[Rd] Problem using findVar( ) in combination with R's lazy evaluation

[Duncan Murdoch]

On 8/20/2009 8:41 AM, MarcelK wrote:
> Hi All,
> 
> I have a few small questions about the usage of the C findVar( ) function
> when used in C code called with '.Call'. In my case I create an R function
> with an argument. This function calls some C code in which I use findVar( )
> to retrieve the values from the argument. Ofcourse normally I would just
> give the values as argument to .Call, but in my project I need to use
> findVar for various reasons (can elaborate if needed, but it's not really
> relevant..) Below is a small example of an R function and the C code in
> which the problem shows:  (skip to bold 'question' part if this is allready
> known)
> 
> ----------# R function #--------
> 
> f <- function(x) {
>   .Call("envtest", environment())
> }
> 
> ----------# C function #--------
> 
> SEXP envtest(SEXP rho) {
>   SEXP data;
>   PROTECT(data = allocVector(REALSXP, 1));
>   data = findVar(install("x"), rho);
>   UNPROTECT(1);
>   return(data);
> }
> 
> ----------# END #-----------
> 
> The result of this function is that it should print the contents of whatever
> is passed to the function f(x). But when executing the function, the result
> is:
> 
> 
>> f(10)
> <promise: 0x142db20>
> 
> Ofcourse this is due to lazy-evaluation as I've also read in the thread 
> http://www.nabble.com/confusion-about-evaluation.-to18561135.html#a18563086
> here . 
> 
> To 'fix' it, I would need to do 'something' with 'x', so changing the
> function f() to:
> 
> f <- function(x) {
>    x = x
>   .Call("envtest", environment())
> }

The standard thing to do is to call force(), i.e. instead of x = x, call 
force(x).  But Simon's solution is better, especially if you don't know 
in advance which arg you're going to be getting.

Duncan Murdoch

> 
> works as can be shown with:
> 
>> f(10)
> [1] 10
> 
> ----------
> 
> My question on this subject is, how can I work around this 'x = x' or other
> similar statement (print(x) to prevent lazy-evaluation? To summarize, I
> would like to use findVar to retrieve values from R objects which are given
> as function arguments to the R-function and not to the C-function. 
> 
> Other than the above question, I'd also like to know if findVar( ) by
> default also checks parent environments (inherits argument?) when the object
> can not be found in the given environment (if not, should I look into
> passing the environment Hash etc?). 
> 
> And, as a last question, how can I retrieve the value from the R-constant
> 'pi' using findVar? If I try this I receive the error "REAL can only be
> applied to 'numeric' not a 'promise'".
> 
> Thank you for any solutions or insights in these subjects, if more details
> are needed I'm happy to give them!
> 
> Marcel Kempenaar
> 
> ----------------
> 
>> sessionInfo()
> R version 2.9.1 (2009-06-26)
> x86_64-pc-linux-gnu
> 
> locale:
> LC_CTYPE=en_US.UTF-8;LC_NUMERIC=C;LC_TIME=nl_NL.UTF-8;LC_COLLATE=C;LC_MONETARY=C;LC_MESSAGES=en_US.UTF-8;LC_PAPER=nl_NL.UTF-8;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_US.UTF-8;LC_IDENTIFICATION=C
> 
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>