Friday, 18 November 2011

Introduction to VBA Loops - Learning Access VBA Tutorial 3

This is the third tutorial in the Learning Access VBA series. In the first tutorial we learnt how to create a simple VBA Sub Procedure and set up up Variables; and in the second, we looked at Evaluating Conditions with the IF THEN ELSE statement.  Today we are going to move on to VBA Loops.

Loops are are common feature of most programming languages, and VBA is no exception.  Loops enable lines within in a defined section of code to be repeated as many times as is necessary, until a specific point is reached when the program flow may move on to the next section of code.  There are different kinds of loops in VBA.  The FOR ... NEXT loop, for example, repeats the loop a set number of times.  We would use  this if we wanted the loop to repeat, say, 10 times, and then move on.  With the DO ... UNTIL loop, on the other hand, the defined section of code would repeat indefinitely, until a specified condition arises.  This may be something to use if you want to repeat the loop until a user enters a specific value into an input box.

The FOR ... NEXT Loop

Let take a look at how the FOR ... NEXT loop is constructed.

For I = 1 To 10
    lines of code to be repeated
Next I

The FOR NEXT loop has a counter which is referred to above as I.  This is basically a variable name - we could have called it a different name if we had wanted.  The first line of the statement (For I = 1 to 10) sets the counter at 1, and tells the procedure to continue repeating the lines of code until the counter reaches 10.  Each time the program flow executes the NEXT keyword at the end of the statement, the counter is increased by 1.  As it does so, it also checks whether the counter has reached 10 (or whatever level it had been set).  If it has not, the lines of code repeat another time until the Next keyword executes again, increasing the count by another 1.  Once it reaches the level set, the program flow continues past the NEXT keyword into the next section of code.

So let's try an exercise to see how the FOR ... NEXT loop works in practice.

  1. Create a new unbound form in Design View.
  2. Add a Command Button.
  3. Add the VBA code (listed below) to the button's OnClick Event (please see the first Learning VBA Tutorial if you need help with this, or any of the previous steps):
  4. Click the SAVE icon.

Dim I As Integer
Dim x As Integer    
For I = 1 To 10    
    x = (9 * Rnd() + 1)
    Debug.Print "Counter = " &  I  & ";  Random Number = " &  x        
Next I

Before we try out the code, we are going to open the VBA Editor's Immediate Window.  To do this,  click the VBA VIEW menu, and then click IMMEDIATE WINDOW from the menu list.  When we run the code, the STOP statement will re-open the VBA editor during runtime, to allow us to Step Into each
line of code as it executes.  Then when the program flow reaches the Debug.Print line, the Counter value and the value of variable x (a random number generated by the code) is displayed in the immediate window.

When you are ready, open the Access form and click the command button we created earlier.  The VBA editor will then open.  The current line being executed is highlighted in yellow.  To advance to the next line we click Step Into from the DEBUG menu (alternative use the F8 key which is far more convenient).  Then watch what happens as the program flow progresses through the loop.  Also take note of the counter value which is displayed before the random number in the immediate window.

Figure 1: The VBA Editor with Immediate Window.  The editor was opened during
run time due to the STOP command which acted as a breakpoint.
Another way of adding a break point is to click on a line
and then select TOGGLE BREAKPOINT from the DEBUG menu.
The DO ... UNTIL Loop

Let's take a quick look at the how the DO ... UNTIL loop is constructed.

Do Until variableName = thisValue
    lines of code to be repeated

The first line of the DO ... UNTIL loop is testing whether the specified condition is true or not.  If it is not true, the lines of code within the loop are executed until the LOOP keyword is reached at the end of the statement. Then the program flow returns to the first line of the statement where the condition is tested again.  If the lines of code within the loop had changed the value of the variable being tested so the condition becomes true, the program flow exits the loop at this top line.

Here is an example of a DO ... UNTIL Loop that you may  like to try for yourself.  Just change the code from the first exercise so that this runs in its place.

Dim varNumber As Integer
Dim varResponse As Integer
varNumber = 9 * Rnd() + 1

Do Until varResponse = varNumber
    varResponse = InputBox("Enter a Number Between 1 and 10", "Guess A Number")
    If varResponse > varNumber Then
        MsgBox "Your Guess is too High"
    ElseIf varResponse < varNumber Then
        MsgBox "Your Guess is too Low"
        MsgBox "Congratulations: Your Guess is Correct."
    End If
    MsgBox "Game Over!!!"

(NB for the sake of clarity, this code does not deal with the possibility that the user may click cancel or enter an empty value in the input box.  If this happens an error results, and the procedure crashes!)

This code is for a simple Guess the Number Game.  The user clicks the command button to begin.  The code then generate a random number and stores it in a variable called varNumber.  Once the program flow gets to the loop, the variable varResponse is tested to determine whether it is equal to the random number stored in varNumber.  If it is not, the lines of code within the loop are executed.  The user is asked to guess the number in a pop up input box.  The code then enters an IF ... THEN ... ELSE statement where the user is informed whether his guess is too high, too low, or is indeed correct.  When the LOOP keyword is reached, the program flow returns to the DO UNTIL keyword where the value of the guess is tested against the random number to determine whether the lines of code should be repeated, or whether the program flow can continue past the LOOP keyword.

When you try this code, please feel free to enter a BREAKPOINT so you can follow the program flow for yourself.  You can do this by entering the STOP statement as we did in the first exercise.  Alternatively, click the line where you want to insert the breakpoint, and then click TOOGLE BREAKPOINT from the DEBUG menu.  The line is then highlighted in red., but the action is the same.

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