1 Chapter 7 VBA Modules, Functions, Variables, and Constants

1Chapter 7Chapter 7

VBA Modules, Functions, Variables,

and Constants

VBA Modules, Functions, Variables,

and Constants

7Chapter ObjectivesChapter Objectives

• Design VBA code that is organized into standard modules, independent class modules, and form and report class modules

• Determine when each type of module is appropriate

• Develop simple sub procedures, functions, and property procedures

7Chapter ObjectivesChapter Objectives

• Determine when a sub procedure or a function is most appropriate

• Declare arguments, variables, and constants

• Invoke procedures and pass arguments between procedures

• Cancel and manage events through VBA

7ModulesModules

• Procedure Collection of VBA statements combined in a

manner to accomplish a processing task

• Module Group of procedures related to a specific

process Makes maintaining the application easier in the

long run

7ModulesModules

• Form and report class modules Include code for all event procedures triggered by

events occurring on a specific form or report, or the controls on that form or report

Figure 7-1 Procedures within modules

7ModulesModules

• Standard module Module you can place sub and function procedures and

declarations in to make available to other procedures throughout your application

Procedures can be executed whenever the application is running

• Independent class modules Not connected to a form or report Procedures inside an independent class module can be

executed as long as the application is open

7Selecting Module TypesSelecting Module Types

• Selection of the best module type for a procedure depends on: Runtime efficiency of the application Amount of primary memory expected on the

computer executing the application Number of forms and reports that use a

procedure

7Selecting Module TypesSelecting Module Types

• Standard modules and independent class modules are loaded into primary memory when an application opens Use primary memory even when they are not needed,

yet the time needed to load the module occurs once

• Form and class modules are loaded into primary memory when the form or report opens Do not use memory until form or report is opened, but

the form or report takes longer to open because module must be loaded

7Sub ProceduresSub Procedures

• Sub procedures Containers for programming code Invoked at any time by writing a statement

within a different procedure to invoke the necessary sub procedurePlace its name, and if needed, the values for its

arguments on a line within the invoking procedure

7FunctionsFunctions

• Functions Return values Invoked by placing it in an expression or using

it as a value to display in a controlApplication can use the value returned by a function

in the expression

7Property ProceduresProperty Procedures

• Property procedure Defines a property that is attached to an object

through dot notation Activated by assigning or using the value of a

property Used if the procedure will correspond to

something that could be a property

7 Creating and Using Sub Procedures

Creating and Using Sub Procedures

Table 7-1 Data types in VBA and their corresponding types in Access tables

7 Creating and Using Sub Procedures

Creating and Using Sub Procedures

Table 7-1 Data types in VBA and their corresponding types in Access tables (continued)

7 Passing Arguments by Reference and by ValuePassing Arguments by

Reference and by Value

• When arguments are processed by reference: The memory address containing the value to be

used is “referenced” by the argument

• When arguments are processed by value: A copy of the value, instead of a reference to

the memory location containing the value, is supplied to the procedure

7Optional KeywordOptional Keyword

• Optional keyword Can be used with arguments Specifies that the invoking procedure does not

need to provide a value for the argument

• Named arguments approach Arguments can be placed in any order and

commas do not have to be used to represent blank arguments

7Nuances of Argument UseNuances of Argument Use

• A procedure must be invoked using the same number of actual arguments as formal arguments in the sub statement, unless optional arguments are used The actual arguments will supply values for the

formal arguments in the order in which they are listed, so the data types must be compatible

7Event Procedure ArgumentsEvent Procedure Arguments

• Event procedures Special types of sub procedures located in form and

report class modules Most do not take arguments, but some have one or

more predefined arguments

• Flags Arguments that contain values that Access evaluates

during and after the procedure executes to determine how to continue the application’s processing


Table 7-2 Event procedures with single arguments


• Intrinsic constants Represent the legal values for arguments that

specify actions other than canceling events True

Intrinsic constant that represents value –1


Table 7-3 Event procedure with multiple or special arguments


Table 7-3 Event procedure with multiple or special arguments (continued)

7Creating and Using FunctionsCreating and Using Functions

• Function Procedures Similar to sub procedures Use formal arguments, can be declared as Public or

