[R] about eval and eval.parent

[Thomas Lumley]

On Fri, 18 Nov 2005, ronggui wrote:

> x<-1
> f<-function(){
> x<-3
> eval(substitute(x+y,list(y=10)))
> }
> f() #13
>
> x<-1
> f<-function(){
> x<-3
> eval(substitute(x+y,list(y=10)), envir = sys.frame(sys.parent()))
> }
> f() #11
>
> x<-1
> f<-function(){
> x<-3
> eval.parent(substitute(x+y,list(y=10)))
> }
> f()#11
>
>
> the help page says:
>    "If 'envir' is
>     not specified, then 'sys.frame(sys.parent())', the environment
>     where the call to 'eval' was made is used. "
>
>     But it seems that the first one is not equal to the second one.

No, it isn't.  This is because default arguments are evaluated inside the 
function but explicit argumnets are evaluated in the calling frame

In the first one, sys.frame(sys.parent()) is evaluated in the environment 
inside f(), and so refers to the frame from which f() was called.  That's 
what you would expect from pass-by-value

In the second one, sys.frame(sys.parent()) is evaluated inside 
substitute() and refers to the environment inside f(). That might be 
surprising, which is why it is documented.

 	-thomas