On 09/02/2010 12:18 PM, bluesky...@gmail.com wrote:
In ?eval, it says
... If ‘envir’ is not
specified, then the default is ‘parent.frame()’ (the environment
where the call to ‘eval’ was made).
I tried the following example with "eval(expr)" and "eval(expr,
parent.frame())" in f(). The results are different, which are not
consistent with the help. Could somebody let know whether I
misunderstand the help? Or there is something with the help?
The help is correct, you misunderstood it. Default values for arguments
are evaluated in the evaluation frame of the function. Explicit
arguments are evaluated in the evaluation frame of the caller.
parent.frame() has different meanings in those two contexts.
Duncan Murdoch
> expr=expression(print(paste('x =', x)))
> x=0
> f=function() {
+ x=2
+ eval(expr)
+ eval(expr, parent.frame())
+ eval(expr, parent.frame(2))
+ eval(expr, parent.frame(3))
+ }
> g=function() {
+ x=1
+ f()
+ }
>
> g()
[1] "x = 2"
[1] "x = 1"
[1] "x = 0"
[1] "x = 0"
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and provide commented, minimal, self-contained, reproducible code.
