Prime Numbers

userxyz · Jun 12, 2007

How can u find prime numbers, and how to write in Visual Basic ?

Like, if u enter a number in a textfield, the code must calculate if it's a prime number, yes or no...

?

panic mode · Jun 12, 2007

Option Explicit

Private Sub Command1_Click()
Dim n As Long
Dim i As Long

On Error Resume Next
n = CLng(Text1.Text)
i = 2
While (i < n) And (n / i <> n \ i)
i = i + 1
Wend

If i = n Then
MsgBox "this seam to be prime number"
Else
MsgBox "i don't think so..."
End If

End Sub

but there are so many ways to speed this up so it works nice and fast for larger numbers. (no need to test all even numbers, numbers greater than 1/2 of entered value etc.)

panic mode · Jun 12, 2007

here is compiled version:

Peter Nachtwey · Jun 12, 2007

That isn't very efficient and over 2000 years behind.

http://www.math.utah.edu/~pa/Eratosthenes.html

panic mode · Jun 12, 2007

yepp and that's what I said already...but

...lord, please forgive me for not killing myself over such poor reply...

OZEE · Jun 12, 2007

Out of curiosity... unless it's homework, why?????

panic mode · Jun 13, 2007

precisely...:nodi:

Corbis · Jun 13, 2007

Just another variation....

Code:

Private Sub Command1_Click()
  Dim Counter As Long
  Dim Value As Long
  Dim Results As Boolean
  Value = Abs(Val(Me.Text1.Text))
  Results = True
  For Counter = 2 To Value - 1
	If Value Mod Counter = 0 Then
	  Results = False
	  Exit For
	End If
  Next Counter
  MsgBox "Prime Number Is " & Results
End Sub

userxyz · Jun 13, 2007

hi

I'm study Visual Basic on my own, so it's not homework, I just hadn't any idea how to begin on this exercise. It's an exercise of www.bisonline.be

panic mode said:
precisely...:nodi:

Thanks for the code Corbis and Panic mode, I understand the code and thanks a lot, now I have a start to write it in my own way.

rdrast · Jun 13, 2007

For simple loop-based prime number determination, you can save half the loop time by only checking integral divisors from 2 to ((Value/2) + 1). There are other tricks to shortcut loop-checking even further, but they will not be able to recognize all primes in a mathmatical manner.

bernie_carlton · Jun 13, 2007

Actually you can stop when your trial number passes the square root of the target. This would save a lot of passes on very large numbers.

kamenges · Jun 13, 2007

I think I am misunderstanding your post, bernie. The square root of 13 is 3.6. I would miss 5, 7 and 11.

Keith

bernie_carlton · Jun 13, 2007

The original post was to test if a given number was prime, not to find all the primes up to that number.

kamenges · Jun 13, 2007

Correct. My bad.

Prime Numbers

userxyz

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panic mode

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Peter Nachtwey

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OZEE

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Corbis

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userxyz

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rdrast

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bernie_carlton

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kamenges

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kamenges

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