Question

Using a random library with these functions:

randomChance(p) Returns true with the probability indicated by p.

randomInteger(low, high) Returns a random integer in the range low to high, inclusive.

what is the easiest way to implement a “random selector” that takes consideration of percentage, 1/4 or 1/3 etc… I got a array with key/value pairing. For example "a" migth have the value 2 and "b" have the value 2. 1/2 chance for both.

The max value will be the size of the array, cause it only contains unique items. The randomChance() function ranges between 0.0 - 1.0 where 1 = 100%. If my array size is, say 4. What is the best way of “letting 4 be 1”.

Answer 1

Lets say you have:

a = 2, b = 2, c = 1, d = 3

now make it:

a = 2, b = 4, c = 5, d = 8

Create a random number from 1 to MaxVal (value of the last key, 8 in this example). Select the first Key where Value >= RandomNum

EDIT

I made a small VB.Net to show the algorithm and how it works. The code is not meant to be: Good, elegant, performant or readable.

Module Module1

    Private Class Value
        Public vOrg, vRecalc, HitCount As Integer
        Public Key As String
        Public Sub New(s, v1, v2, c)
            Key = s : vOrg = v1 : vRecalc = v2 : HitCount = c
        End Sub
    End Class

    Sub Main()

        ' set initial values
        Dim KVP() As Value = {New Value("A", 2, 0, 0),
                              New Value("B", 2, 0, 0),
                              New Value("C", 1, 0, 0),
                              New Value("D", 3, 0, 0)}
        ' recalc values
        For i = 0 To KVP.Length - 1
            If i = 0 Then KVP(0).vRecalc = KVP(0).vOrg Else KVP(i).vRecalc = KVP(i).vOrg + KVP(i - 1).vRecalc
        Next
        ' do test
        Dim r As New Random
        Dim runs As Integer = 1000 * 1000, maxval As Integer = KVP(KVP.Length - 1).vRecalc
        For i = 1 To runs
            Dim RandVal = r.Next(1, maxval + 1)
            Dim chosen As Integer = (From j In Enumerable.Range(0, KVP.Length) Where KVP(j).vRecalc >= RandVal Take 1 Select j)(0)
            KVP(chosen).HitCount += 1
        Next
        ' ouput results
        For Each kv In KVP
            Console.WriteLine("{0} was chosen with {1:F3} propability, expected was {2:F3}", kv.Key, kv.HitCount / CDbl(runs), kv.vOrg / CDbl(maxval))
        Next

        Console.ReadLine()

    End Sub

End Module

An output sample:

A was chosen with 0.250 propability, expected was 0.250
B was chosen with 0.251 propability, expected was 0.250
C was chosen with 0.124 propability, expected was 0.125
D was chosen with 0.375 propability, expected was 0.375