Lesson 7 : Controlling Program Flow

7.1 Conditional Operators
To control the VB program flow, we can use various conditional operators. Basically, they resemble mathematical operators. Conditional operators are very powerful tools, they let the VB program compare data values and then decide what action to take, whether to execute a program or terminate the program and etc. These operators are shown in Table 7.1.

Table 7.1: Conditional Operators






Operator
Meaning
=
Equal to
>
More than
<
Less Than
>=
More than and equal
<=
Less than and equal
<>
Not Equal to

* You can also compare strings with the above operators. However, there are certain rules to follows: Upper case letters are less than lowercase letters, "A"<"B"<"C"<"D".......<"Z" and number are less than letters.

7.2 Logical Operators
In addition to conditional operators, there are a few logical operators which offer added power to the VB programs. There are shown in Table 7.2.

Table 7.2
Operator
Meaning
And
Both sides must be true
or
One side or other must be true
Xor
One side or other must be true but not both
Not
Negates truth


7.3 Using If.....Then.....Else Statements with Opreators
To effectively control the VB program flow, we shall use If...Then...Else statement together with the conditonal operators and logical operators.
The general format for the if...then...else statement is

If conditions Then

VB expressions

Else

VB expressions

End If

* any If..Then..Else statement must end with End If. Sometime it is not necessary to use Else.

Example:

Private Sub OK_Click()

firstnum = Val(usernum1.Text)
secondnum = Val(usernum2.Text)
total = Val(sum.Text)
If total = firstnum + secondnum And Val(sum.Text) <> 0 Then
correct.Visible = True
wrong.Visible = False
Else
correct.Visible = False
wrong.Visible = True
End If

End Sub