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In this lesson, you will learn how to write Visual Basic 2012 code that can make decisions when it processes input from the user and controls the program flow.
9.1 Decision Making using If…Then…Else
For example, we can write a Visual Basic 2012 program that can ask the computer to perform a certain task until a certain condition is met or a program that will reject non-numeric data. In order to control the program flow and to make decisions, we use the If…Then…Else control structure. The if…Then…Else control structure employs the conditional operators and the logical operators to make decision
9.2 Conditional Operators
The conditional operators are powerful tools that resemble mathematical operators. These operators allow a Visual Basic 2012 program to compare data values and then decide what actions to take, whether to execute a program or terminate the program and more. They are also known as numerical comparison operators. Normally they are used to compare two values to see whether they are equal or one value is greater or less than the other value. The comparison will return a true or false result. These operators are shown in Table 9.1.
Table 9.1 Conditional Operators
| Operator | Description |
|---|---|
| = | Equal to |
| > | Greater than |
| < | Less than |
| >= | Equal to or Greater than |
| <= | Less than or Equal to |
| <> | Not equal to |
9.2 Logical Operators
Sometimes we might need to make more than one comparisons before a decision can be made. In this case, using numerical comparison operators alone is not sufficient, we need to use the logical operators. These logical operators are shown in Table 9.2.
Table 9.2 Logical Operators
| Operator | Description |
|---|---|
| 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 true |
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* In making strings comparison, 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.
9.3 Using the If control structure with the Comparison Operators
To effectively control the Visual Basic 2012 program flow, we shall use the If control structure together with the conditional operators and logical operators. There are basically three types of If control structures, namely If….Then statement, If….Then… Else statement and If….Then….ElseIf statement.
9.3(a) If….Then Statement
This is the simplest control structure which instructs the computer to perform a certain action specified by the Visual Basic 2012 expression if the condition is true. However, when the condition is false, no action will be performed. The syntax for the if…then.. statement is
If condition Then Visual Basic 2012 expression End If
Example 9.1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim myNumber As Integer myNumber = TextBox1.Text If myNumber > 100 Then Label2.Text = " You win a lucky prize" End If End Sub
* When you run the program and enter a number that is greater than 100, you will see the “You win a lucky prize” statement. On the other hand, if the number entered is less than or equal to 100, you don’t see any display.
9.3(b) If….Then…Else Statement
Using just If….Then statement is not very useful in programming and it does not provide choices for the users. In order to provide a choice, we can use the If….Then…Else Statement. This control structure will ask the computer to perform a certain action specified by the Visual Basic 2012 expression if the condition is met. And when the condition is false, an alternative action will be executed.
The syntax for the if…then… Else statement is
If condition Then
Visual Basic 2012 expression
Else
Visual Basic 2012 expression
End If
Example 9.2
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim myNumber As Integer myNumber = TextBox1.Text If myNumber > 100 Then Label2.Text = " Congratulation! You win a lucky prize" Else Label2.Text = " Sorry, You dif not win any prize" End If End Sub
* When you run the program and enter a number that is greater than 100, the statement “Congratulation! You win a lucky prize” will be shown. On the other hand, if the number entered is less than or equal to 100, you will see the “Sorry, You did not win any prize” statement
Example 9.3
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim myNumber, MyAge As Integer myNumber = TextBox1.Text MyAge = TextBox2.Text If myNumber > 100 And myAge > 60 Then Label2.Text = " Congratulation! You win a lucky prize" Else Label2.Text = " Sorry, You did not win any prize" End If End Sub
* This program use the logical operator And besides the conditional operators. This means that both the conditions must be fulfilled in order for the conditions to be true, otherwise, the second block of code will be executed. In this example, the number entered must be more than 100 and the age must be more than 60 in order to win a lucky prize, any one of the above conditions not fulfilled will disqualify the user from winning a prize.
9.3(c) If….Then…ElseIf Statement
If there are more than two alternative choices, using just If….Then….Else statement will not be enough. In order to provide more choices, we can use the If….Then…ElseIf Statement. executed. The general format for the if…then… Else statement is
If condition Then Visual Basic 2012 expression ElseIf condition Then Visual Basic 2012 expression ElseIf condition Then Visual Basic 2012 expression . Else Visual Basic 2012 expression End If
Example 9.4
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim Mark As Integer Dim Grade as String Mark = TextBox1.Text If myNumber >=80 Then Grade="A" ElseIf Mark>=60 and Mark<80 then Grade="B" ElseIf Mark>=40 and Mark<60 then Grade="C" Else Grade="D" End If End Sub