers are actual ly m et hods of t he DoCmdobj ect (see Sect ion 18.4.1 for more in format ionon t he evolut ion of VBA and i t s incr easing use ofobj ect-or i ented concept s). Al t hough theinclusion of so many di f ferent t ypes of

st atem ent s result s in a conceptual m ess, i tprovides programers with a great deal off lexib i l i ty.

At this point, you should ignoretheoretical language issues and acceptVBA as a pow erf ul-but -inconsist ent

f r iend.

Type the f o l lowi ng int o the debug wi ndow:NL MsgBox Hello, world!

The MsgBox macr o act ion pr ovides an easy w ayt o cre ate cust om messages using t he st andard

WINDOWS m essage box f orm at .

The DoCmd obj ect i s j ust a kludge t hat br i ngsVBA, obj ect- or ientat ion, and t he macrolanguage t oget her. Ugly as it is, t he DoCmdobj ect has many met hods t hat are wort hlearning about .

FIGURE 18. 1: Int eract ing wit h t he VISUAL

BASIC in terpreter .

The debug window in version 8.0is shared with the locals window.?

Type VISUAL BASICstatements into theinterpreter.

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The statements executeimmediately and results areshown in the debug window.

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Type the f o l lowi ng int o the debug wi ndow:NL DoCmd.OpenForm frmOrders

You wi l l not ice t hat your order f orm opens in

the background behind the debug window.

I f you do not have a form cal ledfrmOrders, t h is met hod wi l l r esu l t in anerror.

18.3.2.3 Variables and assignment

A variable i s space in mem ory t o which youassign a name and a value. When you use t hevariable name in expressions, the programminglanguage replaces t he variable name w it h i t sassigned value at t hat part i cular inst ant.

Type t he f o l low ing:

NL s = Hello

NL ? s & world

NL ? s & world

In t he f i rst st atem ent, t he variable named s iscreated and the st r ing Hel lo is assigned t o i t .Recal l t he funct ion of t he concatenat ionoperator f rom Sect ion 11.4.1. When the secondst at ement is execut ed, VBA recognizes t hat s isa variable, not a st r ing (since i t is not i nquotat ions marks). The int erpret er replaces swi t h i ts va lue ( Hel lo ) before execut ing t hePrint comm and. In t he f ina l st a tement , s is in

quotat ion marks so i t is int erpret ed as a literalstring.

Contrary to the practice in languages l ikeC and PASCAL, t he equals sign (=) is used t oassign values to variables. It is also usedas t he equivalence operator (e. g., does x= y?).

With in t he debug window, any st r ing of

characters in quotations marks (e.g., Hel lo ) is int erpre ted as a l i t e ral st r ing.Any st r ing wit hout quot at ion m arks (e.g. ,strName) is int erpr et ed as a vari able orsome ot her obj ect s (such as a f iel d) t hatis def ined wi t h in the ACCESS environment.

18.3.2.4 Predefined functionsYou were i ntr oduced t o predef i ned funct ions inSect ion 11.4.2. In t his sect ion, y ou are going t oexplore some basic predefined functions forworki ng wi t h numbers and text . The result s ofthese exercises are shown in Figure 18 .2.

Pri nt t he cosine of 2 radians and then nestt he cosine f unct ion w it h in a conversionfunct ion to round to t he nearest int eger:

NL pi = 3.14159

NL ? cos(2*pi)

NL ? CInt(cos(2*pi))

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Convert a st ring of charact ers t o uppercase:NL s = basic or cobol

NL ? UCase(s)

Ext ract t he middle six charact ers f r om ast r ing st ar t ing at t he f i f t h charact er :

NL ? mid (s,5,6)

Calculate t he monthly payment f or a$12,000 loan over f our years w it h 8.5%

int erest (nom inal) compounded mont hly:NL ? Pmt(0.085/12, 4*12, -12000)

In VBA, as in ACCESS, case is ignored. Assuch MID(S,5,6) is ident ical to

mid(s,5,6). In addi t ion, t he amount o f whi t espace (space betw een elements

of t he st a tement) is i r re levant .

18.3.2.5 Remark statements

When creati ng large programs, it is consider edgood programm ing pract ice t o include adequateint ernal documentat ion. In ot her words, youshould include comments throughout your codet o explain t o others and your fut ure self whatt he program is doing.

