VBA随机数以规则的间隔产生重复序列

Question

该代码应该在VBA中生成10,000个随机数的序列.出于某种原因,我只能生成一个长度为5842的独特序列,然后重复.但是,这是最奇怪的部分,每次运行代码时,序列都会在不同的地方开始.例如,在一次运行中,元素2660之后的元素与元素8502之后的元素相同(8502-2660 = 5842).下一次运行,我得到一个重复以下元素3704和9546(9546-3704 = 5842)的序列.等等.

Function NormRand() As Double
' NormRand returns a randomly distributed drawing from a
' standard normal distribution i.e. one with:
' Average = 0 and Standard Deviation = 1.0
Dim fac As Double, rsq As Double, v1 As Double, v2 As Double
Static flag As Boolean, gset As Double

' Each pass through the calculation of the routine produces
'  two normally-distributed deviates, so we only need to do
'  the calculations every other call. So we set the flag
'  variable (to true) if gset contains a spare NormRand value.
If flag Then
    NormRand = gset
    ' Force calculation next time.
    flag = False
Else
    ' Don't have anything saved so need to find a pair of values
    ' First generate a co-ordinate pair within the unit circle:
    Do
        v1 = 2 * Rnd - 1#
        v2 = 2 * Rnd - 1#
        rsq = v1 * v1 + v2 * v2
    Loop Until rsq <= 1#

    ' Do the Math:
    fac = Sqr(-2# * Log(rsq) / rsq)

    ' Return one of the values and save the other (gset) for next time:
    NormRand = v2 * fac
    gset = v1 * fac
    flag = True
End If

End Function

Answer 1

由于某种原因，我只能生成长度为 5842 的唯一序列，然后它会重复。但是，这是最奇怪的部分，每次我运行代码时，序列都会在不同的位置开始

这是设计使然，也是众所周知的——这就是为什么数字生成被标记为伪随机而不是随机的。

顺便说一句，我注意到您正在将两个值相乘。这可能不是一个好主意——正如这里提到的。

在您的函数中，您可以尝试替换Rnd为RndDbl：

Public Function RndDbl(Optional ByRef Number As Single) As Double

    ' Exponent to shift the significant digits of a single to
    ' the least significant digits of a double.
    Const Exponent  As Long = 7

    Dim Value       As Double

    ' Generate two values like:
    '   0.1851513
    '   0.000000072890967130661
    ' and add these.
    Value = CDbl(Rnd(Number)) + CDbl(Rnd(Number) * 10 ^ -Exponent)

    RndDbl = Value

End Function

然后通过调用以下命令修改您的代码以包含动态种子Timer：

Do
    v1 = 2 * RndDbl(-Timer) - 1#
    v2 = 2 * RndDbl(-Timer) - 1#
    rsq = v1 + v2
Loop Until rsq <= 1#

生成的值仍然不是真正的随机值，但不应采用重复序列的形式。