From m_kempenaar at planet.nl  Thu Aug 20 14:41:27 2009
From: m_kempenaar at planet.nl (MarcelK)
Date: Thu, 20 Aug 2009 05:41:27 -0700 (PDT)
Subject: [Rd] Problem using findVar( ) in combination with R's lazy
 evaluation
Message-ID: <25061253.post@talk.nabble.com>


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())
}

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

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