sys.frame() and variables from parent frame

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

   }

metaCall()

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()
}

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.

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()
}

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

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.