[R] Can you recover default argument values of a function?
Gabor Grothendieck
ggrothendieck at gmail.com
Thu Dec 27 00:27:27 CET 2007
Yes, that is what I was referring to. In fact my second example shows it can be
even worse than in your example since v is computed by f so you
can't get v's value without running f.
On Dec 26, 2007 6:09 PM, Talbot Katz <topkatz at msn.com> wrote:
>
> Thank you, that's just what I wanted. By the way, I found an interesting "gotcha" that can occur with expression arguments:
>
> > x = 7
> > z = 2
> > mxy <- function( x = 4, y = x + z ) { return(x*y) }
> > eval( formals( mxy )[[1]] )
> [1] 4
> > eval( formals( mxy )[[2]] )
> [1] 9
> > mxy()
> [1] 24
> > mxy( eval( formals( mxy )[[1]] ), eval( formals( mxy )[[2]] ) )
> [1] 36
> >
>
> The problem is "confusion" about whether the "x" in the second argument expression refers to the first argument, or the environment variable. When the function is evaluated, the argument value of x is used, but when the argument is evaluated (using eval and formals) the environment value of x is used. This is a reasonable choice, and mixing up arguments and environment variables in a function definition probably should be considered bad programming.
>
> -- TMK --
> 212-460-5430 home
> 917-656-5351 cell
>
>
> > Date: Wed, 26 Dec 2007 17:21:46 -0500
> > From: ggrothendieck at gmail.com
> > To: topkatz at msn.com
> > Subject: Re: [R] Can you recover default argument values of a function?
> > CC: r-help at stat.math.ethz.ch
> >
> > 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 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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> >> and provide commented, minimal, self-contained, reproducible code.
> >>
>
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