Visual Basic Helper Routines

Pure VB: Generating a Random Array of Unique Numbers
     
Posted:   Monday October 18, 1999
Updated:   Monday December 26, 2011
     
Applies to:   VB4-32, VB5, VB6
Developed with:   VB6, Windows NT4
OS restrictions:   None
Author:   Rick Rothstein
     
Related:     Pure VB: Preventing Duplicates in a Random Number Array
 Prerequisites
None.

When you need an array populated with a set of random numbers, where each number is unique (not duplicated as another array member), try this quick routine posted to the newsgroups by Rick Rothstein. 

Perfect for lotteries and card games, the following is a generalized shuffling routine; pass an array of numbers and it will put them in random order.

Typically, to generate a random number list of n numbers where each number must be unique, it is necessary to generate the number then perform a search of the already-assigned array members to check if the new value is a duplicate. This method, unlike those more complicated generate-and-search methods, is very fast because once the array is initialized (pre-filled) with numbers in sequentially order (array element 1 is 1, element 2 is 2, element 3 is 3, and so on), the Randomize routine never has to visit any array element more than once because we know from the outset that each initial number is unique. The result of the code therefore is a random shuffling the array with no duplicate data.

 BAS Module Code
None.

 Form Code
Toss a command button (Command1) and two list boxes (List1, List2) onto a form, along with the following code:

Option Explicit
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' Copyright 1996-2011 VBnet/Randy Birch, All Rights Reserved.
' Some pages may also contain other copyrights by the author.
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' Distribution: You can freely use this code in your own
'               applications, but you may not reproduce 
'               or publish this code on any web site,
'               online service, or distribute as source 
'               on any media without express permission.
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Private Sub Form_Load()

  'assure random numbers don't
  'repeat each run       
   Randomize
	  
End Sub
         
         
Private Sub Command1_Click()

  'Set the number of elements needed. This demo 
  'uses 1 to 52 to simulate a deck of cards.
   Dim cnt As Long
   Dim myArray(1 To 52) As Long
   
  'lists are for info only
   List1.Clear
   List2.Clear
   
  'fill the array with consecutive numbers from 1 to 52
   For cnt = 1 To UBound(myArray)
      myArray(cnt) = cnt

     'debug/info only - not needed for routine
      List1.AddItem cnt & vbTab & myArray(cnt)
   Next
   
  'randomize (suffle) the array values
   RandomizeArray myArray
   
  'debug/info only - not needed for routine
   For cnt = 1 To UBound(myArray)
      List2.AddItem cnt & vbTab & myArray(cnt)
   Next
   
End Sub


Private Sub RandomizeArray(ArrayIn As Variant)
   
   Dim cnt As Long
   Dim RandomIndex As Long
   Dim tmp As Variant
   
  'only if an array was passed
   If VarType(ArrayIn) >= vbArray Then
        
     'loop through the array elements in reverse
      For cnt = UBound(ArrayIn) To LBound(ArrayIn) Step -1
      
        'select a random array index
         RandomIndex = Int((cnt - LBound(ArrayIn) + 1) * _
                       Rnd + LBound(ArrayIn))
                                    
        'cnt represents one array member 
        'index, and RandomIndex represents
        'another, so swap the data held in 
        'myarray(cnt) with that in myarray(RandomIndex)
         tmp = ArrayIn(RandomIndex)
         ArrayIn(RandomIndex) = ArrayIn(cnt)
         ArrayIn(cnt) = tmp
         
      Next
      
   Else
   
     'The passed argument was not an
     'array; error handler goes here
      
   End If
  
End Sub


Private Sub List1_Scroll()

  'if List2 is scrolled, keep List1 in sync
   List2.TopIndex = List1.TopIndex
   
End Sub


Private Sub List2_Scroll()

  'if List1 is scrolled, keep List2 in sync
   List1.TopIndex = List2.TopIndex

End Sub
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