Need Help

Jason Baggett

I'm learning VB.Net and a problem I am trying to do asks the following: Write a procedure that will print all prime numbers from 1 to N, where N is passed to the procedure as a parameter. A prime number is one that can be evenly divided only by 1 or itself. The smallest prime numbers are 1,2,3 and 5. Write a program to test your function. Hint: Identify the primes from smallest to the largest and store all the identified prime numbers in an array. Determine whether a new number is a prime number by checking the remainder of this number divided by each of the currently identified prime numbers, except for 1. If a new number cannot be evenly divided (i.e. no remainder) by any prime numbers that are less than the square root of the number, the new number is a prime number. I'm lost!!! Can anyone help me? Thanks JB

Marc 0

:wtf: ! Nice hint is that, it only makes it harder to understand... X| Okay, maybe this will help you in the right direction: Make a loop that goes from 2 (not 1!) to N. Something like For i As Integer = 2 To N. If i is a prime, store it in an array. i is a prime if for each number stored in the array (lets call that p), i / p is not a round number. I have a code snippet if you want. :cool:

Marc 0

Ok, here's the function:

Public Shared Function GetPrimes(ByVal n As Integer) As Integer()
    Dim primes As New ArrayList
    'Print 1 at the start, don't add to primes!
    Debug.WriteLine(1)

    For i As Integer = 2 To n
        'Calculate the square root of i
        Dim root As Integer = CInt(Math.Sqrt(i))
        'Is this number a prime?
        Dim prime As Boolean = True
        'Go to each previously calculated prime
        For Each p As Integer In primes
            'Check to see if the prime is smaller than the square root
            If (p <= root) Then
                'Check if rounded division and not-rounded division are the same
                If (i / p) = (i \\ p) Then
                    'Not a prime
                    prime = False
                    'No need to check any further
                    Exit For
                End If
            End If
        Next p

        If prime Then
            Debug.WriteLine(i)
            'Add the prime to the arraylist
            primes.Add(i)
        End If
    Next i

    'Insert 1 at the start of primes
    'Can't do that at the beginning, because 100 / 1 = 100 \\ 1
    primes.Insert(0, 1)
    'Return the primes
    Return CType(primes.ToArray(GetType(Integer)), Integer())
End Function

Hope this helped you out! :cool:

Jason Baggett

Thanks dude, I'll look at it later today. I might be hollering at you for further help if you don't mind....??? email me direct at jbaggett@hotfish.com for my Instant Message names