Private Can contain variable and constant declaration

statements

• In a function procedure, the function is assigned a value Assignment is fundamental difference between sub

procedures and functions

7 Property Procedures –Let, Set, and Get

Property Procedures –Let, Set, and Get

• Property Let and Set procedures Accept information from an invoking

procedure, but do not return a value

• Property Get procedures Returns a value to the calling procedure

• Property Let and Get usually appear in pairs

7 Property Procedures – Let, Set, and Get

Property Procedures – Let, Set, and Get

Figure 7-2 Property procedures for frmIDSearchWindow

7Declaring VariablesDeclaring Variables

• Dim Statements Define variables that will be used within

procedures

• Variables are frequently used to store immediate results as execution of the procedure moves from one line to another


Figure 7-3 Part of txtPaymentNo_BeforeUpdate in frmProcessPayment


• Private Used in the Declarations section of a module to

declare variables that are used only within the procedure contained by the module

• Public Used in the Declarations section to declare

variables that can be used by any procedure in any module


• Static Used in a procedure to declare variables that

retain their value even after the procedure finishes executing

May be declared only within procedures

7The Variant Data TypeThe Variant Data Type

• Variant data type Can contain almost any type of data, including

numbers, strings, and dates Only data type in VBA that can store null and

empty values

• VarType built-in function Used to determine the type of data stored in a

Variant

7The Variant Data TypeThe Variant Data Type

Table 7-4 Values returned by VarType and variable testing functions

7The User-Defined TypeThe User-Defined Type

• VBA provides a way to create a user-defined type with a Type statement Allows you to define a complex data structure Useful when the values of many similar

variables need to be copied to other variables

7 Single- and Multiple- Dimension Arrays

Single- and Multiple- Dimension Arrays

• Arrays Collections of variables given a single name but

indexed by one or more values

• Array indexes begin at zero

• ReDim statement Can change the size of the array

7 Scope, Life, and the Dim Statement

Scope, Life, and the Dim Statement

• Where and how variables are declared determine which other procedures can use the variable Scope of a variable

• How long the variable actually exists Life of a variable



Table 7-5 Variable and constant scope prefixes



Figure 7-4 Procedure that facilitates the reuse of values



• Any variable declared in the Declarations section of a module maintains its stored value as long as: The application continues to run or The form or report remains open

• Variables declared with the Dim statement in a procedure are available to that procedure only as long as the procedure is running

7 Scope, Life, and the Static Statement

Scope, Life, and the Static Statement

• Variables declared with the Static statement in a procedure continue to exist after the procedure finishes

• Programmers frequently use static variables to determine whether a newly entered value is the same as a previous value

7 Intrinsic and Symbolic Constants

Intrinsic and Symbolic Constants

• Constant Named item that retains the same value

throughout the execution of a program

• Intrinsic constant Integer values that have some particular

meaning Makes programming easier for the developer

and facilitates program maintenance



• Symbolic constant Constant that is defined by the programmer Retains a constant value throughout the

execution of a procedure Similar to a variable except that its value is

specified as soon as the constant is declared



Figure 7-5 Public constants in the Declarations section of the basUtilities module

7 Controlling Processing in the Declarations Section

Controlling Processing in the Declarations Section

• Option Compare statement Automatically included in Declarations section Defines the default method to use when

comparing text data

• Three types of methods used when comparing text data: Binary method

Comparisons will be case-sensitive



• Three types of methods used when comparing text data (continued): Text method

Comparisons will not be case sensitive

Database method Comparisons are based on character’s relative

positions according to the sort order specified when the database was created or compacted



• If the Declarations section of the module containing the procedure contains an Option Explicit statement: Variables must be explicitly defined prior to

their use

• If it does not: Variables do not need to be declared before

they are used

7Chapter SummaryChapter Summary

• VBA statements are written inside functions procedures, sub procedures, and property procedures

• Standard modules Contain functions and sub procedures Loaded into memory as soon as the application opens

• Form and Report class modules Contain all three types of procedures


• Functions differ from sub procedures in two ways A function returns a value A function has an associated data type

• Functions should be written in cases where a value is being returned


• All procedures have arguments• Argument values can be passed

by reference Changes made to the value passed into the

procedure are reflected in the environment where the procedure was originally invoked

by valueMake a copy of the actual argument value available

to the procedure


• Variables Can be local or they can be public Usually only available while the code in which

they have been declared is executing Deleted from memory unless they have been

declared as static May be declared as one of a number of

different data types


• Arrays Collections of indexed variables with a

common name, and user defined types, which allow programmers to create their own data structures

• Constants Help make the program code more

understandable

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1 Chapter 7 VBA Modules, Functions, Variables, and Constants