VB.NET - Code Samples - Find Prime Numbers Algorithm

VB.NET Prime Number Searching

Synopsis:

This is an algorithm created for a contest on vbforums.com . Pass the function the number of primes you want to generate and it will return them as an int32 array.

The Code:

   Private Function GeneratePrimeList2(ByVal numOfPrimes As Int32) As Int32()
        Dim primes(numOfPrimes - 1) As Int32
        Dim cntr As Int32 = 5
        Dim pcntr As Int32 = 2
        Dim i As Int32

        Dim maxPrimeSqrRoot As Double


        primes(0) = 2
        primes(1) = 3

        Do Until pcntr = numOfPrimes



            'get the highest factor to check against if under the 
            'predetermined maximum
            maxPrimeSqrRoot = Math.Sqrt(cntr)
            'for each prime other than two
            For i = 1 To pcntr - 1
                'check for clean division
                If cntr Mod primes(i) = 0 Then
                    'not prime
                    Exit For
                Else
                    'if all primes up to maxfactor used or all primes used then it is prime
                    If primes(i + 1) > maxPrimeSqrRoot OrElse i = pcntr - 1 Then
                        primes(pcntr) = cntr
                        pcntr += 1
                        Exit For
                    End If
                End If
            Next
            cntr += 2
        Loop
        Return primes

    End Function