VB.NET Do Loop

A Loop is used to repeat the same process multiple times until it meets the specified condition in a program. By using a loop in a program, a programmer can repeat any number of statements up to the desired number of repetitions. A loop also provides the suitability to a programmer to repeat the statement in a program according to the requirement. A loop is also used to reduce the program complexity, easy to understand, and easy to debug.

Advantages of VB.NET loop

It provides code iteration functionality in a program.
It executes the statement until the specified condition is true.
It helps in reducing the size of the code.
It reduces compile time.

Types of Loops

There are five types of loops available in VB.NET:

Do While Loop
For Next Loop
For Each Loop
While End Loop
With End Loop

Do While Loop

In VB.NET, Do While loop is used to execute blocks of statements in the program, as long as the condition remains true. It is similar to the While End Loop, but there is slight difference between them. The while loop initially checks the defined condition, if the condition becomes true, the while loop's statement is executed. Whereas in the Do loop, is opposite of the while loop, it means that it executes the Do statements, and then it checks the condition.

Syntax:

Do
[ Statements to be executed]
Loop While Boolean_expression
// or
Do 
[Statement to be executed]
Loop Until Boolean_expression

In the above syntax, the Do keyword followed a block of statements, and While keyword checks Boolean_expression after the execution of the first Do statement.

Flowchart of Do loop

The above flow chart represents the flow of Do While loop. It is used to control the flow of statements, such that it executes the statement at least once before checking the While or Until condition. If the condition is true, the next iteration will be executed till the condition become false.

Example 1. Write a simple program to print a number from 1 to 10 using the Do While loop in VB.NET.

Do_loop.vb

Imports System
Module Do_loop
    Sub Main()
        ' Initializatio and Declaration of variable i
        Dim i As Integer = 1
        Do
            ' Executes the following Statement
            Console.WriteLine(" Value of variable I is : {0}", i)
            i = i + 1 'Increment the variable i by 1
        Loop While i <= 10 ' Define the While Condition

        Console.WriteLine(" Press any key to exit...")
        Console.ReadKey()
    End Sub
End Module

Now compile and execute the above program by clicking on the Start button, it shows the following output:

In the above program, the Do While loop executes the body until the given condition becomes false. When the condition becomes false the loop will be terminated.

Use of Until in Do Until Loop statement

In the VB.NET loop, there is a Do Until loop statement, which is similar to the Do While loop. The Do Statement executes as long as Until condition becomes true.

Example: Write a program to understand the uses of Do Until Loop in VB.NET.

Do_loop.vb

Imports System
Module Do_loop
    Sub Main()
        ' Initialization and Declaration of variable i
        Dim i As Integer = 1
        Do
            ' Executes the following Statement
            Console.WriteLine(" Value of variable i is : {0}", i)
            i = i + 1 'Increment variable i by 1
        Loop Until i = 10 ' Define the Until Condition

        Console.WriteLine(" Press any key to exit...")
        Console.ReadKey()
    End Sub
End Module

Output:

In the above program, a Do Until loop is executed their statement until the given condition Until (i =10) is not meet. When the counter value of the variable i becomes 10, the defined statement will be false, and the loop will be terminated.

Nested Do While Loop Statement

In VB.NET, when we use one Do While loop inside the body of another Do While loop, it is called Nested Do While loop.

Syntax

Do
// Statement of the outer loop
Do
           // Statement of outer loop
While (condition -2)
// Statement of outer loop
While (condition -1)

Example 2: Write a simple program to use the Do While loop Statement in VB.NET.

Nest_Do_While.vb

Imports System
Module Nest_Do_While
    Sub Main()
        ' Declare i and j as Integer variable
        Dim i As Integer = 1
        Do
            ' Outer loop statement
            Console.WriteLine(" Execution of Outer loop is {0}", i & " times")
            
            Dim j As Integer = 1

            Do
                'Inner loop statement
                Console.WriteLine(" Execution of Inner loop is {0}", j)
                j = j + 1 ' Increment Inner Counter variable by 1
            Loop While j < 3

            Console.WriteLine()
            i = i + 1 ' Increment Outer Counter variable by 1
        Loop While i < 4
        Console.WriteLine(" Exit from the loop")
        Console.WriteLine(" Press any key to exit...")
        Console.ReadKey()
    End Sub
End Module

Output:

In the above example, each iteration of the outer loop also executes the inner loop repeatedly until the inner condition becomes false. When the condition of the outer loop becomes false, the execution of the outer and inner loop will be terminated.

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