Loops in VB.NET

Loops

For Loop

The For loop is the most popular loop. For loops enable us to execute a series of expressions multiple numbers of times. The For loop in VB .NET needs a loop index which counts the number of loop iterations as the loop executes. The syntax for the For loop looks like this:

For index=start to end[Step step]
[statements]
[Exit For]
[statements]
Next[index]

The index variable is set to start automatically when the loop starts. Each time in the loop, index is incremented by step and when index equals end, the loop ends.

Example on For loop

Module Module1

Sub Main()
Dim d As Integer
For d = 0 To 2
System.Console.WriteLine("In the For Loop")
Next d
End Sub

End Module

The image below displays output from above code.



While loop

While loop keeps executing until the condition against which it tests remain true. The syntax of while loop looks like this:

While condition
[statements]
End While

Example on While loop

Module Module1

Sub Main()
Dim d, e As Integer
d = 0
e = 6
While e > 4
e -= 1
d += 1
End While
System.Console.WriteLine("The Loop ran " & e & "times")
End Sub

End Module

The image below displays output from above code.



Do Loop

The Do loop can be used to execute a fixed block of statements indefinite number of times. The Do loop keeps executing it's statements while or until the condition is true. Two keywords, while and until can be used with the do loop. The Do loop also supports an Exit Do statement which makes the loop to exit at any moment. The syntax of Do loop looks like this:

Do[{while | Until} condition]
[statements]
[Exit Do]
[statements]
Loop

Example on Do loop

Module Module1

Sub Main()
Dim str As String
Do Until str = "Cool"
System.Console.WriteLine("What to do?")
str = System.Console.ReadLine()
Loop
End Sub

End Module

The image below displays output from above code.

Conditional Statements in VB.NET

Conditional Statements

If....Else statement

If conditional expression is one of the most useful control structures which allows us to execute a expression if a condition is true and execute a different expression if it is False. The syntax looks like this:

If condition Then
[statements]
Else If condition Then
[statements]
-
-
Else
[statements]
End If

Understanding the Syntax

If the condition is true, the statements following the Then keyword will be executed, else, the statements following the ElseIf will be checked and if true, will be executed, else, the statements in the else part will be executed.

Example

Imports System.Console
Module Module1

Sub Main()
Dim i As Integer
WriteLine("Enter an integer, 1 or 2 or 3")
i = Val(ReadLine())
'ReadLine() method is used to read from console

If i = 1 Then
WriteLine("One")
ElseIf i = 2 Then
WriteLine("Two")
ElseIf i = 3 Then
WriteLine("Three")
Else
WriteLine("Number not 1,2,3")
End If

End Sub

End Module

The image below displays output from above code.



Select....Case Statement

The Select Case statement executes one of several groups of statements depending on the value of an expression. If your code has the capability to handle different values of a particular variable then you can use a Select Case statement. You use Select Case to test an expression, determine which of the given cases it matches and execute the code in that matched case.

The syntax of the Select Case statement looks like this:

Select Case testexpression
[Case expressionlist-n
[statements-n]] . . .
[Case Else elsestatements]
End Select

Example

Imports System.Console
Module Module1

Sub Main()
Dim keyIn As Integer
WriteLine("Enter a number between 1 and 4")
keyIn = Val(ReadLine())

Select Case keyIn
Case 1
WriteLine("You entered 1")
Case 2
WriteLine("You entered 2")
Case 3
WriteLine("You entered 3")
Case 4
WriteLine("You entered 4")
End Select

End Sub

End Module

The image below displays output from above code.

Methods,Sub Procedures

Methods

A Method is a procedure built into the class. They are a series of statements that are executed when called. Methods allow us to handle code in a simple and organized fashion. There are two types of methods in VB .NET: those that return a value (Functions) and those that do not return a value (Sub Procedures). Both of them are discussed below.

Sub Procedures

Sub procedures are methods which do not return a value. Each time when the Sub procedure is called the statements within it are executed until the matching End Sub is encountered. Sub Main(), the starting point of the program itself is a sub procedure. When the application starts execution, control is transferred to Main Sub procedure automatically which is called by default.

Example of a Sub Procedure

Module Module1

Sub Main()
'sub procedure Main() is called by default
Display()
'sub procedure display() which we are creating
End Sub

Sub Display()
System.Console.WriteLine("Using Sub Procedures")
'executing sub procedure Display()
End Sub
End Module

The image below displays output from above code.



Functions

Function is a method which returns a value. Functions are used to evaluate data, make calculations or to transform data. Declaring a Function is similar to declaring a Sub procedure. Functions are declared with the Function keyword. The following code is an example on Functions:

Imports System.Console
Module Module1

Sub Main()
Write("Sum is" & " " & Add())
'calling the function
End Sub

Public Function Add() As Integer
'declaring a function add
Dim i, j As Integer
'declaring two integers and assigning values to them
i = 10
j = 20
Return (i + j)
'performing the sum of two integers and returning it's value
End Function

End Module

The image below displays output from above code.



Calling Methods

A method is not executed until it is called. A method is called by referencing it's name along with any required parameters. For example, the above code called the Add method in Sub main like this:
Write("Sum is" & " " & Add()).

Method Variables

Variables declared within methods are called method variables. They have method scope which means that once the method is executed they are destroyed and their memory is reclaimed. For example, from the above code (Functions) the Add method declared two integer variables i, j. Those two variables are accessible only within the method and not from outside the method.

Parameters

A parameter is an argument that is passed to the method by the method that calls it. Parameters are enclosed in parentheses after the method name in the method declaration. You must specify types for these parameters. The general form of a method with parameters looks like this:

Public Function Add(ByVal x1 as Integer, ByVal y1 as Integer)
------------
Implementation
------------
End Function