[R] Help creating the IBM Randu function

[Martin Maechler]

>>>>> Richard M Heiberger <rmh at temple.edu>
>>>>>     on Mon, 14 Aug 2017 14:36:40 -0400 writes:

    > Please look at ?datasets::randu
    > for David Donoho's translation of RANDU into R.

Thanks a lot, Rich,  for pointing to this:

Indeed, the RANDU  aka 'randu' data set has been part of R since
the last millennium (and hence before R reached version 1.0.0).
The help page does mention where we got the data set originally,
namely Dave Donoho.  However, I would bet he did not use R at
the time, and indeed it was Prof Brian Ripley who added the
R code to R sources in 1999

------------------------------------------------------------------------
r5632 | ripley | 1999-08-27 08:59:05 +0200 (Fri, 27. Aug 1999)

improve clarity (I hope), add an R example to re-generate this
------------------------------------------------------------------------

and yes, indeed

    example(randu)

already creates the RANDU data set for you .. since 1999.

One curious thing:
Martin .. Pedersen's Wikipedia page (1) seems to make it clear it
stems from IBM whereas in R's help page   help(randu)
we only mention that we got it through VAX VMS code.


    > On Mon, Aug 14, 2017 at 12:49 PM, Martin Møller Skarbiniks Pedersen
    > <traxplayer at gmail.com> wrote:
    >> Dear all,
    >> 
    >> I am trying to learn functions in R and 3D plotting so I decided to try
    >> to plot
    >> the famous bad PRNG Randu from IBM(1).
    >> However something is not correct in the function I have created.
    >> First I define the function RANDU like this:
    >> 
    >>> RANDU <-  function(num) { return (65539*num)%%(2^31) }
    >> 
    >> and test that it works for a seed of 1:
    >>> RANDU(1)
    >> [1] 65539
    >> 
    >> but if I want the next value in the sequence I get this number.
    >>> (65539*65539)%%(2^31)
    >> [1] 393225
    >> 
    >> However using the RANDU function twice doesn't give the same result as
    >> above.
    >> 
    >>> RANDU(RANDU(1))
    >> [1] 4295360521
    >> 
    >> I expect these two values to be the same but that is not the case.
    >> 393225 should be the correct.
    >> 
    >> I guess it might be something with local vs. global environment ?!
    >> 
    >> Please advise and thanks.
    >> 
    >> Regards
    >> Martin M. S. Pedersen
    >> 
    >> (1) https://en.wikipedia.org/wiki/RANDU
    >> 
    >> [[alternative HTML version deleted]]