[R] get: problem with environments

[R. Michael Weylandt]

On Sun, Oct 7, 2012 at 12:33 PM, Rui Barradas <ruipbarradas at sapo.pt> wrote:
> Hello,
>
> No, I can't confirm the behavior the op sees.
> I've tried it in an R terminal on ubuntu 12.04, rkward on ubuntu, and RGui
> on Windows 7. R version 2.15.1.
>
> Here's the rkward sessionInfo.
>
> sessionInfo()
> R version 2.15.1 (2012-06-22)
> Platform: x86_64-pc-linux-gnu (64-bit)

On Sun, Oct 7, 2012 at 1:28 PM, arun <smartpink111 at yahoo.com> wrote:
> I am getting the same error message as Michael got (R terminal (R 2.15) on Ubuntu 12.04).
>
>   testfun(x=5)
> #Error in sys.frame(which = x) : not that many frames on the stack
>  print(testfun(x=6))
> #Error in sys.frame(which = x) : not that many frames on the stack
>

From: R. Michael Weylandt <michael.weylandt at gmail.com>
> To: Martin Ivanov <tramni at abv.bg>
> Cc: r-help at r-project.org
> Sent: Sunday, October 7, 2012 5:34 AM
> Subject: Re: [R] get: problem with environments
>
> On Sun, Oct 7, 2012 at 10:16 AM, Martin Ivanov <tramni at abv.bg> wrote:
>>  Dear R users,
>>
>> I am running R-2.15.1 in Linux Slackware64-14.0. Here is my minimal working example:
>>
>> testfun <- function (x) {
>>  a <- 0;
>>  sapply(X="a", FUN=get, envir=sys.frame(which=x));
>> }
>>
>> Inside R, that is R called from within a Linux terminal, the following code works:
>> testfun(x=5)
>> print(testfun(x=6))
>> But within rkward the above code fails and the following works:
>> testfun(x=1)
>> print(testfun(x=2))
>>
>> As you can see, the number of contexts up the call stack that contain the variable "a"
>> varies depending on the implementation. If I call testfun() from within print(), I have to go
>> one context up the call stack than if I call testfun() alone by itself. This implies
>> some inherent instability of my code.
>
> Unlike you, I don't get testfun(x = 5) to work in a terminal emulator.
> As expected, I only get x =1 and print(... x = 2) to work and I just
> tried this with R 2.15.0 and R devel, both in Terminal and by way of
> ESS.  It could be a buglet introduced later into the 2.15 branch, but
> that seems unlikely. As expected,
>
>> testfun(5)
> Error in sys.frame(which = x) : not that many frames on the stack
>

So this all suggests something very strange has happened to your
setup, Martin. Does this persist after a new session (perhaps running
as R --vanilla) and/or reinstall?

To your original questions:

> 1. Do You have an idea how I could implement the code
> more stably, so that the variable "a" is always visible to get, regardless of whether testfun
> is used alone by itself or called from within another function?

Well, you can do it without get() and just trust in the scoping rules
to make it work. Depending on your application, this might simplify
the code nicely.

If you really do need to compute something on the fly, tricks like
eval(parse(text = "")), as.name(), call() etc might work, but they're
occasionally difficult to get right.

a <- 54
eval(as.name("a")) + 2

testfun2 <- function(n, letter) eval(as.name(letter)) + n

testfun2(3, "a") # 57

>
> 2. Why does the implementation with parent.frame() not work?

You didn't show us how you tried to use parent.frame()

>
> 3. Why does the number of contexts in the call stack differ in R and in rkward?

It shouldn't. This is an issue that needs further sorting out.

Cheers,
Michael