by Bradley Plies (2 Submissions)
Category: Math/Dates
Compatability: VB 6.0
Difficulty: Unknown Difficulty
Originally Published: Sun 8th April 2001
Date Added: Mon 8th February 2021
Rating: 
(1 Votes)
A much more probabilistically accurate random number generation function for the domain of 1..N. The problem with most textbook VB random
VBcoders
' getRandomNumber
'
'Inputs:
' int numSides // Range limit of 1..N inclusive
'
'Outputs:
' int result
'
'Assumptions:
' System randomizer has already been initialized because
' it should not be initilized here since it will be reinitialized
' every time this function is called.
'
'Notes:
' The standard VB random number generation code is unfair to endpoint
' numbers in the range. My own analysis has deteremined that d20 (1..20)
' that numbers 2..19 are greater than their theoretical occurance of 5%
' and numbers 1 & 20 are below their theoretical occurance usually 2.5%
'
' Analysis:
' # Of dice: 500,000
' # Of sides: 20
'
'Side # %
'1 13204 2.6408
'2 26357 5.2714
'3 26120 5.224
'4 26198 5.2396
'5 26033 5.2066
'6 26257 5.2514
'7 26378 5.2756
'8 26365 5.273
'9 26515 5.303
'10 26362 5.2724
'11 26231 5.2462
'12 26334 5.2668
'13 26297 5.2594
'14 26337 5.2674
'15 26655 5.331
'16 26121 5.2242
'17 26498 5.2996
'18 26243 5.2486
'19 26373 5.2746
'20 13122 2.6244
' This algorithm will attempt to correct the unfairness of
' the random number generation using the hypothesis that the endpoint values
' are less likely. The solution will be to add an extra 2 numbers to the
' number range (ex> 1..20 becomes 1..23) These numbers will be the "new"
' endpoints. When an endpoint roll occurs, it is ignored and re-rolled.
' The resulting number range would be 2..21
' 1 is then subtracted from the result, yielding the proper range of 1..20
Public Function RollDice(numDice As Integer, numSides As Integer) As Integer
Const RANGE_PADDING As Integer = 2
Dim ROLL_ADJUSTMENT As Integer
Dim modifiedRange As Integer
Dim rollTotal As Integer ' Sum of the dice rolls
Dim rollValue As Integer ' Value of one die roll
Dim lcv As Integer ' Loop control variable
' Explicitly initialize variables (unnecessary, but good style)
ROLL_ADJUSTMENT = Round(RANGE_PADDING ^ 0.5)
rollTotal = 0
rollValue = 0
modifiedRange = numSides + RANGE_PADDING
' For every roll...
For lcv = 1 To numDice
' Get a random number from 1..numSides
rollValue = Round((modifiedRange - 1) * Rnd) + 1
Do While (rollValue = 1 Or rollValue = modifiedRange)
rollValue = Round((modifiedRange - 1) * Rnd) + 1
Loop
' Adjust the rollValue to remove the padding
rollValue = rollValue - ROLL_ADJUSTMENT
' Add this number to the current total
rollTotal = rollTotal + rollValue
Next lcv
' Set return value and exit
RollDice = rollTotal
End Function
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