Re: [R] Function to find prime numbers
On 10/13/2009 09:59 AM, AJ83 wrote: I need to create a function to find all the prime numbers in an array. Can anyone point me in the right direction? Thank you. AJ There is isprime in package gmp. data.frame( number = 1:10, is.prime = isprime( 1:10 ) 1, is.probably.prime = isprime(1:10) 0 ) number is.prime is.probably.prime 1 1FALSE FALSE 2 2 TRUE TRUE 3 3 TRUE TRUE 4 4FALSE FALSE 5 5 TRUE TRUE 6 6FALSE FALSE 7 7 TRUE TRUE 8 8FALSE FALSE 9 9FALSE FALSE 10 10FALSE FALSE See this: http://jostamon.blogspot.com/2009/02/goldbachs-comet.html for an example of its use. Romain -- Romain Francois Professional R Enthusiast +33(0) 6 28 91 30 30 http://romainfrancois.blog.free.fr |- http://tr.im/BcPw : celebrating R commit #5 |- http://tr.im/ztCu : RGG #158:161: examples of package IDPmisc `- http://tr.im/yw8E : New R package : sos __ 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.
Re: [R] Function to find prime numbers
library(gmp) ?isprime /Gustaf On Tue, Oct 13, 2009 at 9:59 AM, AJ83 aljense...@gmail.com wrote: I need to create a function to find all the prime numbers in an array. Can anyone point me in the right direction? Thank you. AJ -- View this message in context: http://www.nabble.com/Function-to-find-prime-numbers-tp25868633p25868633.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. -- Gustaf Rydevik, M.Sci. tel: +46(0)703 051 451 address:Essingetorget 40,112 66 Stockholm, SE skype:gustaf_rydevik [[alternative HTML version deleted]] __ 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.
Re: [R] Function to find prime numbers
On Tue, 13 Oct 2009, AJ83 wrote: I need to create a function to find all the prime numbers in an array. Can anyone point me in the right direction? It depends a bit on how big the numbers are. If the array is large but the numbers are not very large the fastest approach is probably to create a vector of small primes and use my_array %in% smallprimes. For example, the first 1000 primes are at http://primes.utm.edu/lists/small/1000.txt and the first 1 are on the same site. -thomas Thomas Lumley Assoc. Professor, Biostatistics tlum...@u.washington.eduUniversity of Washington, Seattle __ 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.
Re: [R] Function to find prime numbers
On Tue, Oct 13, 2009 at 2:41 PM, Thomas Lumley tlum...@u.washington.edu wrote:
On Tue, 13 Oct 2009, AJ83 wrote:
I need to create a function to find all the prime numbers in an array. Can
anyone point me in the right direction?
This almost sounds like a homework problem to me... So here's a
solution that you can happily present to a tutor - if you can explain
how it works, then you deserve full marks!
primer=function(v){
return(regexpr(^1$|^(11+?)\\1+$,unlist(lapply(v,function(z){paste(rep(1,z),sep='',collapse='')})),perl=TRUE)
== -1)
}
Test:
(1:30)[primer(1:30)]
[1] 2 3 5 7 11 13 17 19 23 29
I'm not sure how big a number this works for
R golf anyone?
Barry
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Re: [R] Function to find prime numbers
On Tue, Oct 13, 2009 at 5:30 PM, Barry Rowlingson
b.rowling...@lancaster.ac.uk wrote:
On Tue, Oct 13, 2009 at 2:41 PM, Thomas Lumley tlum...@u.washington.edu
wrote:
On Tue, 13 Oct 2009, AJ83 wrote:
I need to create a function to find all the prime numbers in an array. Can
anyone point me in the right direction?
This almost sounds like a homework problem to me... So here's a
solution that you can happily present to a tutor - if you can explain
how it works, then you deserve full marks!
This almost sounds like an extra assignment to me :-)
primer=function(v){
return(regexpr(^1$|^(11+?)\\1+$,unlist(lapply(v,function(z){paste(rep(1,z),sep='',collapse='')})),perl=TRUE)
== -1)
}
Test:
(1:30)[primer(1:30)]
[1] 2 3 5 7 11 13 17 19 23 29
I'm not sure how big a number this works for
R golf anyone?
I take the challenge and give AJ some more stuff to think about.
First the obvious :
primer=function(v){
return(regexpr(^1$|^(11+?)\\1+$,sapply(v,function(z){paste(rep(1,z),sep='',collapse='')}),perl=TRUE)
== -1)
}
Test :
(1:30)[primer(1:30)]
[1] 2 3 5 7 11 13 17 19 23 29
Shorter, if I keep the same use of the function primer :
primer=function(v){
return(sapply(v,function(z){sum(z/1:z==z%/%1:z)})==2)
}
(1:30)[primer(1:30)]
[1] 2 3 5 7 11 13 17 19 23 29
nchar(primer=function(v){ . . . (1:30)[primer(1:30)])
[1] 97
This works for all numbers. It also allows for a function primes :
primes=function(v){
return(v[sapply(v,function(z){sum(z/1:z==z%/%1:z)==2})])
}
primes(1:30)
[1] 2 3 5 7 11 13 17 19 23 29
nchar(primes=function(v){ . . . primes(1:30))
[1] 91
Cheers
Joris
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