3-5: Variable Declarations (continued)

Declaring Global Variables

In cases like this Visual Basic allows for variables to be declared outside of any subprogram, but within the Form class. Usually you make such declarations at the very beginning of the program code but after the Public Class Form1 declaration. The syntax is slightly different - you should use the keyword Private rather than Dim as shown next:

Public Class Form1
Private year as Integer

Private Sub btnNextYr_Click( ... )
year = year + 1
txtYear.Text = year
End Sub

... (etc)

End Class

Run this program and you should find that the value in the Year textbox increases as expected every time the button is clicked.

Unlike a local variable, the variable year, because it is declared outside any subprogram exists as long as the form object is active. Moreover, the variable is accessible inside any event procedure that is defined in the code. Thus in the example, although year is not declared inside the btnNextYr_Click event it is still possible to use the variable in an expression and to change its value within the event.

The calculation of the capital must be done in a similar fashion. The capital from the previous year must have the interest for the current year added to it, so the statement will be

capital = capital + interestAmount

Therefore the variable capital cannot be declared locally because its value will be initialised to 0 each time the event is executed. The code therefore becomes

Public Class Form1
Private year as Integer
Private capital as Double

Private Sub btnNextYr_Click( ... )
Dim interestAmount as Double
Dim interestRate As Double
'
' Assign value to InterestRate variable from text property
'
interestRate = CDbl(txtRate.Text)
'
' Calculate new values for year, interest amount and capital
'
year = year + 1
interestAmount = capital * interestRate
capital = capital + interestAmount
'
' Assign the values to the textboxes
'
txtYear.Text = year
txtInterest.Text = interestAmount
txtCapital.Text = capital
End Sub

... (etc)

End Class

where we have also declared the local variable interestAmount and assigned it the value

capital * interestRate

and assigned the calculated values to the Text properties of the textboxes.