For Next Loop

A For Next loop is used to repeatedly execute a sequence of code or a block of code until a given condition is satisfied. A For loop is useful in such a case when we know how many times a block of code has to be executed. In VB.NET, the For loop is also known as For Next Loop.

Syntax

For variable_name As [ DataType ] = start To end [ Step step ]
[ Statements to be executed ]
Next

Let's understand the For Next loop in detail.

For: It is the keyword that is present at the beginning of the definition.
variable_name: It is a variable name, which is required in the For loop Statement. The value of the variable determines when to exit from the For-Next loop, and the value should only be a numeric.
[Data Type]: It represents the Data Type of the variable_name.
start To end: The start and end are the two important parameters representing the initial and final values of the variable_name. These parameters are helpful while the execution begins, the initial value of the variable is set by the start. Before the completion of each repetition, the variable's current value is compared with the end value. And if the value of the variable is less than the end value, the execution continues until the variable's current value is greater than the end value. And if the value is exceeded, the loop is terminated.
Step: A step parameter is used to determine by which the counter value of a variable is increased or decreased after each iteration in a program. If the counter value is not specified; It uses 1 as the default value.
Statements: A statement can be a single statement or group of statements that execute during the completion of each iteration in a loop.
Next: In VB.NET a Next is a keyword that represents the end of the For loop's

Flowchart of For Next loop

The following flowchart represents the functioning of the For Next loop in the VB.NET programming language.

In the above flow chart, the first step is to initialize the variable name with the start value. And then, the value of the variable will be compared to the end expression or value. If the condition is true, the control enters the loop body and executes the statements. After that, the value of a variable will be automatically incremented by the compiler. Upon completion of each iteration, the current value of a variable will be again compared to the end expression. If the condition is not true, the controlled exit from the loop.

Example 1. Write a simple program to print the number from 1 to 10 using the For Next loop.

Number.vb

Imports System
Module Number
    Sub Main()
        ' It is a simple print statement, and 'vbCrLf' is used to jump in the next line.
        Console.Write(" The number starts from 1 to 10 " & vbCrLf)
        ' declare and initialize variable i
        For i As Integer = 1 To 10 Step 1
            ' if the condition is true, the following statement will be executed
            Console.WriteLine(" Number is {0} ", i)
            ' after completion of each iteration, next will update the variable counter
        Next
        Console.WriteLine(" Press any key to exit... ")
        Console.ReadKey()
    End Sub
End Module

Output:

In the above example, we have initialized an integer variable i with an initial value 1. The For loop will continuously execute its body until the value of i is smaller or equal to 10. After each iteration, the value of i is automatically increased with 'Step 1'. If the value of i reached 10, the loop would be terminated and control transfer to the Main() function.

Further, we can change the Step in For Next loop. Write the following program to skip the number is 2.

Number.vb

Imports System
Module Number
    Sub Main()
        ' declaration of variable i
        Dim i As Integer
        Console.Write(" Skip one number at the completion of each iteration in between 1 to 10 " & vbCrLf)
        ' initialize i to 1 and declare Step to 2 for skipping a number
        For i = 1 To 10 Step 2
            ' if condition is true, it skips one number
            Console.WriteLine(" Number is {0} ", i)
            ' after completion of each iteration, next will update the variable counter to step 2
        Next
        Console.WriteLine(" Press any key to exit... ")
        Console.ReadKey()
    End Sub
End Module 

Output:

As we can see in the above output, the value of the variable i is initialized with 1, and the value of i is skipped by 'Step 2' in the loop for each iteration to print the skipped number from 1 to 10.

Example 2: Write a simple program to print a table in the VB.NET.

Table.vb

Imports System
Module Table
    Sub Main()
        'declaration of i and num variable
        Dim i, num As Integer
        Console.WriteLine(" Enter any number to print the table")
        num = Console.ReadLine() ' accept a number from the user
        Console.WriteLine(" Table of " & num)
        'define for loop condition, it automatically initialize step to 1
        For i = 1 To 10
            Console.WriteLine(num & " * " & i & " = " & i * num)
        Next
        Console.ReadKey()
    End Sub
End Module

Output:

Nested For Next Loop in VB.NET

In VB.NET, when we write one For loop inside the body of another For Next loop, it is called Nested For Next loop.

Syntax:

For variable_name As [Data Type] = start To end [ Step step ]
	For variable_name As [Data Type] = start To end [ Step step ]
		[ inner loop statements ]
	Next
	[ Outer loop statements ]
Next

Following is the example of Nested For Next loop in VB.NET.

Nested_loop.vb

Imports System
Module Nested_loop
    Sub Main()
        Dim i, j As Integer
        For i = 1 To 3
            'Outer loop
            Console.WriteLine(" Outer loop, i = {0}", i)
            'Console.WriteLine(vbCrLf)

            'Inner loop
            For j = 1 To 4
                Console.WriteLine(" Inner loop, j = {0}", j)
            Next
            Console.WriteLine()
        Next
        Console.WriteLine(" Press any key to exit...")
        Console.ReadKey()
    End Sub
End Module

Output:

In the above example, at each iteration of the outer loop, the inner loop is repeatedly executed its entire cycles until the condition is not satisfied.

Example 2: Write a program to print a pattern in VB.NET.

Pattern.vb

Imports System
Module Pattern
    Sub Main()
        Dim i, n, j As Integer
        Console.WriteLine(" Enter a number to show rows in a Pattern")
        ' take a number from user
        n = Console.ReadLine()

        'Outer loop
        For i = 1 To n
            'Inner loop
            'value of j should be less than i
            For j = 1 To i
                Console.Write(" * ")
            Next
            Console.WriteLine("")
        Next
        Console.WriteLine(" Press any key to exit...")
        Console.ReadKey()
    End Sub
End Module

Output: