1 Chapter 9 Writing, Testing, and Debugging Access Applications

9 Chapter Objectives Write VBA procedures that use a variety of assignment statements Choose and write appropriate conditional execution statements Create program loops Write disk- and file-related statements

9 Chapter Objectives Invoke error handlers and write error- handling routines Test Access applications Debug Access applications using message boxes and the debugging facilities

9 Operators Many VBA statements support mathematical, string, and comparison operations Integer division  Indicated by \ Exponentiation  Indicated by the ^ operator

9 Operators Comparison operators , >=, =, and <> And and Or  Logical operators Not  Negative operator And, Or, and Not have traditional precedence relationships  Parentheses can be used to control the precedence

9 Operators String concatenation  Uniting of two or more string values  Can be performed using either the + or & operator  + operator produces a null result when either operand is null  & operator treats a null operand as a zero-length string

9 Operators Figure 9-1 Constructing an SQL command by using strings in VBA

9 Comments  Indicate the purpose of each section of code Rem statements  Used to include explanatory comments in a program  Can be represented in code with a single quote (‘)

9 Built-in Functions Built-in functions  Perform trigonometric calculations, financial calculations, and common mathematical calculations  Handle the conversion of data from one data type to another  InputBox  Used to prompt a user for information

9 Assignment and With Statements Assignment statements  Change the value of an object’s properties  Use them to update fields in recordsets and to place values in VBA variables With statements  Allow you to abbreviate assignment statements that reference object properties, because the object does not need to be specified

9 Assignment and With Statements Table 9-1 Summary of assignment statements

9 Assignment and With Statements Table 9-1 Summary of assignment statements (continued)

9 Conditional Execution Statements (If Statement) If statement  Can evaluate multiple conditions and thus selectively execute multiple sections of code  Can also contain multiple ElseIf parts  When it does include such parts, conditions occur for the initial If part and for each ElseIf part

9 Iterative (Looping) Statements Iterative (or looping) statements  Cause blocks of statements to repeat VBA supports multiple looping statement forms  The Do statement is a commonly used “looping” statement

9 Iterative (Looping) Statements Table 9-2 Looping statements

9 Unconditional Branching Statements Unconditional branching statements  Cause the program to skip execution of code that would have otherwise been executed  Created using line labels and numbers Exit Function  Exits a function

9 VBA Statements and Related Functions that Perform Access Actions VBA provides many statements that may not appear elsewhere which include statements that:  Manipulate the Access environment  Manipulate files  Reference the Windows environment  Manipulate other Windows applications

9 Statements Related to the Access Environment SendKeys statements are useful when you want to trigger a series of events Table 9-3 Statements that perform actions within the Access environment

9 Statements Related to the Access Environment Table 9-3 Statements that perform actions within the Access environment (continued)

9 Windows Disk- and File- Related Statements ChDrive  Changes the current drive ChDir  Changes the current directory CurDir[] function  Returns the current directory

9 Windows Disk- and File- Related Statements Table 9-4 Disk- and file-related statements

9 Windows Disk- and File- Related Statements Table 9-5 File I/O statements

9 Determining and Setting the Applications Operating Environment An application can:  Determine the path to an application by reading the Windows Registry  Access information about the registry through the GetSetting and GetAllSettings functions

9 Determining and Setting the Applications Operating Environment Table 9-6 Commonly used SysCmd specifications

9 Using SysCmd to Implement a Progress Meter A Progress meter should be used whenever an operation is expected to take a long time  acSysCmdInitMeter constant  Used to start the progress meter and length of argument indicates the total time of the operation  acSysCmdRemoveMeter constant  Used when the operation is complete  acSysCmdUpdateMeter constant  Used to show progress at a given point in time

9 Compilation Errors Compilation errors  Due to syntax mistakes Compiling a procedure tells Access to translate VBA code into a format that is efficiently executed by the computer A procedure will not compile when one or more lines of code violate the syntax rules of VBA

9 Runtime Errors When a statement combination is invalid, errors may occur when the procedure executes Figure 9-2 Runtime error example

9 Logic Errors A logic error occurs when a procedure executes successfully but produces an incorrect result Common logic errors include:  Procedures associated with events that do not execute  Procedures associated with events that execute at an unplanned time

9 Logic Errors Common logic errors (continued):  Events execute in an unplanned sequence  Focus of a control, form, or report changes earlier or later than a procedure expects  Value of a control changes earlier or later than an executed procedure expects  Criteria string used by a method is incorrect  An expression that does not accommodate null values o empty strings receives one

9 The On Error Statement On Error statement  Principal means for controlling error processing in VBA When VBA encounters an On Error statement, it enables an error handler  Error handler  A mechanism for transferring process control to specific VBA statements when an error occurs

