[R] Question about "evalq"

[ronggui]

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


>    f1 <- function(x,digits=5) lapply(x, f2)
>    f2 <- function(x) {
>       pf2 <- parent.frame(2)
>       evalq(print(digits), list(x=x), pf2)
>    }
>    f1(list(x1=1)) # 5
>
>
>
> On 5/26/07, ronggui <ronggui.huang at gmail.com> wrote:
> > The help page of eval says: The 'evalq' form is equivalent to
> > 'eval(quote(expr), ...)'.  But the following is not equivalent.  Can
> > anyone give me some explaination? Thanks very much.
> >
> > > f1 <- function(x,digits=5) lapply(x, f2)
> > > f2 <- function(x) eval(quote(print(x+1,digits=digits)),list(x=x),parent.frame(2))
> > > f1(list(x1=1))
> > [1] 2
> > $x1
> > [1] 2
> >
> > >
> > > f1 <- function(x,digits=5) lapply(x, f2)
> > > f2 <- function(x) evalq(print(x+1,digits=digits),list(x=x),parent.frame(2))
> > > f1(list(x1=1))
> > Error in print.default(x + 1, digits = digits) :
> >  object "digits" not found
> >
> >
> >
> > --
> > Ronggui Huang
> > Department of Sociology
> > Fudan University, Shanghai, China
> >
> > ______________________________________________
> > 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
> > and provide commented, minimal, self-contained, reproducible code.
> >
>


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