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Language: Visual Basic
Expertise: Intermediate
Jul 24, 1998

Random Values Without Duplicates

This code produces a "random" sequence of integers between specified Lower and Upper values without duplication. On the first call to the Shuffle routine with new values for Upper and Lower, Shuffle performs an efficient prime factorization, computes the constants for a linear sequence generator, and uses Randomize to generate a seed value. Each subsequent call with the same value of Upper and Lower uses this generator to return the next "random" number in the sequence. The sequence does not repeat until all possible values have been returned. This code fills the array with 20 random numbers between 1 and 100, without duplication:

 For I = 1 To 20
next I

Here is the Shuffle routine:

 Static Function Shuffle (Lower As Integer, _
	Upper As Integer) As Integer
Static PrimeFactor(10) As Integer  
Static a As Integer, c As Integer, b As Integer
Static s As Integer, n As Integer
Dim i As Integer, j As Integer, k As Integer
Dim m As Integer

If (n <> Upper - Lower + 1) Then
	n = Upper - Lower + 1
	i = 0
	n1 = n
	k = 2
	Do While k <= n1
		If (n1 Mod k = 0) Then
			If (i = 0 Or PrimeFactor(i) <> k) Then
				i = i + 1
				PrimeFactor(i) = k 
			End If
			n1 = n1 / k
			k = k + 1
		End If

	b = 1
	For j = 1 To I
		b = b * PrimeFactor(j)
	Next j
	If n Mod 4 = 0 Then b = b * 2
	a = b + 1
	c = Int(n * .66)
	t = True
	Do While t
		t = False
		For j = 1 To I
			If ((c Mod PrimeFactor(j) = 0) Or _
				(c Mod a = 0)) Then t = True
		Next j
		If t Then c = c - 1
	s = Rnd(n)
End If

s = (a * s + c) Mod n
Shuffle = s + Lower

End Function
John W.
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