Scriptorium

Sub Euler_0003()
'The prime factors of 13195 are 5, 7, 13 and 29.
'
'What is the largest prime factor of the number 600851475143 ?
 
 
    Dim i As Long, i2 As Long
    Dim arrNums() As Long
    Dim sNum As String
    Dim cPrimes As Collection
    
    'Store enough prime numbers in a collection
    i2 = Int(Sqr(Len("600851475143")) + 1)
    For i = 1 To i2
        sNum = sNum & "9"
    Next i
    Set cPrimes = New Collection
    For i = 1 To CLng(sNum) Step 2
        cPrimes.Add i, CStr(i)
    Next i
 
    For i = cPrimes.Count To 1 Step -1
        If cPrimes(i) / 3 = Int(cPrimes(i) / 3) Then
            cPrimes.Remove i
        End If
    Next i
 
    For i = cPrimes.Count To 1 Step -1
        If cPrimes(i) / 5 = Int(cPrimes(i) / 5) Then
            cPrimes.Remove i
        End If
    Next i
    
    For i = cPrimes.Count To 1 Step -1
        If cPrimes(i) / 7 = Int(cPrimes(i) / 7) Then
            cPrimes.Remove i
        End If
    Next i
 
    'Remove squares of primes
    For i = cPrimes.Count To 1 Step -1
        If Sqr(cPrimes(i)) = Int(Sqr(cPrimes(i))) Then
            cPrimes.Remove i
        End If
    Next i
 
    For i = cPrimes.Count To 1 Step -1
        If 600851475143# / cPrimes(i) = Int(600851475143# / cPrimes(i)) Then
            Debug.Print cPrimes(i)
            Exit Sub
        End If
    Next i
End Sub