sys.frame() and variables from parent frame
On Feb 27, 2013, at 10:16 AM, Bert Gunter wrote:
Some additional comments inline below to add to David's clarifications, proffered largely in response to the poster's remark about coming to R from another language. On Wed, Feb 27, 2013 at 9:44 AM, David Winsemius <dwinsemius at comcast.net> wrote:
On Feb 27, 2013, at 1:11 AM, Z? Miguel wrote:
Dear David,
thanks for your reply. Sorry for the ambiguous example. I guess the get() will do it, but I still have a question regarding the parent frame. If I declare both function g and f in the global environment:
g <- function()
{
get(N,envir = ???)
write(N)
}
f<-function()
{
N<-99
g()
}
While David shows you below how you **can** do this, I would suggest that you **don't.** The reason is that R largely follows a functional programming paradigm (e.g like Scheme or Lisp, for those in the know). Among other things, this means that essentially a function environment should be its own little world, and that anything needed for it to do its job in that world should be explicitly passed to it as a function argument. The practical danger of circumventing this paradigm and reaching back into a parent or global environment is that you don't know what will be there. Functions can be called by other functions and, if created to serve a purpose as an independent entity, might be called under circumstances entirely unforeseen by the function author. There are certainly situations where one may wish to violate this dictum, and I am sure R experts could provide compelling examples of them. But for the great mass of us unwashed, I think hewing to the R functional paradigm is a safer and more sensible course.
Oh, yes. I quite agree. I was taking that as an opportunity to illustrate why the N of 99 in inside his f() was NOT in the global environment (as it appeared that JMD expected). I was not suggesting that he try to shoehorn R into behaving like SAS or SPSS or BASIC. or Minitab or .... where everything is global.
David.
>
> Cheers,
> Bert
>
>>>
>>> What is the environment I should be evoking in the get() function? Excuse me for the bad coding, I jst migrated from another language, please interpret it as a generic language.
>>>
>>
>> You can specify the global environment with either a function call or by a consant name
>>
>> globalenv()
>>
>> .GlobalEnv
>>
>>
>> In your code however, 'N' is not in the global environment and you are not passing a character argument to get() as you had been in your first example. Furthermore you have chosen to use write which does not gte the desired effect of reporting from inside a function.
>>
>> g <- function()
>> {
>> get("N", envir = .GlobalEnv) # character argument to get
>> cat(N)
>> }
>>
>> f<-function()
>> {
>> N<-99 # not in the global environment
>> g()
>> }
>>
>> N <- 42 # assigned in the global environment
>> f()
>> # returns 42
>>
>> --
>> David.
>>
>>
>>
>>> Cheers,
>>>
>>> Jos?
>>>
>>>
>>>
>>>
>>> On 27 February 2013 01:20, David Winsemius <dwinsemius at comcast.net> wrote:
>>>
>>> On Feb 26, 2013, at 2:40 AM, Jos? Miguel Delgado wrote:
>>>
>>>> Dear R-help,
>>>>
>>>> I wrote the following lines in order to use a variable from a parent frame in my callM() function. I would like to solve this by only editing the callM(function). When I run this code I obtain a empty list, meaning that eval(ls(),sys.frame(-1)) could not access the variables in the parent function. Any suggestions? Thanks in advance!
>>>>
>>>> metaCall <- function()
>>>> {
>>>> NN <- 99
>>>> callM()
>>>> }
>>>>
>>>> callM <- function()
>>>> {
>>>> Ls <- eval(ls(),sys.frame(-1))
>>>> print(Ls)
>>>> ### use Ls to call a certain model named M
>>>
>>> That doesn't make much sense.
>>>
>>>> }
>>>>
>>>> metaCall()
>>>>
>>>
>>> I don't think you need the eval() call:
>>>
>>> metaCall <- function()
>>> {
>>> NN <- 99
>>> callM()
>>> }
>>>
>>> callM <- function()
>>> {
>>> Ls <- ls(env=sys.frame(-1))
>>> cat(Ls)
>>> }
>>>
>>> metaCall()
>>> NN
>>>
>>> You should be aware that metaCall will not return anything since the last value is from the cat()-call or the print()-call and they each return NULL. If you wanted to go on from there and do something with 'NN", which is no longer a number but rather a character vector, you could, however.
>>>
>>> Oh, I think I finally see .... the eval() was an attempt to retrieve the object named "NN"" in the parent.frame? That means you need :
>>>
>>> ?get
>>>
>>> metaCall <- function()
>>> {
>>> NN <- 99
>>> callM()
>>> }
>>>
>>> callM <- function()
>>> {
>>> Ls <- ls(env=sys.frame(-1))
>>> str(get(Ls, env=sys.frame(-1)))
>>> }
>>>
>>> metaCall()
>>> # num 99
>>>
>>> --
>>> David Winsemius
>>> Alameda, CA, USA
>>>
>>>
>>>
>>>
>>> --
>>> Jos? Miguel Delgado
>>> Reichenberger Str. 52
>>> 10999 Berlin
>>> Tel. 0176 9633 92 56
>>
>> David Winsemius
>> Alameda, CA, USA
>>
>> ______________________________________________
>> R-help at r-project.org 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.
>
>
>
> --
>
> Bert Gunter
> Genentech Nonclinical Biostatistics
>
> Internal Contact Info:
> Phone: 467-7374
> Website:
> http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm
David Winsemius
Alameda, CA, USA