Visual Studio 2013 Lesson 13: Making Decisions Using If…Then…Else
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Decision making process is an important part of programming in Visual Studio 2013 because it can solve practical problems intelligently and provide useful output or feedback to the user. For example, we can write a Visual Studio 2013 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..Then…Else control structure.
13.1 Conditional Operators
The conditional operators are powerful tools that resembles mathematical operators . These operators allow a Visual Studio 2013 program to compare data values and then decides what actions to take, whether to execute a program or terminate the program and more. They are also known as numerical comparison operators which 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 following table:
| Operator | Description |
|---|---|
| = | Equal to |
| > | Greater than |
| < | Less than |
| >= | Equal to or Greater than |
| <= | Less than or Equal to |
| <> | Not equal to |
13.2 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.
The logical operators are shown in the following table:
| 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 |
the above operators can be used to compare numerical data as well as non-numeric data such as text(string). 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.
13.3 Using the If control structure with the Comparison Operators
To effectively control the Visual Studio 2013 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.
13.3(a) If….Then
Statement
This is the simplest control structure which instructs the computer to
perform a certain action specified by the Visual Studio 2013
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 Studio 2013 expressions
End If
Example 13.1
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.
13.3(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 Studio 2013 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
Example 13.2
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 13.3
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.
13.3(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.The general structure for the if…then.. Else statement is
If condition Then
Visual Studio 2013 expression
ElseIf condition Then
Visual Studio 2013 expression
ElseIf condition Then
Visual Studio 2013 expression
.
.
Else
Visual Studio 2013 expression
End If
Example 13.4
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
Grade=”B”
ElseIf Mark>=40 And Mark <60
Grade=”C”
Else
Grade=”D”
End If
End Sub