if you really really need to have it done from within r, you may want to
use an external facility such as bc, the 'basic calculator' [1,2]. for
example, use the (experimental!) r-bc:
source('http://r-bc.googlecode.com/svn/trunk/R/bc.R')
(you can also download the zipped package which will install on windows,
where you're likely not to have bc yet; see
http://code.google.com/p/r-bc/downloads/)
# an intuitive but slow approach implemented mostly in r
# (alternatively, you may want to have it recursive)
factorial.r = function(n) {
result = bc(1)
while (n > 1) {
result = result*n
n = n-1 }
result }
# an alternative, faster approach implemented mostly in bc
factorial.bc = function(n)
bc(sprintf('define fact(n) { if (n < 2) return 1; return n *
fact(n-1) }; fact(%d)', n))
library(rbenchmark)
benchmark(replications=10, columns=c('test', 'elapsed'),
r=factorial.r(500),
bc=factorial.bc(500))
# test elapsed
# 2 bc 0.101
# 1 r 34.181
this gives you factorials for arbitrary input, but note that the result
is not an integer, but an object of class 'bc' backed by a *character
string*:
result = factorial.bc(10^4)
is(result)
# "bc"
nchar(result)
# 35660
vQ
[1] http://www.gnu.org/software/bc/manual/html_mono/bc.html
[2] http://www.opengroup.org/onlinepubs/9699919799/utilities/bc.html
Murray Cooper wrote:
> You don't say what the error was, for the R factorial function,
> but it is probably irrelevant for your question.
>
> Factorials get to be big numbers rather quickly and unless you
> are using a program that does arbitrary precission arithmetic
> you will quickly exceed the precission limits, for storing a number.
> If you have Maple, do 170! and count the number of digits in the
> result. You will see what I mean.
>
> There are some tricks when working with large factorials, depending
> on what you are doing with them. I'd first try the log factorial function
> in R I think its called lfactorial. Just do a ?factorial and you'll find
> documentation. If this doesn't work, for you, repost with a clear
> description of what you're trying to do and someone may be able
> to help.
>
> Murray M Cooper, Ph.D.
> Richland Statistics
> 9800 N 24th St
> Richland, MI, USA 49083
> Mail: richs...@earthlink.net
>
> ----- Original Message ----- From: "molinar" <sky...@hotmail.com>
> To: <r-help@r-project.org>
> Sent: Wednesday, April 22, 2009 3:21 PM
> Subject: [R] large factorials
>
>
>>
>> I am working on a project that requires me to do very large factorial
>> evaluations. On R the built in factorial function and the one I created
>> both are not able to do factorials over 170. The first gives an
>> error and
>> mine return Inf.
>>
>> Is there a way to have R do these larger calculations (the calculator in
>> accessories can do 10000 factorial and Maple can do even larger)
>> --
>> View this message in context:
>> http://www.nabble.com/large-factorials-tp23175816p23175816.html
>> Sent from the R help mailing list archive at Nabble.com.
>>
>> ______________________________________________
>> R-help@r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
> ______________________________________________
> R-help@r-project.org mailing list
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> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
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