Visual Basic 6 Programming Blue Book: The Most Complete, Hands-On Resource for Writing Programs with Microsoft Visual Basic 6!:Visual Design + Basic Code = Visual Basic
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Listing 5.9 The Operator button Click event procedure.


Private Sub cmdOperators_Click(Index As Integer)

Dim Result As Double

‘ See if there is at least 1 value on the stack.
‘ We cannot perform an operation otherwise.

If StackPointer < 1 Then Exit Sub

Select Case Index
Case 0 ‘ plus
Result = Stack(StackPointer) + Val(txtDisplay.Text)
Case 1 ‘ minus
Result = Stack(StackPointer) - Val(txtDisplay.Text)
Case 2 ‘ multiply
Result = Stack(StackPointer) * Val(txtDisplay.Text)
Case 3 ‘ divide
If (Val(txtDisplay.Text) <> 0) Then
Result = Stack(StackPointer) / Val(txtDisplay.Text)
Else
MsgBox “Cannot divide by 0!”
Exit Sub
End If
End Select

StackPointer = StackPointer - 1
NewEntry = True
DisplayHasData = True
DisplayResult (Result)

End Sub


The calculator is now complete. (Well, at least it’s working, as shown in Figure 5.6.) Take it for a spin, and see how it works. We will be adding some enhancements later in the chapter. But, first, let’s sit back and think about the miles we’ve covered.

Is That Cool Or What?
Well, I certainly think so. Just look at what Visual Basic has allowed us to accomplish. While it may have taken you—a Visual Basic programming newcomer—an hour or two to complete this calculator project, even a moderately experienced Visual Basic programmer would have knocked it off in 15 to 30 minutes. So you can see that for a very modest expenditure of time, we have a fully functional (albeit basic) calculator with a slick visual interface.
Just thinking about how long this would have taken with the old ways of programming makes me sweat. If you are new to programming, you can’t really appreciate the incredible gains. Just imagine the world of difference between a horse and buggy and a 400-horse power Jaguar with a wet bar and mink seats—that should give you some idea. For those who are unfamiliar with any other way of programming, the speed and power of Visual Basic are simply the way programming ought to be. I agree completely—I just wish it had been around 10 years ago.

Figure 5.6  The calculator in action.

But enough complaining. Let’s take a moment to review what we have accomplished so far. The calculator project helps to reinforce some of the Visual Basic tools and concepts you learned in previous chapters: drawing controls on a form, setting control properties, and writing event procedures. You’ve also been introduced to several powerful new techniques. Let’s summarize:


•  Data arrays are a useful way of storing large quantities of data in an indexed fashion. Combining arrays with loops is a powerful technique that you’ll see frequently in all kinds of Visual Basic programs.
•  A control array lets you group two or more controls under the same name. By sharing a name and event procedures, control arrays greatly reduce the coding required for certain tasks.
•  General procedures are used to isolate individual program tasks, reducing coding and debugging effort and producing simpler and more robust programs.
•  Visual Basic’s built-in functions perform a variety of commonly needed tasks.
•  The Debug window and the Debug.Print method provide a useful way for peeking at what’s going on inside your program.

At this point, I think you’re ready to start experimenting with Visual Basic on your own. Don’t give up reading this book, of course, but by combining further readings with experimentation, you’ll learn more quickly than by doing just one or the other. I recommend that you spend some time browsing through Visual Basic’s function library; it contains a great deal of useful stuff. If you’re feeling ambitious, a good project to try is a calculator that uses algebraic logic. (Hint: It’s more difficult than an RPN calculator.)

While our new calculator is quite nice, a few more features would make it even better. We’ll spend the remainder of this chapter polishing our project.
Enhancing The Calculator
The enhancements that I have in mind will add more functionality to the calculator. We would like the ability to calculate the trigonometric functions Sin, Cos, and Tan, as well as the inverse (1/X). The capability of raising a number to a power would also be a valuable addition to the calculator. Another useful capability would be copying a number in the calculator’s display to the Windows Clipboard for use in other Windows applications. This would permit the user to perform calculations and then paste the answer directly into any application, such as a letter you are writing with Word or WordPerfect. Finally, having a Last X button that displays the top value from the stack would be useful for certain calculations.

For the trig calculations, we will use Visual Basic’s built-in Sin, Cos, and Tan functions. As you may already have guessed, we’ll use a control array of Command Buttons for these three calculations. Using the techniques you learned earlier, create this array of three buttons on the Calculator form. You may need to enlarge the form and rearrange the other buttons to accommodate new buttons, unless you planned ahead better than I did.
Assign the array of buttons the Name property cmdTrig and set the Caption properties to Sin, Cos, and Tan. The code for the event procedure is shown in Listing 5.10. Again, you must be sure that the Index property of each button matches the corresponding Case statement in the procedure. After calculating the correct value, all that is required is setting the two flags and calling the DisplayResult function. (I told you we would get a lot of mileage out of this function.)
Listing 5.10 The Trigonometric Command Button Click event procedure.


Private Sub cmdTrig_Click(Index As Integer)

‘ Trigonometric calculations.

Dim Result As Double

Select Case Index
Case 0 ‘ Sin
Result = Sin(txtDisplay.Text)
Case 1 ‘ Cos
Result = Cos(txtDisplay.Text)
Case 2 ‘ Tan
Result = Tan(txtDisplay.Text)
End Select

DisplayResult (Result)
NewEntry = True
DisplayHasData = True

End Sub


The remaining buttons we will put on the calculator are all single buttons and not part of a control array. Add four more Command Buttons, assigning the Name and Caption properties shown here:

•  Command Button 1

Caption: 1/X
Name: cmdInverse

•  Command Button 2

Caption: Y^X
Name: cmdPower

•  Command Button 3

Caption: Last X
Name: cmdLastX

•  Command Button 4

Caption: Copy
Name: cmdCopy


When you have finished adding the buttons, your form will look similar to Figure 5.7. Although I have rearranged some of the buttons, the final layout is up to you. If you find the default font too small, just change the Font property of the new buttons.
The event procedure for the Last X button is shown in Listing 5.11. After verifying that the stack has at least one value on it, the value on the top of the stack is displayed and the flagsare set.

Figure 5.7  The completed Calculator form.

Listing 5.11 The Last X button Click event procedure.


Private Sub cmdLastX_Click()

‘ Display the value at the top of the stack.

If StackPointer > 0 Then
DisplayValue (Stack(StackPointer))
End If

NewEntry = True
DisplayHasData = True

End Sub


The code to calculate the inverse of a number (1/X) is simple. The only possible catch would be if the user tries to invert 0, so we’ll test for that condition and display a message box if it occurs. Otherwise, all that’s necessary is to divide 1 by the value in the display, calling DisplayResult to display it. As usual, the NewEntry and DisplayHasData flags need to be set as well. The code for this event procedure is shown in Listing 5.12.
Listing 5.12 The 1/X button Click event procedure.


Private Sub cmdInverse_Click()

‘ Displays the inverse of the
‘ number in the display.

If txtDisplay.Text = 0 Then
MsgBox “Cannot take inverse of 0!”
Exit Sub
Else
DisplayResult (1 / txtDisplay.Text)
NewEntry = True
DisplayHasData = True
End If

End Sub






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