Comm ent l ines are ignored by the int erpret erw hen t he program i s run. To designate acomm ent in VBA, use an apost rophe t o st art t hecomment , e .g . :

NL This is a comment line!

NL Print Hello the comment starts

here

The original REM (remark) statement fromBASIC can also be used, but is less common.

NL REM This is also a comment (remark)

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FIGURE 18. 2: Using t he V ISUAL BASICin terpr eter t o test pr edef ined funct ions.

The argument forcos() contains anexpression.

UCase() converts astring to uppercase.

Mid() extracts

characters from astring.

The debug window can beenlarged by dragging thedividing line.

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Many specialized financial andscientific functions are available.
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18.3.3 Creating a module

So far, you have w ri t t en and execut ed VBAst atem ents one at a t ime. A more useful

t echnique is t o bundle a number of VBAcommands t oget her in a procedure and runthem in sequence. A module is a collection ofst atem ents that is saved w it h t he database ( l ikea form or a query).

Close the debug window so that the

declarat ion page of t he new m odulecreated in Sect ion 18.3.3 is visib le (seeFigure 18.3).

The tw o l ines:NL Option Compare Database

NL Option Explicit

are included in t he module by default . TheOption Compare st atem ent specif i es t he way inwhich st r ings are compared (e.g. , are wi rew hisk and WIRE WHISK consider edidentical?). The Option Explicit statementforces you to declare all your variables beforeusing t hem (variabl e declar at ion i s discussed inSect ion 18.3.4 .1) .

In version 2.0, ACCESS does not

aut omat ical ly add t heOption Explicit

statement. You should add it yourself tothe declarations section.

A module cont ains a declarat ion page and oneor more pages containing procedures. In VBA,t here are tw o t ypes of pr ocedures: subrout ines

and funct ions. The primary di f f erence bet weent he t wo is that subrout i nes simpl y executewhereas funct ions execut e and ret urn a value(e.g. , cos()) .

In version 2.0, only one subrout ine orfunct ion shows in t he window at a t im e.

You m ust use t he Page Up an d Page Downkeys to navigate the module.

FIGURE 18.3: The de clarat ions page of aVISUAL BASIC module.

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18.3.4 Creating subroutines with looping and

branching

In th is sect i on, you wi l l explore t wo of t he most

powerf ul const ruct s in comput er programming:looping and conditional branching.

Create a new subrout ine by t yping t hefol l owing anywhere on the declarat ionspage of t he open module:

NL Sub LoopingTest()

Not ice t hat ACCESS creates a new sect i on/ pagein t he module f or t he subrout ine, as shown inFigure 18.4.

18.3.4.1 Declaring variables

When you declare a variable, you te l l t heprogramm ing environm ent t o reserve some

space in memory for t he variable. Since t heamount of space t hat i s requi red depends ont he type of data t he variable is expected t ocontain (e. g. , st r ing, i nt eger, Boolean, double-precision f loat i ng-point , etc. ) , you have toinc lude data t ype in format ion in t he declara t ionsta tement .

In VBA, you use t he Dim st a tement t o declarevariables.

Type t he fo l l owing into t he space betw eent he Sub End Sub pair:


NL Dim i as integerNL Dim s as string

NL End Sub

Save t he m odule as basTesting.

One of t he most usef ul loopi ng const ruct s isFor Next. A l l st a tementsbetw een t he For an d Next part s are repeatedas long as the part is t rue. Theindex i is automat ical ly increment ed af t er eachi tera t ion.

Enter t he remainder o f t he LoopingTestprogram:

FIGURE 18.4: Creat e a new subrout ine.

You can use the procedurecombo box to switch betweenprocedures in a module.

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NL Dim i as integer

NL Dim s as string

NL s = Loop number:

NL F o r i = 1 T o 1 0

NL Debug.Print s & i

NL Next i

NL End Sub

Save t he m odule.

I t i s cust omary in m ost programm inglanguages t o use t he Tab key to indentt he element s wi t h in a loop sl ight l y. Thismakes t he program more readable.

