Topic 5.3.2: Conditional Constructs
IF....ELSE
- IF is the simplest form of control statement, frequently used in decision making and changing the control flow of the program execution.
Syntax:
If condition Then
[Statement(s)]
End If
- Where, condition is a Boolean or relational condition and Statement(s) is a simple or compound statement.
Example:
Dim a As Integer = 10
' check the boolean condition using if statement
If (a < 20) Then
' if condition is true then print the following
Console.WriteLine("a is less than 20")
End If
- If the condition evaluates to true, then the block of code inside the If statement will be executed. If condition evaluates to false, then the first set of code after the end of the If statement (after the closing End If) will be executed.
IF....THEN....ELSE
- An If statement can be followed by an optional Else statement, which executes when the Boolean expression is false.
Syntax:
If (boolean_expression) Then
'statement(s) will execute if the Boolean expression is true
Else
'statement(s) will execute if the Boolean expression is false
End If
- Where, condition is a Boolean or relational condition and Statement(s) is a simple or compound statement.
Example:
Dim a As Integer = 100
If (a < 20) Then
Console.WriteLine("a is less than 20")
Else
Console.WriteLine("a is not less than 20")
End If
- If the condition evaluates to true, then the block of code inside the If statement will be executed. If condition evaluates to false, then the ELSE part will be executed.
ELSEIF
- An If statement can be followed by an optional Else if...Else statement, which is very useful to test various conditions using single If...Else If statement.
- When using If... Else If... Else statements, there are few points to keep in mind.
- An If can have zero or one Else's and it must come after an Else If's.
- An If can have zero to many Else If's and they must come before the Else.
- Once an Else if succeeds, none of the remaining Else If's or Else's will be tested.
Syntax:
If (boolean_expression 1) Then
' Executes when the boolean expression 1 is true
ElseIf ( boolean_expression 2) Then
' Executes when the boolean expression 2 is true
ElseIf ( boolean_expression 3) Then
' Executes when the boolean expression 3 is true
Else
' executes when the none of the above condition is true
End If
- Where, condition is a Boolean or relational condition and Statement(s) is a simple or compound statement.
Example:
Dim a As Integer = 100
If (a = 10) Then
' if condition is true then print the following '
Console.WriteLine("Value of a is 10") '
ElseIf (a = 20) Then
' if else if condition is true '
Console.WriteLine("Value of a is 20") '
ElseIf (a = 30) Then
' if else if condition is true
Console.WriteLine("Value of a is 30")
Else
' if none of the conditions is true
Console.WriteLine("None of the values is matching")
End If