[R] Question about "evalq"

[ronggui]

That's great. I got it. Million thanks.

On 5/28/07, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
> On 5/27/07, ronggui <ronggui.huang at gmail.com> wrote:
> > Hi,Gabor Grothendieck, Thanks very much.
> >
> > On 5/27/07, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
> > > evalq looks like this:
> > >
> > >    > evalq
> > >    function (expr, envir, enclos)
> > >    eval.parent(substitute(eval(quote(expr), envir, enclos)))
> > >    <environment: namespace:base>
> > >
> > > so it seems the difference is that
> > >
> > > - eval(quote(), envir, enclos) evaluates envir and enclos
> > >   in the current frame but
> > > - evalq evaluates them in the parent.frame.
> > >
> > > This may be easier to see in the following example:
> >
> > Yeah, This example make the question easier to understand.
> >
> > >    x <- "G"
> > >    f1 <- function() eval(quote(x), parent.frame())
> > >    f2 <- function() evalq(x, parent.frame())
> > >    f11 <- function() {
> > >         x <- "a"
> > >         f1()
> > >    }
> > >    f22 <- function() {
> > >         x <- "b"
> > >         f2()
> > >    }
> > >    f11() # a
> > >    f22() # G
> > >
> > > To avoid this problem pass a variable whose value is
> > > to be enclos= rather than an expression to compute it:
> >
> > --This is a good idea.
> > --If "evalq evaluates them in the parent.frame", I expected that if I
> > change parent.frame(2) to parent.frame(1), I will get the answer.But I
> > can not actually. So what's wrong with my understanding?
> >
> >    f1 <- function(x,digits=5) lapply(x, f2)
> >    f2 <- function(x) {
> >       evalq(print(digits), list(x=x), parent.frame(1))
> >    }
> >    f1(list(x1=1)) ##Error in print(digits) : object "digits" not found
>
>
> Good point.  Insert a browser statement where the parent.frame
> call was and when it stops do a traceback.  That will show you
> what the call stack looks like at that point in time.
>
>
> > f1 <- function(x,digits=5) lapply(x, f2)
> > f2 <- function(x) evalq(print(digits), list(x=x), { browser() } )
> > f1(list(x1=1)) ##Error in print(digits) : object "digits" not found
> Called from: eval(quote(print(digits)), list(x = x), {
>     browser()
> })
> Browse[1]> traceback()
> 10: print(digits)
> 9: eval(expr, envir, enclos)
> 8: eval(quote(print(digits)), list(x = x), browser())
> 7: eval(expr, envir, enclos)
> 6: eval(expr, p)
> 5: eval.parent(substitute(eval(quote(expr), envir, enclos)))
> 4: evalq(print(digits), list(x = x), browser())
> 3: FUN(X[[1L]], ...)
> 2: lapply(x, f2)
> 1: f1(list(x1 = 1))
> Browse[1]>
>


-- 
Ronggui Huang
Department of Sociology
Fudan University, Shanghai, China