[R] Question about "evalq"

Prof Brian Ripley ripley at stats.ox.ac.uk
Sun May 27 08:19:46 CEST 2007

The meaning of parent.frame depends on where it is evaluated.  So one 
should not expect it to do the same thing in two equivalent expressions 
(and nor should one expect deparse to do so, for example).

A pretty close analogy is that using a symbolic link in a file system is 
equivalent to using the original file path, at least until you try '..' or 
'pwd' on the path.  (In the case of 'pwd' it depends on the OS: POSIX 
only requires '_an_ absolute pathname'.)

On Sun, 27 May 2007, ronggui 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

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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