[R] Problems with eval() in connection with match.call()

[Douglas Bates]

On 7/4/05, Søren Højsgaard <Soren.Hojsgaard at agrsci.dk> wrote:
> Dear all, I have a problem when passing parms from one function to another when the argument list is just '...'. Consider this example:
> 
> foo<-function(){
>  xx <- 111222
>  bar(x=xx)
> }
> bar <- function(...){
>   cl <- match.call(expand.dots=TRUE)
>   print(cl)
>   x <- eval(cl$x)
>   print(x)
> }
> foo()
> 
> > bar(x = xx)
> > Error in eval(expr, envir, enclos) : Object "xx" not found
> 
> My expectation was, that xx would be evaluated to 111222 in foo before being passed on to bar, but obviously it is not so. 

Welcome to the world of lazy evaluation.  Arguments are evaluated only
when needed (i.e. when first referenced) not at the time of building
the function call.   That's why the default value of an argument can
depend on previously calculated results within the function.

> Should I do something explicitely in foo() to 'evaluate' xx or need I do something special in bar()??

Change the eval(cl$x) to eval(cl$x, parent.frame())

> bar
function(...){
    cl <- match.call(expand.dots=TRUE)
    print(cl)
    x <- eval(cl$x, parent.frame())
    print(x)
}
> foo()
bar(x = xx)
[1] 111222

> Thanks in advance, Søren
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
>