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 <[EMAIL PROTECTED]> 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: [EMAIL PROTECTED]
> > To: [EMAIL PROTECTED]
> > Subject: Re: [R] Can you recover default argument values of a function?
> > CC: [EMAIL PROTECTED]
> >
> > 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]]
> >>
> >> ______________________________________________
> >> R-help@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.
> >>
>
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