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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