[R] a question about frame

[Gabor Grothendieck]

On 6/15/05, ronggui <0034058 at fudan.edu.cn> wrote:
> gg <- function(y) {
>         ggg <- function() {
>             cat("current frame is", sys.nframe(), "\n")
>             cat("parents are", sys.parents(), "\n")
>             print(sys.function(0)) # ggg
>             print(sys.function(2)) # gg
>         }
>         if(y > 0) gg(y-1) else ggg()
>     }
> 
> 
> >  gg(3)
> current frame is 5
> parents are 0 1 2 3 4
> function() {
>             cat("current frame is", sys.nframe(), "\n")
>             cat("parents are", sys.parents(), "\n")
>             print(sys.function(0)) # ggg
>             print(sys.function(2)) # gg
>         }
> <environment: 071C62FC>
> function(y) {
>         ggg <- function() {
>             cat("current frame is", sys.nframe(), "\n")
>             cat("parents are", sys.parents(), "\n")
>             print(sys.function(0)) # ggg
>             print(sys.function(2)) # gg
>         }
>         if(y > 0) gg(y-1) else ggg()
>     }
> 
> 
> my question is ,why the current frame is 5?why the sys.function is ggg.i am quite confused.
> anyone can give me some clue?i have read the ?sys.parent,but i still can not get the point exactly.


We start out at frame 0 and then:
1. we call gg(3) so now we are at frame 1
2. which calls gg(2) so now we are at frame 2
3. which calls gg(1) so now we are at frame 3
4. which calls gg(0) so now we are at frame 4
5. which calls ggg() so now we are at frame 5

sys.function(0) seems to be referring to the current frame
(not frame 0 as the which= documentation in ?sys.function
seems to imply).  Since sys.function(0) is called within ggg
we get the result. Also sys.function(2) refers to frame 2 
and referring to the numbered list above we see it is gg.