The three-dot question

Dear all,

Why does not the three-dot accept arguments from the parent environment?
I am just confused with this error, can someone give me a hint?

rm(list=ls())
testFun <- function(a, ...)

+   {
+     if(a){
+         print(a)
+       }else
+       {
+         print(b)
+       }
+   }

myTask <- function(a)

+   {
+     b <- 3
+     testFun(a, b = b)
+   }

myTask(FALSE)

Error in print(b) : object 'b' not found


Thanks in advance!

Feng

--
Feng Li
Department of Statistics
Stockholm University
SE-106 91 Stockholm, Sweden
http://feng.li/

Hi Feng,

I'm afraid I don't entirely understansd your question -- the `...`
construct only allows you to pass variable numbers of arguments, not
to have arbitrary access to the parent frames. You need to manually
extract "b" from the dots inside of testFun.

Also, it's quite frowned upon to put ##rm(list = ls())## in your
examples: it's the mailing list equivalent of asking a buddy to come
over to help you move and then punching him in the face when he tries
to lift your sofa.

Cheers,
Michael


On Mon, Jan 14, 2013 at 10:21 AM, Feng Li <m at feng.li> wrote:

Dear all,

Why does not the three-dot accept arguments from the parent environment?
I am just confused with this error, can someone give me a hint?

rm(list=ls())
testFun <- function(a, ...)

+   {
+     if(a){
+         print(a)
+       }else
+       {
+         print(b)
+       }
+   }

myTask <- function(a)

+   {
+     b <- 3
+     testFun(a, b = b)
+   }

myTask(FALSE)

Error in print(b) : object 'b' not found


Thanks in advance!

Feng

--
Feng Li
Department of Statistics
Stockholm University
SE-106 91 Stockholm, Sweden
http://feng.li/

Hi Michael,

Thanks for the reply.

On Mon, 2013-01-14 at 10:33 +0000, R. Michael Weylandt wrote:

Hi Feng,

I'm afraid I don't entirely understansd your question -- the `...`
construct only allows you to pass variable numbers of arguments, not
to have arbitrary access to the parent frames. You need to manually
extract "b" from the dots inside of testFun.

Also, it's quite frowned upon to put ##rm(list = ls())## in your
examples: it's the mailing list equivalent of asking a buddy to come
over to help you move and then punching him in the face when he tries
to lift your sofa.

The rm line just wants to declare there is no "b" in the global
environment. I did not mean people ought to try it. Also notice that
there is ">" in front of the rm. You won't get hurt if you just copy and
paste it. If it bites you, the mouse and keyboard did it:)

Cheers,

Feng

Cheers,
Michael


On Mon, Jan 14, 2013 at 10:21 AM, Feng Li <m at feng.li> wrote:

Dear all,

Why does not the three-dot accept arguments from the parent environment?
I am just confused with this error, can someone give me a hint?

rm(list=ls())
testFun <- function(a, ...)

+   {
+     if(a){
+         print(a)
+       }else
+       {
+         print(b)
+       }
+   }

myTask <- function(a)

+   {
+     b <- 3
+     testFun(a, b = b)
+   }

myTask(FALSE)

Error in print(b) : object 'b' not found


Thanks in advance!

Feng

--
Feng Li
Department of Statistics
Stockholm University
SE-106 91 Stockholm, Sweden
http://feng.li/

Hi: If you want testFun to know about b, then you would have to do
b<-list(...)$b inside
TestFun itself.  But the dot dot dot argument is not really for that
purpose. 

The use of dotdotdot is for the case where a function INSIDE testFun
has a formal argument named say b. Then you can pass the ... at the
top level and the function inside will receive the ... but
automatically slurp the b out of the dot dot dot and know about it.
Below is an example of the use I'm talking about. It was created by a
mentoR when I was trying to understand the dotdotdot concept. 
hope it helps you. 

#======================================================================

Note that f does not directly know about x and g does.  That is g
knows about x even though f does
not know and g got it from f.

g <- function(x, ...) { cat("g: exists('x') = ", exists("x"), "\n");
list(...) }
f <- function(...) { cat("f: exists('x') = ", exists("x"), "\n");
g(...) }
f(x = 3, y = 4)
f: exists('x') =  FALSE
g: exists('x') =  TRUE
$y




On Mon, Jan 14, 2013 at 5:21 AM, Feng Li <m at feng.li> wrote:
        Dear all,
        
        Why does not the three-dot accept arguments from the parent
        environment?
        I am just confused with this error, can someone give me a
        hint?
        

        > rm(list=ls())
        > testFun <- function(a, ...)

        +   {
        +     if(a){
        +         print(a)
        +       }else
        +       {
        +         print(b)
        +       }
        +   }

        >
        > myTask <- function(a)

        +   {
        +     b <- 3
        +     testFun(a, b = b)
        +   }

        > myTask(FALSE)

        Error in print(b) : object 'b' not found
        
        
        Thanks in advance!
        
        Feng
        
        --
        Feng Li
        Department of Statistics
        Stockholm University
        SE-106 91 Stockholm, Sweden
        http://feng.li/