Binaryworld - IsPrime - Determine whether a number is prime ... [ VB -> Math ]

' Return True if the number is prime

Function IsPrime(ByVal number As Long) As Boolean
  ' manually test 2 and 3
  If number > 3 Then
    If number Mod 2 = 0 Then Exit Function
    If number Mod 3 = 0 Then Exit Function
  End If
  ' we can now avoid to consider multiples
  ' of 2 and 3. This can be done really simply
  ' by starting at 5 and incrementing by 2 and 4
  ' alternatively, that is:
  '  5, 7, 11, 13, 17, 19, 23, 25, 29, 31, 35, 37, ...
  Dim divisor As Long
  Dim increment As Long
  Dim maxDivisor As Long

  divisor = 5
  increment = 2
  ' we don't need to go higher than the square
  ' root of the number
  maxDivisor = Sqr(number) + 1

  Do Until divisor > maxDivisor
    If number Mod divisor = 0 Then Exit Function
    divisor = divisor + increment
    ' this modifies 2 into 4 and viceversa
    increment = 6 - increment
  Loop

  ' if we get here, the number is prime
  IsPrime = True

End Function