[Rd] CHAR(STRING_ELT( - OK but CHAR(asChar(STRING_ELT( - not, why?

[Prof Brian Ripley]

I think you are asking why calling asChar on a CHARSXP gives NA_STRING. 
In particular, the calls you mention *are* perfectly OK and work as 
intended.

As barely documented in R-exts, asChar is designed for vector arguments: a 
CHARSXP is not a vector.  It gives NA_STRING for invalid inputs.
The asXXXX family of functions are designed to coerce as necessary user 
inputs, and CHARSXPs are not visible at R level.

In general, with internal functions you are expected to read the code to 
find out what they do before using them.

On Mon, 23 Jul 2007, Oleg Sklyar wrote:

> Any idea why CHAR(asChar(STRING_ELT( produces NA whereas
> CHAR(STRING_ELT( gets a pointer to a string? It's generally expected
> that STRING_ELT should already be a character,

It is required to be a CHARSXP, but 'character' usually refers to STRSXP.

> but why the coercion does not work? Here is a simple example (consistent 
> over R2.5.1-R2.6 rev 42284, I didn't check earlier versions, but it used 
> to be different in 2.4):

There is no R '2.4', but the behaviour of asChar was the same in R 2.4.0
except for the adding of SYMSXP at

-------------
r40358 | maechler | 2007-01-04 11:07:04 +0000 (Thu, 04 Jan 2007) | 1 line

eliminate CHAR_STAR in methods/src/ as per old "todo"
-------------

which message does not help me at all.  Perhaps Martin can explain?


> install.packages("inline")
>
> library(inline)
>
> sig <- signature(x="character")
> code1 <- "
>  for (int i = 0; i < LENGTH(x); i++ )
>    Rprintf(\"%s\\n\", CHAR(STRING_ELT(x, i)));
>  return R_NilValue;
> "
>
> code2 <- "
>  for (int i = 0; i < LENGTH(x); i++ )
>    Rprintf(\"%s\\n\", CHAR(asChar(STRING_ELT(x, i))));
>  return R_NilValue;
> "
>
> setCMethod(c("p1","p2"), list(sig,sig), list(code1,code2))
>
>
> #----------------------------------------------------------
> p1(c("str1", "str2"))
> # str1
> # str2
> # NULL
>
> p2(c("str1", "str2"))
> # NA
> # NA
> # NULL
>
>
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595