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

Chapter 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

Chapter 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

Modules Procedure Module 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

Modules 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

Modules Standard module Independent class modules 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

Selecting 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

Selecting 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

Sub Procedures Sub procedures Containers for programming code Invoked at any time by writing a statement within a different procedure to invoke the necessary sub procedure Place its name, and if needed, the values for its arguments on a line within the invoking procedure

Functions Functions Return values Invoked by placing it in an expression or using it as a value to display in a control Application can use the value returned by a function in the expression

Property 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

Creating and Using Sub Procedures Table 7-1 Data types in VBA and their corresponding types in Access tables

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

Passing 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

Optional Keyword Optional keyword Named arguments approach 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

Nuances 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

Event 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

Event Procedure Arguments Table 7-2 Event procedures with single arguments

Event Procedure Arguments Intrinsic constants Represent the legal values for arguments that specify actions other than canceling events True Intrinsic constant that represents value –1

Event Procedure Arguments Table 7-3 Event procedure with multiple or special arguments

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

Creating 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

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

Property Procedures – Let, Set, and Get Figure 7-2 Property procedures for frmIDSearchWindow

Declaring 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

Declaring Variables Figure 7-3 Part of txtPaymentNo_BeforeUpdate in frmProcessPayment

Declaring Variables Private Public 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

Declaring Variables Static Used in a procedure to declare variables that retain their value even after the procedure finishes executing May be declared only within procedures

The Variant Data Type Variant data type VarType built-in function 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

The Variant Data Type Table 7-4 Values returned by VarType and variable testing functions

The 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

Single- and Multiple- Dimension 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

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

Scope, Life, and the Dim Statement Table 7-5 Variable and constant scope prefixes

Scope, Life, and the Dim Statement Figure 7-4 Procedure that facilitates the reuse of values

Scope, Life, and the Dim Statement 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

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

Intrinsic and Symbolic Constants 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

Intrinsic and Symbolic Constants 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

Intrinsic and Symbolic Constants Figure 7-5 Public constants in the Declarations section of the basUtilities module

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

Controlling Processing in the Declarations Section 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

Controlling Processing in the Declarations Section 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

Chapter 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

Chapter Summary 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

Chapter Summary 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 value Make a copy of the actual argument value available to the procedure

Chapter Summary 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

Chapter Summary Arrays Constants 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