[R] question on function arguments

[baptiste Auguié]

Many thanks, this clarifies both points perfectly.

baptiste



On 19 Feb 2008, at 11:48, Duncan Murdoch wrote:

> On 18/02/2008 4:53 AM, baptiste Auguié wrote:
>> Hi,
>> I have two small issues with my R code, no big deal but curiosity   
>> really. Here is a sample code,
>>> x <- rnorm(1:10)
>>>
>>> foo <- function(a = 1, b = list(x = c(1:10), y = c(1:10))){
>>> 	
>>> 	for (ii in seq(along=b$y)){
>>> 		
>>> 		print(x[ii] + b$x[ii])
>>> 	}
>>> 	
>>> 	
>>> }
>>>
>>> foo() # default OK
>>>
>>> foo(b=list(x=1, y=c(1:10))) # only the first term works
>>>
>>> foo(b$y = 1:5) # error
>>>
>> In the last call, i wish to use all default arguments but b$y  
>> (that  is, using the default a and b$x). Is this possible?
>
> No, arguments are a simple list where you use the defaults or you  
> don't.  You can't treat them as a hierarchical structure.
>> The second call is more related to indices: i would like my  
>> argument b $x to be either a vector (default), or a scalar. In the  
>> latter, the  loop b$x[ii] breaks when i would like it to recycle  
>> the single value.  I can check the length of b$x with a "if"  
>> statement, but it becomes  intricate when several arguments have  
>> this option of being vector or  scalar. Is there a way to use b$x  
>> that would work in both cases?
>
> The normal way to do this is to standardize the length of the  
> object early in the function, e.g.
>
>  len <- max(length(b$y), length(b$x))
>  b$y <- rep(b$y, length=len)
>  b$x <- rep(b$x, length=len)
>
> Another way (which is the way used internally in the C  
> implementation) is to compute an index using the recycling rules,  
> but that's probably not efficient in R code, e.g. you'd need
>
> b$x[ (ii - 1) %% length(b$x) + 1 ]
>
> in place of b$x[ii] (and this solution is not general; it doesn't  
> handle zero or negative indices properly).
>
> Duncan Murdoch
>> Many thanks,
>> baptiste
>> _____________________________
>> Baptiste Auguié
>> Physics Department
>> University of Exeter
>> Stocker Road,
>> Exeter, Devon,
>> EX4 4QL, UK
>> Phone: +44 1392 264187
>> http://newton.ex.ac.uk/research/emag
>> http://projects.ex.ac.uk/atto
>> ______________________________________________
>> R-help at 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.
>

_____________________________

Baptiste Auguié

Physics Department
University of Exeter
Stocker Road,
Exeter, Devon,
EX4 4QL, UK

Phone: +44 1392 264187

http://newton.ex.ac.uk/research/emag
http://projects.ex.ac.uk/atto