R help - Dec 2007 - Can you recover default argument values of a function?

Hi.
 
Maybe this is a stupid question.  If so, I apologize, but here goes.  Suppose I
have a function f1(x,...) that calls a function f2(y1,y2,...,yn) in the
following way: if x satisfies a certain condition, then I want to call
f2(x,y2,...,yn); otherwise I want to use the default value of y1, if there is
one.  I could do something like the following:
 
v <- ifelse ( is.null(x), f2( , y2,..., yn), f2( x, y2,..., yn) )
 
but I'm doing this in a loop (where the y2,...,yn variables may change), and
I'd prefer not to execute the ifelse statement each time, so I'd like an
initial pre-loop ifelse such as the following:
 
y0 <- ifelse ( is.null(x), default(y1), x )
 
where default(y1) is the default value of the y1 argument of f2.  Then, inside
the loop I'd have
 
v <- f2( y0, y2,..., yn )
 
Is there any mechanism that tells me how many arguments a function has, and the
default values of each one, if there are default values?
 
Thanks!
 
--  TMK  --212-460-5430 home917-656-5351 cell
	[[alternative HTML version deleted]]

If the default value is a constant such as in

f <- function(x = 1) x

then formals(f)[[1]] will give it to you but it could be an expression
referring to other variables (other arguments, other variables in
the function, free variables) such as

f <- function(x = u, u) x

or

f <- function(x = u+v+w, u) { v <- u+1; x }

in which case you would get an object of class
"name" in the first case or "call" in the second.


On Dec 26, 2007 4:42 PM, Talbot Katz <topkatz at msn.com>
wrote:
>
> Hi.
>
> Maybe this is a stupid question.  If so, I apologize, but here goes. 
Suppose I have a function f1(x,...) that calls a function f2(y1,y2,...,yn) in
the following way: if x satisfies a certain condition, then I want to call
f2(x,y2,...,yn); otherwise I want to use the default value of y1, if there is
one.  I could do something like the following:
>
> v <- ifelse ( is.null(x), f2( , y2,..., yn), f2( x, y2,..., yn) )
>
> but I'm doing this in a loop (where the y2,...,yn variables may
change), and I'd prefer not to execute the ifelse statement each time, so
I'd like an initial pre-loop ifelse such as the following:
>
> y0 <- ifelse ( is.null(x), default(y1), x )
>
> where default(y1) is the default value of the y1 argument of f2.  Then,
inside the loop I'd have
>
> v <- f2( y0, y2,..., yn )
>
> Is there any mechanism that tells me how many arguments a function has, and
the default values of each one, if there are default values?
>
> Thanks!
>
> --  TMK  --212-460-5430 home917-656-5351 cell
>        [[alternative HTML version deleted]]
>
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