[AccessD] Rnd as Double (was: Create a random date/time)

[Gustav Brock]

Hi Stuart

Yes. Sort of. Without the parameter, it wouldn't work as a direct replacement for Rnd.

But it was rather if anyone could find a trap in this simple method?

/gustav

-----Oprindelig meddelelse-----
Fra: AccessD <accessd-bounces at databaseadvisors.com> På vegne af Stuart McLachlan
Sendt: 5. september 2018 15:51
Til: Access Developers discussion and problem solving <accessd at databaseadvisors.com>
Emne: Re: [AccessD] Rnd as Double (was: Create a random date/time)

Can be simplified to just:

    Randomize Number
    RndDbl = Rnd() / 10000000 + Rnd()



On 5 Sep 2018 at 13:00, Gustav Brock wrote:

> Hi all
> 
> I wondered how to generate a pseudo random value as a double - like 
> Rnd does for a single.
> 
> This, I believe, can be done by calling Rnd twice, shifting one of the 
> values "below the range of a single", and add the two values, thus 
> obtaining more significant digits.
> 
> It is simpler than it may sound:
> 
> <code>
> 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)
> 
>     ' Return value like:
>     '   0.185151372890967
>     RndDbl = Value
> 
> End Function
> </code>
> 
> Then call it with a dynamic seed using Timer:
> 
>     PseudoRandomDouble = RndDbl(-Timer)
> 
> What do you think?
> 
> /gustav
> 
> PS: I posted here on 2015-08-31 a replacement for the date function 
> below.
> 
> -----Oprindelig meddelelse-----
> Fra: AccessD <accessd-bounces at databaseadvisors.com> På vegne af Stuart 
> McLachlan Sendt: 29. august 2015 03:22 Til: Access Developers 
> discussion and problem solving <accessd at databaseadvisors.com> Emne:
> Re: [AccessD] Create a random date/time
> 
> Rnd * Rnd is NOT a "trick to create a random date/time".
> 
> It is a trick to create "more significant digits" than the Single 
> returned by a Rnd.
> 
> Unfortunately it has a VERY  bad side effect. Multiplying random 
> numbers together alters the probablility distribution.
> http://mathworld.wolfram.com/UniformProductDistribution.html
> 
> A better way would be to use Rnd twice in a different way. First to 
> generate the Date part and a second time to generate the time part, 
> then add then together.
> 
> --
> Stuart
> 
> On 28 Aug 2015 at 15:29, Gustav Brock wrote:
> 
> > Hi all
> > 
> > Have you ever wondered how to create a random date/time?
> > 
> > Well I hadn't, but it is not that difficult. The trick is using Rnd
> > twice:
> > 
> >     RandomDate = CDate((CLng(#12/31/9999#) - CLng(#1/1/100#)) * Rnd
> >     * Rnd + CLng(#1/1/100#))
> > 
> > The full story is here:
> > 
> >     http://stackoverflow.com/a/32265346/3527297
> > 
> > If you want a value within a given range, you can use this simple
> > function:
> > 
> > <code>
> > Public Function DateRandom( _
> >     Optional ByVal UpperDate As Date = #12/31/9999#, _
> >     Optional ByVal LowerDate As Date = #1/1/100#) _
> >     As Date
> > 
> > '   Generates a random date/time - optionally within the range of
> > LowerDate and/or UpperDate. ' '   2015-08-28. Gustav Brock, Cactus
> > Data ApS, CPH.
> > 
> >     Dim RandomDate  As Date
> > 
> >     RandomDate = CDate((CLng(UpperDate) - CLng(LowerDate)) * Rnd *
> >     Rnd
> >     + CLng(LowerDate))
> > 
> >     DateRandom = RandomDate
> > 
> > End Function
> > </code>
> > 
> > /gustav