On 22/10/2015 10:20 AM, david.kaeth...@gmail.com wrote:
> Hello,
>
> I’m trying to solve an exercise, where I want to walk through the search path
> recursively (http://adv-r.had.co.nz/Environments.html
> <http://adv-r.had.co.nz/Environments.html>).
>
> I’m puzzled by a certain behavior and hope somebody can give me an
> explanation.
>
> This code works:
>
> listenv <- function(env = parent.frame()) {
> if (identical(env, emptyenv())) {
> #stop("reached emptyenv", call. = FALSE)
> return(env)
> } else {
> print(env)
> listenv(parent.env(env))
> }
> }
>
> Here, the calling environment is determined with a default parameter in the
> function’s formals.
>
> However, if I want to assign the calling environment within the function’s
> body, I get the error message „infinite recursion“. Also, I never get actual
> environments (with attributes, that is), only memory addresses like this:
> <environment: 0x10da46630>.
I'm not sure what you were looking for, but "<environment: 0x10da46630>"
is the normal way to print an environment, unless it happens to be one
of the special named ones (like .GlobalEnv).
>
> listenv <- function(env) {
> env <- parent.frame()
> if (identical(env, emptyenv())) {
> #stop("reached emptyenv", call. = FALSE)
> return(env)
> } else {
> print(env)
> listenv(parent.env(env))
> }
> }
>
> Any explanation of what’s going on here would be greatly appreciated. I
> suspect it has to do with when exactly the parent.frame()-expression is
> evaluated, but that’s not an actual explanation.
Your function completely ignores the "env" argument. It never recurses.
In the first case, "parent.frame()" is only a default value, so
recursion happens properly. If you change the first line in the body to
these two lines
if (missing(env))
env <- parent.frame()
it would be equivalent.
Duncan Murdoch
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