[R] question on function arguments

[Duncan Murdoch]

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
> 
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