[R] Simulation using parts of density function
Prof Brian Ripley
ripley at stats.ox.ac.uk
Wed May 2 09:45:48 CEST 2007
Please do not send everything twice: you are using R-help in both the To:
and Cc: fields.
I disagree with Ted: it _is_ much easier to create a generator for this
rtgamma <- function(n, ..., tr = log(5000000))
p <- pgamma(tr, ...)
as inversion (especially at C level) is plenty fast enough.
On Wed, 2 May 2007, Thür Brigitte wrote:
> Thanks for your code! It is not exactly what I really want - but it is my fault, because my description was wrong...
> It is not "sim" but rhater exp(rgamma(...)) that should not exceed 5000000. So I tried to modify your code but it doesn't really work. "sim.test" returns just 1 value and not 999.
> My modified code:
> sim.test <- NULL
> for(i in 1:999)
> remain <- rpois(1,2000)
> x <- remain
> exp(rgamma(1, scale = 0.5, shape = 12))
> remain<-(x - length(sim))
> sim.test <- rbind(sim.test, c(value=sum(sim)))
> Thanks for any help,
> -----Ursprüngliche Nachricht-----
> Von: ted.harding at nessie.mcc.ac.uk [mailto:ted.harding at nessie.mcc.ac.uk]
> Gesendet: Dienstag, 1. Mai 2007 20:18
> An: Thür Brigitte
> Cc: r-help at stat.math.ethz.ch
> Betreff: RE: [R] Simulation using parts of density function
> On 01-May-07 17:03:46, Thür Brigitte wrote:
>> My simulation with the followin R code works perfectly:
>> sim <- replicate(999, sum(exp(rgamma(rpois(1,2000),
>> scale = 0.5, shape = 12))))
>> But now I do not want to have values in object "sim" exceeding
>> 5'000'000, that means that I am just using the beginning of
>> densitiy function gamma x < 15.4. Is there a possibility to
>> modify my code in an easy way?
>> Thanks for any help!
>> Regards, Brigitte
> A somewhat extreme problem!
> The easiest way to modify the code is as below -- certiainly easier
> than writing a special function to draw random samples from the
> truncated gamma distribution.
> A bit of experimentation shows that, from your code above, about
> 10% of the results are <= 5000000. So:
> remain <- 999
> sum(exp(rgamma(rpois(1,2000), scale = 0.5, shape = 12)))
> remain<-(999 - length(sim))
> Results of a run:
>  0
>  4999696
>  999
> It may be on the slow side (though not hugely -- on a quite slow
> machine the above run was completed in 2min 5sec, while the
> 999-replicate in your original took 15sec. So about 8 times as long.
> Most of this, of course, is taken up with the first round.
> Hoping this helps,
Brian D. Ripley, ripley at stats.ox.ac.uk
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 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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