What is the recommended way to generate a random first random number
using the Rnd function?

I tried using the entire value of Now,

?rnd(-cdbl(now))

but I kept getting the same first random number.

After a little experimenting, I discovered that Rnd returns the same
first random number for about 5 minutes and 38 seconds:

?rnd(-cdbl(cdate("09/18/05 23:57:10")))
0.225379
?rnd(-cdbl(cdate("09/18/05 23:57:11")))
0.225379

?rnd(-cdbl(cdate("09/18/05 23:57:12")))
0.2146977
?rnd(-cdbl(cdate("09/18/05 23:57:13")))
0.2146977
....
?rnd(-cdbl(cdate("09/19/05 00:02:47")))
0.2146977
?rnd(-cdbl(cdate("09/19/05 00:02:48")))
0.2146977

?rnd(-cdbl(cdate("09/19/05 00:02:49")))
0.2040164
?rnd(-cdbl(cdate("09/19/05 00:02:50")))
0.2040164
....
?rnd(-cdbl(cdate("09/19/05 00:08:25")))
0.2040164
?rnd(-cdbl(cdate("09/19/05 00:08:26")))
0.2040164

?rnd(-cdbl(cdate("09/19/05 00:08:27")))
0.1933351
?rnd(-cdbl(cdate("09/19/05 00:08:28")))
0.1933351

Is this because Rnd only uses a single precision value? and the
difference gets lost for less than 5:38 (338 seconds)?


Anyway, what is the recommended way to get a random first random
number?

Have a look at the Randomize function. Here is what the help file says about
it:

Randomize uses number to initialize the Rnd function's random-number
generator, giving it a newseed value. If you omit number, the value returned
by the system timer is used as the new seed value. If Randomize is not used,
the Rnd function (with no arguments) uses the same number as a seed the
first time it is called, and thereafter uses the last generated number as a
seed value. Note: To repeat sequences of random numbers, call Rnd with a
negative argument immediately before using Randomize with a numeric
argument. Using Randomize with the same value for number does not repeat the
previous sequence.

In your case you can simply use Randomize Timer (or just Randomize). Don't
use it repeatedly though. Use it just once at the start of your program. For
example, use it just once in your Form's Load event (if the Form is your
startup object).

Mike

On Tue, 20 Sep 2005 08:38:40 +0100, "Mike Williams"
<M***@WhiskyAndCoke.com> wrote:

Yes. I knew that, too, but got carried away with trying to figure out
why the date argument wasn't working.

I just like to make things harder. ;-)

Thanks

Show quoteHide quote

On Tue, 20 Sep 2005 00:10:10 -0700, LurfysMa <invalid@invalid.invalid>
wrote:

OK, I got so caught up in trying to figure out why the date wasn't
working, I overlooked the obvious.

Just use Randomize, right?

I would still like to know why the date doesn't work as a negative
argument to rnd().

Probably not a coincidence that 5 min and 38 sec is 1/256'th of a day.
Sounds like it the result of a bit-wise shift in the algorithm that uses
that numeric argument.

        Tony Proctor

Show quoteHide quote

LurfysMa wrote:
I'm sure it's as you suspect -- the argument to Rnd() is converted to a Single, which uses fewer bits.  And I believe that the single is mapped to a 24-bit integer internally -- max value 16777216 -- which is used as the highest 24 bits of a 32-bit integer.

And I guess that would account for there being only 1 new Rnd for every 256 possible Double seed values (16777216 << 8 leaves the lowest 8 bits empty).

Rnd() is only nominally a single, since what's calculated is a 24-bit integer value, which is then mapped to the mantissa of a single.

You'll know if you grasp this, when you can explain the result of this experiment:

? (rnd(-2^23) - rnd(-2^24))

Newbie: FOR loop on dates
Declaring a Constant
Common controls
Tidy Code software
Do The Runtime DLLs Come With Win2000?
Question on printing with Snapshot from VB 6
VB6 IDE VBE Object reference
XMLdoc for VB
event propagation
change "break on" setting at runtime