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Event Driven Programming

sOperator = sNewOperator

sRight = “”
bLeft = False
End If
End Sub

If bLeft is true, meaning the user has just entered the left part of the calculation, this subroutine
sets the sOperator variable we created in step 5 to equal the operator entered, which is passed
to AddOperator as the string sNewOperator. The second step is to set bLeft to False, because the
entry of an operator means the user is done entering the left side of the equation. In order to
handle entries that string multiple operators together, such as 9 * 3 * 2 * 6, we need to also check
whether bLeft is false, meaning the user has entered an operator where we were expecting an
equals. First we call the Click event for the equals button (described in the next step), which does
the calculation and sets tbResult to the result of what has already been entered. Then we clear
sRight so the user can enter the next number, and set bLeft to False so the program knows we
are entering the right hand side of the calculation next. Finally, add an AddOperator call to the
Click event of each operator button, using the same method as we used in step 7 to create the
Click events for the number buttons. Code for the plus button will look like this:

Private Sub btnPlus_Click()
AddOperator (“+”)
End Sub
Likewise, the code for the minus button will look like this:
Private Sub btnMinus_Click()
AddOperator (“-”)

End Sub
9: Create the Click event for the equals button, which is the most complex code in this program.
Create the subroutine structure like we did for the other buttons, by double-clicking the equal’s
button on our form. Our subroutine will look like this when we’ve entered the code:
Private Sub btnEquals_Click()
If sLeft <> “” And sRight = “” And sOperator <> “” Then
sRight = sLeft
End If

If sLeft <> “” And sRight <> “” And sOperator <> “” Then
iLeft = sLeft
iRight = sRight
Select Case sOperator
Case “+”

iResult = iLeft + iRight
Case “-”

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