[Rd] Scoping bug in ftable() (PR#6541)

[Luke Tierney]

On Wed, 4 Feb 2004 dmurdoch at pair.com wrote:

> This bug shows up in ftable() in both r-patched and r-devel:
> 
> > x <- c(1,2)
> > y <- c(1,2)
> > z <- c(1,1)
> > ftable(z,y)
>   y 1 2
> z      
> 1   1 1
> > ftable(z,x)
>   x 1
> z    
> 1   2
> 
> Since x and y are identical, the two ftable results should be the
> same, but they are not.  
> 
> I've only been able to see this when the column variable is named "x",
> so it looks like a scoping problem in ftable.default.  I think the
> problem is in this line:
> 
> x <- do.call("table", c(as.list(substitute(list(...)))[-1],
> list(exclude = exclude)))
> 
> I think this call is finding the local variable "x" (which has been
> used before this line) instead of the argument "x" and thus produces
> an incorrect result.
> 
> How should this be fixed?  What we want is to convert "..." into an
> evaluated list that includes the deparsed arguments as names.  Just
> plain list(...) loses the names.
> 
> I think this works:
> 
> args <- list(...)
> names(args) <-
> as.character(unlist(as.list(substitute(list(...)))))[-1]
> x <- do.call("table", args)
> 
> but isn't there an easier way?

Not sure there is.

The fact that the original code did something close to what was
intended is due to what I still think is a design flaw in do.call
(though others disagree): It does an eval of it's arguents (on top of
what ordinary function calling does to evaluate the expression
producing the argument list).  This is why there is no explicit eval
around the substitute.  It is also why do.call isn't useful when some
elements of the argument list are symbols or expressions.

luke

-- 
Luke Tierney
University of Iowa                  Phone:             319-335-3386
Department of Statistics and        Fax:               319-335-3017
   Actuarial Science
241 Schaeffer Hall                  email:      luke at stat.uiowa.edu
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