9 The Resume, Exit, and End Statements An error-handling routine runs until it encounters a Resume, Exit, or End statement  Resume statement  Causes the system to continue processing  Exit statement  Causes the system to end the error-handling routine  End statement  The last statement in a procedure

9 Advanced Error Handling Err  Commonly used error-handling object  Belongs to VBA object library  Specializes in handling VBA runtime errors Err object properties and methods including:  Description property  Contains the Access default description of the error that just occurred

9 Advanced Error Handling Err object properties and methods including (continued):  Number property  Contains the Access internal error number for the most recent error  Clear method  Automatically sets the Number property to zero

9 Handling Logic Errors by Using MsgBox One technique for understanding the underlying logic of your code is to include MsgBox statements in the code  Display the value of variables as the code executes  When values are displayed, you can see what is actually happening in the procedure and compare that to what you think should be happening

9 Using VBA’s Debug Facility to Find and Solve Complex Logic Errors Simplest way to interact with the Debug facility is to use the Print method in conjunction with the Debug object Debug object  Allows a program to communicate with the Debug facility  Supports Assert and Print methods

9 Using VBA’s Debug Facility to Find and Solve Complex Logic Errors Figure 9-3 The Immediate window of the Debug facility

9 Breakpoints, Step into, and Step Over Breakpoint  Suspends the execution of a procedure Step Into and Step Over menu items  Used to execute one statement in a procedure at a time VBA Toggle Breakpoint  Allows programmer to examine state of the processing at the time when the statement executed Clear All Breakpoints option  Clears all breakpoints set previously

9 Breakpoints, Step into, and Step Over Step Into menu item  Causes execution to continue to progress one statement at a time inside the invoked procedure  If the statement being executed invokes a procedure or a function, then Step Into will enter that procedure or function, allowing you to debug it as well

9 Breakpoints, Step into, and Step Over Step Over menu item  If the statement being executed invokes a procedure or a function, Step Over does not go into the procedure or function Run to Cursor item in the Debug menu  Causes Access to continue processing until it reaches the point where you have placed your cursor and then procedure will then suspend processing

9 Breakpoints, Step into, and Step Over When you select the Continue item on the Run menu, Access will continue processing the procedure until it reaches another breakpoint Reset menu item  Resets the internal representation of the code so that the execution environment returns to what it would be before the procedure is executed

9 Breakpoints, Step into, and Step Over Set Next Statement option  Causes the system to move to the statement that currently contains the cursor and makes it the next executable statement Show Next Statement option  Displays the next executable statement in the procedure

9 Using Watch Expressions to Find Logic Errors Watch window allows your code to break on changes in the variable that you want to analyze Figure 9-4 The Add Watch window

9 Using Watch Expressions to Find Logic Errors You can change or delete watch expressions by using the Edit Watch menu item Figure 9-5 The Edit Watch window

9 Using Watch Expressions to Find Logic Errors Locals window  Displays the current values of all expressions encountered in the procedure currently being executed Figure 9-6 The Locals window

9 Call Stack Window Call Stack window  Shows the sequence of procedures that were invoked, leading to the suspended procedure Figure 9-7 The Call Stack window

9 Repairing and Compacting a Database Corrupt application  Occurs when an error affects the Access internal representation of an application’s tabled, forms, reports, procedures, and macros Symptoms of a corrupt application  Access may recognize when the application is corrupt and notify the user when the application is opened  Application may run in a strange and unpredictable fashion

9 Chapter Summary Statements  Accomplish an objective by organizing the objects, variables, and expressions used in a procedure  Assign values to properties, influence the execution of a procedure, and change aspects of Access environment To build an Access application, a developer must construct statements, order them inside a procedure, and determine when the procedure should execute

9 Chapter Summary VBA supports several types of statements  Assignment statement  Object variables require the Set statement  With statement  Conditional Execution statements  Provided in the form If…Then…Else and If…Else statements  Select Case statement

9 Chapter Summary VBA supports several looping statements  For Next Loop, the Do While loop, and the Do Until loop  Provide different ways to control the number of loop executions  Do statements  Allow the condition controlling the execution of a loop to be placed either at the beginning or the end of the loop

9 Chapter Summary On Error statement  Establishes error-handling capabilities within a procedure Compilation errors  Often identified as soon as statements are entered Runtime errors  Cause Access to display an error message and highlight the line where processing stopped

9 Chapter Summary Logic errors  Can be debugged as an application is executing Breakpoints  Suspend execution of an application Repair database menu item  Used to repair a corrupt database Compacting a database  Can significantly reduce the amount of storage space required by an Access application