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Lesson 9 : Using If...Then...Else


In the previous lessons, we have learned how to write code that accepts input from the user and displays the output without controlling the program flow. In this lesson, you will learn how to write Visual Basic 2012 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 in Visual Basic 2012 because it can solve practical problems intelligently and provide useful output or feedback to the user. For example, we can write a Visual Basic 2012 program that can ask the computer to perform 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 need to use the conditional operators and the logical operators together with the If control structure.

9.1 Conditional Operators

The conditional operators are powerful tools that resembles 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 and an action taken. In this case, using numerical comparison operators alone is not sufficient, we need to use additional operators, and they are 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

* Normally the above operators are use to compare numerical data. However, you can also compare strings with the above operators. 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 dif 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 beside 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


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