[Rd] Random number generation problem (PR#554)
Prof Brian D Ripley
Tue, 23 May 2000 20:03:30 +0100 (BST)
On Tue, 23 May 2000 firstname.lastname@example.org wrote:
> Full_Name: Kjetil Kjernsmo
> Version: 1.0.1
> OS: osf1
> Submission from: (NULL) (18.104.22.168)
> I'm getting strange numbers from a random number generation
> function again. I was wondering if it could be the same problem
> as I reported in (PR#439).
> I'm using the functions (some comments deleted for brevity):
> qamp <- function(p, type=c("point", "nolens"))
> if(type == "point")
> return(1 / (2 * sqrt(1 - p)))
> else return(1)
> ramp <- function(n, type=c("point", "nolens"))
> if(type == "point")
> return(qamp(runif(n), type))
> else return(rep(1, n))
> ...to generate random numbers. I think there might be something wrong
> with runif, possibly the same problem as rpois had.
Not at all, that problem is fixed.
> More details:
> I'm calling this function a number of times through a function
> intensityfromcloud <- function(ncloudsbin, cloudintensity, amptype,
> exptime, ...)
> amps <- ramp(ncloudsbin, amptype)
> write(c(max(amps), sum(amps), sum(amps - 1)),
> "lastamps.dat", ncolumns=3, append=T)
> return(cloudintensity * exptime * amps)
> ...that is called a lot. As you can see, I'm writing the maximum
> (among other things) of the generated numbers to file
> (this is a preliminary hack, for a (quite unrelated) sanity
> check I'm running).
> Now, it turns out that in 8080 maxima, the number 14654.29 appears
> twice. Things like that happen, this _is_ statistics,
> and my pdf doesn't fall that fast, but you know...
> I thought I'd report this sooner the better...
And which random number generator are you using? And have you a proven
stable procedure based on its results? (Doesn't look like it, I have to
Please do read the advice in the FAQ on reporting a bug. There is
npthing here that I can see is a bug, or see how to investigate.
All PRNGs have deficiencies, and you have to program procedures to take
account of them. R provides several alternatives for you to try out.
Do make use of them!
Brian D. Ripley, email@example.com
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272860 (secr)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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