|
|
| In previous lessons, we have learned how to create Visual Basic code that
can
accept input from the user and display the output without controlling
the program flow. In this chapter, you will learn how to crreate VB
code that can make decision when it process input from the user, and
control the program flow in the process. Decision making process is an
important part of programming because it can help to solve practical
problems intelligently so that it can provide useful output or feedback
to the user. For example, we can write a program that can ask the
computer to perform certain task until a certain condition is met. |
|
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
more. These operators are shown in Table 7.1.
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 7.1: Conditional Operators
|
Operator |
Meaning |
|
= |
Equal to |
|
> |
More than |
|
< |
Less Than |
|
>= |
More than and equal |
|
<= |
Less than and equal |
|
<> |
Not Equal to |
|
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 |
|
* 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.
7.3 Using If.....Then.....Else
Statements with Operators
To effectively control the VB program flow, we shall
use If...Then...Else statement together with the conditional 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 |
|
|