Microsoft Access 2010 : TESTING AND DEBUGGING VBA CODE (part 2) - The Debug.Assert Statement, Breakpoints, Stepping through Code

3. The Debug.Assert Statement

In addition to Debug.Print, you can also use Debug.Assert to pause the code execution when a Boolean value passed to the statement evaluated to False. For example, the following code stops the code execution when a blnUnderBudget is False:

Option Compare Database
Private blnUnderBudget As Boolean
Const curBudget = 1000

Sub GoShopping()
    Dim intSuits As Integer
    Dim curSuitPrice As Currency
    Dim curTotalPrice As Currency
    Dim curBudget As Currency
    Dim i as Integer

    curBudget = 1000
    curSuitPrice = 100
    intSuits = InputBox("Enter the desired number of suits", "Suits")

    For i=1To intSuits
        curTotalPrice = curTotalPrice + curSuitPrice
        If curTotalPrice > curBudget Then
            blnUnderBudget = False
        Else
            blnUnderBudget = True
        End If
        Debug.Assert blnUnderBudget
    Next
End Sub                                                    

					  

The code breaks every time you go over budget on your shopping trip. You can use this statement when testing for specific conditions within your code. Although Debug.Assert is a good debugging tool, you should not use it in live code because it's a rather abrupt way to stop an application. The user would get no warning, and because the code stops, you do not get to exit gracefully by providing him with a friendly message or explanation. However, the Debug.Assert statement is essentially ignored in an MDE or ACCDE. That said, we also caution you that you will need to use a conditional compilation to essentially wrap the specified code in If, Then, and Else statements and stipulate that they will be ignored. You can find additional guidance about conditional compilations online by searching the Access communities and help resources. It seems much easier to search and comment out or remove the lines.

4. Breakpoints

Breakpoints are essentially places in your code where you want to pause the execution of the code. For example, if you want to check the value of the variable curTotalCost midway through the following procedure, you can use the Debug.Print statement as shown in the following code snippet or set a breakpoint, which we will demonstrate shortly.

Sub HowMuchCanWeSpend()
    Dim curTotalPrice As Currency
    Dim curUnitPrice As Currency
    Dim intNumSocks As Integer
    Dim i As Integer

    curUnitPrice = 3.5
    intNumSocks = InputBox( _
        "Please enter the number of pairs of socks you want.", _
        "Pairs of Socks")

    For i=1 To intNumSocks
        curTotalPrice = curTotalPrice + curUnitPrice
    Next

    Debug.Print curTotalPrice
End Sub                                                    

This code uses the Immediate window to display the amount you'll spend for the total sock purchase (using the line, Debug.Print curTotalPrice). That's great for a single transaction, but what if you want to see the total accumulating as you go? One option is to add a Debug.Print statement within the For...Next loop. Alternatively, you can set a breakpoint anywhere in the loop. When the breakpoint is reached, the code stops running and you can use the Immediate window to obtain the value of any variable of interest (? <VariableName>). You can set a breakpoint on any line of code except for Dim statements and comments.

Setting a breakpoint is as simple as clicking in the left margin of the Code window. A brick-colored dot appears in the margin and the corresponding line of code is highlighted. To clear a breakpoint, just click on the dot in the left margin. You can also set and clear a breakpoint by placing the cursor in the desired line of code and selecting Debug Toggle Breakpoint or by pressing F9. When you run code that has breakpoints, every time the breakpoint is reached, the code execution stops and waits for you to decide (and tell it) what to do next. At that point, you basically have four options:

• Check the value of variables in the Immediate window. When your code reaches a breakpoint, the value of all variables is retained. You can check the value of any variable by using the Debug.Print statement or the ? character within the Immediate window ..

• Use your mouse to hover over any variable in the current procedure. The value of the variable appears close to the mouse cursor. Keep in mind that variables will show no value when you hover over them if a value was not assigned to them before you reached the break point,

• Press F5 or select Run Continue to continue code execution. The code will run until it reaches the next breakpoint or the end of the procedure.

• Press F8 to step through the code (explained in the following section). When you press F8, the code execution stops at the next line of code — even if it does not have a breakpoint. This enables you to step through the code and identify missed steps or other errors. As you step through the code, you can continue to check the value of the variables or press F5 to resume code execution.

BREAKPOINTS

When VBA encounters a breakpoint, it pauses execution immediately before the line of code executes. The line of code that contains the breakpoint is not executed unless or until you choose to step through the code using the F8 key (or continue running the code).

5. Stepping through Code

In most cases, you design code to run with little or no user intervention. However, when testing the code, it is often helpful or even necessary to do more than insert a couple of breakpoints or include some Debug.Print statements. If your code has several variable changes and/or some intricate looping, it can help clarify the process to step through the code line by line. This allows you to validate the value of variables after each line of code has executed, and it can help you pinpoint any errors or mistakes in the underlying logic.

To start stepping through your code, place the cursor at the point that you want to initiate the process and press F8 (you can also press F8 after the code has entered break mode to step through the remaining code). When you press F8 to begin code execution, the name of the sub or function will be highlighted in yellow. Pressing the F8 key moves the code execution from line to line, sequentially highlighting the next executable line. Much like with breakpoints, comment lines and Dim statements are skipped when stepping through code. As you press F8, the highlighted line executes.

If the current procedure calls another sub or function, F8 also executes the called procedure line-by-line. If you already tested the called procedure and are confident it doesn't contain any errors, you can execute the entire called procedure and then return to line-by-line execution of the calling procedure by pressing Shift+F8. This is called stepping over the procedure.

Stepping over the called procedure executes the entire procedure and then returns to the calling procedure to continue with code execution one step at a time. If you're within a called procedure, you can press Ctrl+Shift+F8 to step out of the current procedure. This may sound like nothing more than semantics. And indeed, if you're already in the called procedure, stepping out has the same effect as stepping over. But, the two actions will not always provide the same results. We'll use the following example to illustrate the difference and to give you some practice with code. Assume you're stepping through the following code:

Option Compare Database
Private blnUnderBudget As Boolean
Const curBudget As Currency = 1000

Private Sub GoShopping()
    Dim intSuits As Integer
    Dim curSuitPrice As Currency
    Dim curTotalPrice As Currency
    Dim curBudget As Currency
    Dim i As Integer

    curBudget = 1000
    curSuitPrice = 100
    intSuits = InputBox("Enter the desired number of suits", "Suits")

    For i=1To intSuits
        curTotalPrice = curTotalPrice + curSuitPrice
        If curTotalPrice > curBudget Then
            blnUnderBudget = False
        Else
            blnUnderBudget = True
        End If
    Next

    If blnUnderBudget = False Then
        OverBudget
    End If
End Sub

Private Sub OverBudget()
    Debug.Print "You've gone over budget."
    Debug.Print "You need to work some overtime."
    Debug.Print "Remember to pay your taxes."
End Sub                                                    				  

Use the F8 key to step through the code until you reach the last If...Then loop (If blnUnderBudget = False Then). When the OverBudget line is highlighted in yellow (meaning it hasn't yet been executed), stepping over the OverBudget procedure returns execution to the line after the OverBudget call (in this case the End If line). If you step out of the procedure, the OverBudget procedure runs, and your code returns to the GoShopping procedure and completes the procedure. If, however, you use the F8 key to step through the code until you reach the first line of the OverBudget procedure, stepping out of the procedure returns you to the line after the OverBudget called (the End If line).