Prime Number Tester

What is a Prime Number?

A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. Prime numbers are the building blocks of number theory and have fascinated mathematicians for centuries.

Key Characteristics:

Must be greater than 1
Divisible only by 1 and itself
2 is the only even prime number
1 is not considered a prime number

Examples of prime numbers: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97

Why Test for Prime Numbers?

Prime number testing is fundamental to:

Cryptography (RSA encryption)
Computer security algorithms
Mathematical research
Optimization problems
Programming education

Visual Basic 6 Implementation

This VB6 program tests whether a number entered by the user is prime. It uses a straightforward approach by checking divisibility from 2 to n-1.

Interface Design:

VB6 Code:

Private Sub Command1_Click()
    Dim N, D As Single
    Dim tag As String
    
    N = Val(TxtNumber.Text)
    
    Select Case N
        Case Is < 2
            Lbl_Answer.Caption = "It is not a prime number"
            
        Case Is = 2
            Lbl_Answer.Caption = "It is a prime number"
            
        Case Is > 2
            D = 2
            Do
                If N / D = Int(N / D) Then
                    Lbl_Answer.Caption = "It is not a prime number"
                    tag = "Not Prime"
                    Exit Do
                End If
                D = D + 1
                
            Loop While D <= N - 1
            
            If tag <> "Not Prime" Then
                Lbl_Answer.Caption = "It is a prime number"
            End If
    End Select
End Sub

How the VB6 Code Works:

Get the input number from the textbox
Handle special cases: numbers less than 2 are not prime
2 is the only even prime number
For numbers greater than 2, check divisibility from 2 to n-1
If any divisor is found, mark as "Not Prime" and exit the loop
If no divisors are found, the number is prime

Visual Basic .NET Implementation

This VB.NET version uses a more efficient algorithm and modern programming techniques. It only checks divisors up to the square root of the number.

Public Class PrimeTester
    Private Sub btnCheck_Click(sender As Object, e As EventArgs) Handles btnCheck.Click
        Dim number As Integer
        If Integer.TryParse(txtNumber.Text, number) Then
            If IsPrime(number) Then
                lblResult.Text = number.ToString() & " is a prime number."
            Else
                lblResult.Text = number.ToString() & " is not a prime number."
            End If
        Else
            lblResult.Text = "Please enter a valid integer."
        End If
    End Sub

    Private Function IsPrime(n As Integer) As Boolean
        ' Handle special cases
        If n <= 1 Then Return False
        If n = 2 Then Return True
        If n Mod 2 = 0 Then Return False
        
        ' Only check odd factors up to square root
        Dim limit As Integer = Math.Sqrt(n)
        For i As Integer = 3 To limit Step 2
            If n Mod i = 0 Then
                Return False
            End If
        Next
        
        Return True
    End Function
    
    Private Sub btnNextPrime_Click(sender As Object, e As EventArgs) Handles btnNextPrime.Click
        Dim current As Integer
        If Integer.TryParse(txtNumber.Text, current) AndAlso current >= 1 Then
            txtNumber.Text = FindNextPrime(current + 1).ToString()
            btnCheck.PerformClick()
        End If
    End Sub
    
    Private Function FindNextPrime(start As Integer) As Integer
        Dim candidate As Integer = If(start Mod 2 = 0, start + 1, start)
        
        While True
            If IsPrime(candidate) Then
                Return candidate
            End If
            candidate += 2
        End While
    End Function
End Class

Advantages of the VB.NET Version:

More efficient algorithm (checks only up to √n)
Only checks odd numbers after 2
Uses modern .NET functions and error handling
Includes helper function to find next prime
Better input validation with Integer.TryParse

The VB.NET algorithm is significantly more efficient for large numbers because it only needs to check divisors up to the square root of the number.

Algorithm Comparison

Basic Algorithm (VB6)

Checks all numbers from 2 to n-1
Simple to understand
Inefficient for large numbers
Time complexity: O(n)
Good for small numbers and educational purposes

Optimized Algorithm (VB.NET)

Checks only up to √n
Skips even numbers after 2
Much faster for large numbers
Time complexity: O(√n)
More suitable for production code

Performance Comparison

Number	VB6 Approach (Iterations)	VB.NET Approach (Iterations)	Time Savings
100	98	4 (check 3,5,7,9)	95.9%
1,000	998	15	98.5%
10,000	9,998	50	99.5%
100,000	99,998	166	99.8%