[R] Most elegant way to use formals() in building functions

[Frank E Harrell Jr]

Duncan Murdoch wrote:
> On 24/07/2009 6:35 PM, Frank E Harrell Jr wrote:
>> Dear Group:
>>
>> I want to create a function having a ... argument and to have the 
>> default arguments evaluated, as thus:
>>
>> g <- function(a, b, ...) a+b
>> formals(g) <- alist(a=,b=2+3,...=)
>> g
>> function (a, b = 2 + 3, ...)
>> a + b
>>
>> But I want the default argument for b to be evaluated as 5.  How can 
>> this be done?  Note: My real need is for a more complex expression to 
>> be evaluated for the default value of an argument.
>>
>> I can use formals(g) <- list(a=NULL, b=2+3, ...=NULL) but then 
>> list(...) doesn't behave properly in the function.
> 
> Not sure if it's "most elegant", but I think this works:
> 
> formals(g) <- eval(substitute(alist(a=,b=default,...=),
>                    list(default=2+3)))
> 
> Duncan Murdoch

Thank you Duncan.  That works perfectly.
Frank

> 


-- 
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University