Listing Prime Numbers in VB.Net

Randomness of Prime Numbers

Prime numbers, in ascending order, are:

2, 3, 5, 7, 11, 13, 17, 19, 23, ...

Because prime numbers are so random in their progression, they are very difficult to predict as they get larger.

Hence writing a Visual Basic (VB.Net) code to list prime numbers will involve checking every number in a range of interest and gathering those that are without factors.

Say we want to list all prime numbers between 2 and 100 inclusive, we would start from 2; check to see whether it has any factors; keep it as a prime number if it has no factors; otherwise discard it.
We would then go to 3, and repeat the same process.
Repeat same for 4, 5, 6, 7, ..., 98, 99, 100.

Code that lists Prime Numbers in VB.Net

But we can always use a few tricks .

We will take the following steps:

Step 1:

First, we'll start our range from 9 (keeping 2, 3, 5, and 7 as prime numbers).

Step 2:

Next, we'll only check through odd numbers.

Step 3:

Next, we'll check against the factors 2, 3, and 5.

Step 4:

Lastly, we'll check with a subset of smaller prime numbers that we'll gather as our check progresses.

Create a new class file; Project, Add Class.
Call it listPrimeNumbers.vb.
Optionally, create a new module file; Project, Add Module.
Call it ListPrimeNumbersModule.vb.

Type out the adjoining VB.Net code for listing prime numbers.

Note: You can reuse the main module from the previous lesson if you have been following..

VB.Net code for listPrimeNumbers Class File.

Public Class PrimeNumbers

    Dim first As Integer
    Dim last As Integer
    Dim progress As Integer
    Dim list_of_primes As New List(Of Integer)(New Integer() {2, 3, 5, 7})
    Dim index As Integer
    Dim square_root As Double
    Dim do_next_iteration As Boolean
    Dim result As New List(Of Integer)

    ' Simulate a constructor
    Public Sub _init_(alpha As Integer, omega As Integer)
        first = alpha
        last = omega
        ' STEP 1:
        progress = 9

        square_root = 0
    End Sub

    ' Returns a list Of the desired Set Of prime numbers
    Public Function getPrimes() As List(Of Integer)

        ' STEP 2:
        For progress = progress To last Step 2

            do_next_iteration = False ' a flag

            ' STEPS 3, 4:
            ' Check through already accumulated prime numbers
            ' to see if any Is a factor of "progress".
            square_root = Val(Math.Ceiling(Math.Sqrt(progress)))

            index = 0
            Do While list_of_primes.Item(index) <= square_root
                If progress Mod list_of_primes.Item(index) = 0 Then
                    do_next_iteration = True
                    Exit Do
                End If

                index += 1
            Loop

            If do_next_iteration = True Then
                Continue For
            End If

            ' if all checks are scaled,then "progress" Is our guy.
            list_of_primes.Add(progress)

        Next progress

        Return list_of_primes

    End Function

End Class

Main module code.

Module Arithmetic_PrimeNumbers

    Sub Main()

        ' Use the even number Class
        Dim lower_boundary = 1
        Dim upper_boundary = 100
        Dim prime_list As New PrimeNumbers
        prime_list._init_(lower_boundary, upper_boundary)
        Dim answer = prime_list.getPrimes()

        Console.WriteLine("Prime numbers between " & lower_boundary & " and " & upper_boundary & " are:")
        Console.WriteLine(String.Join(", ", answer))

    End Sub

End Module

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