Note that t he Print st a tement in Figure 18 .5 is

pref aced by Debug. You can get away w it h t heol d st yl e BASIC Print st a tement in t he debugwindow. But within modules, VBA enforces aform of ob j ect -or ienta t ion. A l l st and-a lonest atem ents are repl aced by m et hods of obj ects.In th is case, t he Print met hod window belongsto the Debug ob jec t .

18.3.4.2 Running the subroutine

Now t hat you have created a subrout ine, youneed to r un i t t o see t hat i t works. To invoke asubrout ine, you sim ply use it s name l ike youwould any statement.

Select View Debug Window f r om themenu (or p ress Ctrl-G in ver sion 7.0 andabove).

Type: LoopingTest in t he debug wi ndow, asshown in Figure 18.5.

18.3.4.3 Conditional branching

We can use a diff erent looping const ruct , DoUntil Loop, and the

condit ional branching const ruct ,

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FIGURE 18.5: Run t he LoopingTestsubrout ine in t he debug wi ndow.

Invoke theLoopingTestsubroutine by typing itsname in the debug window.

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If Then Else, to ach ieve thesame r esult .

Type the f o l lowi ng anywhere under t he EndSub st atem ent in order t o creat e a newpage in t he module:

NL Sub BranchingTest

Ent er t he fo l low ing program:NL Sub BranchingTest

NL Dim i As Integer, s As String

NL Dim blDone As Boolean

NL s = Loop number:

NL i = 1 initialize counter

NL blDone = False

NL Do Until blDone

NL If i > 10 Then

NLDebug.Print All done

NL blDone = True

NL Else

NL Debug.Print s & i

NL i = i + 1

NL End If

NL Loop

NL End Sub

Run t he pr ogram

18.3.5 Using the debugger

ACCESS provi des a very good debugger t o help

you st ep t hrough your programs and underst and

how t hey are execut ing. The t wo basicdebugging const ruct s explor ed here arebreakpoints an d stepping ( l ine-by-l ineexecut ion).

Move to t he Do Until blDone l ine in theBranchingTest subrout ine and selectRun Toggle Breakpoint f r om the menu(you can al so pr ess F9 t o toggle t hebreakpoint on a part icular l ine of code).

Not e t hat t he l ine becomes highl ighted,indicat ing t he presence of an act ive breakpoint .When the program runs, t he in terpre t er wi l lsuspend execut ion at t his breakpoint and passcontrol of t he program back to you.

Run t he subrout i ne f r om t he debug window.

Execut ion should halt at t he breakpoint .

St ep t hrough a couple of l ines in t heprogram line-by-l ine by pressing F8, asshown in Figure 18.6.

By st epping th rough a program l ine by l in e, you

can usually f ind any program bugs. In addit ion,you can use t he debug wi ndow t o examine t hevalue of variables whi l e t he program sexecution is suspended.

Click on t he debug wi ndow and t ype ? i t osee the curr ent value of t he variable i, as

shown in Figure 18.7.

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18.3.6 Passing parameters

In t he BranchingTest subrout i ne, t he loopst art s at 1 and repeats unt i l t he counter ireaches 10. I t m ay be preferable, however, t oset t he st art and f in ish quant i t ies when t hesubrout ine is executed. To achieve thi s, you

pass parameters (or arguments) to thesubroutine.

The main difference between passed

parameters and other variables in a procedureis t hat p assed paramet ers are declar ed in t hef ir st l ine of the subrout ine def i n i t ion. Forexample, f o l lowi ng subrout ine declarat i on

NL Sub BranchingTest(intStart as

Integer, intStop as Integer)

not only declares t he variables intStart andintStop as int egers, i t also tel ls t he subrout ine

FIGURE 18.6: St ep t hrough t he each li ne int he program indiv idual l y.

The highlighted boxindicates the currentlocation of the interpreter inthe program. Press F8 toexecute one line of code.

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Use the F9 key to set abreakpoint where you want theprogram to suspend execution.

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FIGURE 18.7: Use t he debug w indow t opr in t var iab le va lues whi le t he program i s

running.

While the program executionis suspended, you canexecute statements in thedebug window. Type ? i toprint the current value of i.

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In version 8.0, the Localswindow shows the values of allvariables in the current scope.

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E iA i t d t i t VISUAL BASIC
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t o expect t hese t w o number s t o be passed asparameters.

To see how t his w orks, cr eat e a new subrout ine

cal led ParameterTest based onBranchingTest.

Type the declarat ion st atem ent above tocreate t he ParameterTest subroutine.

Swi t ch back to BranchingTest and

highl ight a l l t he code except t he Sub andEnd Sub sta tements

Cut and past e t he code into t heParameterTest procedure.

To incorporate t he parameters into

ParameterTest, you wi l l have to m ake t hefol l owing modif i cat ions t o the past ed code:

Replace i = 1 w i t h i = intStart.

Replace i > 1 0 w i t h i > intStop.

Call t he subrout ine f r om t he debug wi ndowby t yping:

NL ParameterTest 4, 12

The result s are shown in Figure 18.8.

If you prefer enclosing parameters inbracket s, you have to use t he

Call (parameter1, ,

parametern) syntax, e.g. :Call ParameterTest(4, 12). SeeSect ion 18.4.3 for m ore in format ion ont he use of br acket s in VISUAL BASIC.

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FIGURE 18.8: Creat e a param et eri zedsubroutine.

Include the names ofthe parameters in thesubroutinesdeclaration.

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DiscussionAn int r oduct ion t o VISUAL BASIC
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18.3.7 Creating a MinValue() function

In thi s sect ion, you are going t o creat e a user-def ined funct ion that r e turns t he min imum of

t wo number s. Alt hough m ost languages supplysuch a funct ion, ACCESS does not ( t he Min() and

Max() funct ion in ACCESS are f or use w it hin SQLst atem ents only).

Create a new m odule cal led basUtilities.

Type the f o l lowi ng (on one l ine) t o create anew funct ion:

NL Function MinValue(n1 as Single, n2

as Single) as Single

The st atem ent above def ines a funct ion cal ledMinValue that returns a single-precision

number. The funct ion requir es t wo single-preci sion num bers as paramet ers.

Since a funct ion ret urns a value, t he data t ypeof t he ret urn value should be specif ied in t hefunct ion declarat i on. Accordingly, t he basicsyntax of a funct ion declarat ion i s:

NL Function (parameter1As , , parametern As

) A s

The funct ion ret urns a variable named.

Type t he fo l l owing as t he body of t he

funct ion:

NL Function MinValue(n1 as Single, n2

as Single) as Single

NL If n1

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example, recal l t he Debug.Print met hod inSect ion 18.3.4.

18.4.2 Interpreted and compiled languages

VBA is an interpreted language. In int erpre tedlanguages, each l i ne of t he program is

int erpret ed (convert ed int o machine language)

and executed w hen the pr ogram is run. Ot herlan guages (such as C, PASCAL, FORTRAN, etc.)ar e compiled, m eaning t hat t he or ig inal(source) pr ogram is t ranslat ed and saved int o afi le of machine language commands. Theresult ing executable f i le is run inst ead of t hesource code.

FIGURE 18. 9: Test ing t he MinValue() funct ion.

Implement the MinValue()function using conditional branching.1111

These five lines could be replaced with animmediate if function: MinValue =iif(n1

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Predi ct ably, compi led l anguages run m uchfast er t han inter pret ed languages (e.g. ,compiled C++ is generally ten t imes faster thanin terpre ted JAVA) . However, interpreted

languages are generally easier to debug sinceprogram execution can be stopped at any pointand the curr ent l ine of source code viewed andmanipulated.

18.4.3 Brackets and parameters

Recall t hat subrout ines in VISUAL BASIC execut e abatch of commands, whereas funct ions execut ea batch of comm ands and ret urn a single value.The dist inct ion is im port ant because, when i tcomes to p assing paramet ers to procedur es,VISUAL BASIC adopt s an odd convent ion:

I f the procedure re turns a va lue ( i .e . , i t is afunct ion), enclose t he parameters inbrackets, e. g. , MinValue(2, 7).

I f the procedure does not re turn a va lue(i . e. , i t is a subrout ine), do not enclose theparamet ers in brackets, e.g. :ParameterTest 4, 12.

18.5 Application to the project

You w il l need a MinValue() funct ion inSect ion 19.5 when you have to det ermi ne thedefault quantity to ship. Ensure you have

created and t est ed the funct ion inSect ion 18.3.7.
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