On Tue, 7 Jun 2011, Duncan Murdoch wrote:
On 07/06/2011 9:08 AM, oliver wrote:
Hello,
following an advice here from the list I looked into sources of other
packages (xts) and found the TYPEOF() macro/function, which really is
helpful.
It is documented, of course, but actually better alternatives are
described in 'Writing R Extensions'.
We would urge you to study the R sources rather than copy bad habits
from randomly chosen package sources.
I iused the follwong code snippet:
switch( TYPEOF( filename_sexp ) )
{
case STRSXP: filename = CHAR( STRING_ELT(filename_sexp, 0) );
break;
default: error("filename argument must be a string");
break;
}
Here, filename is of type char*
and one function opens a file with that name.
So it is purely intended to just grab out the char* from the
String-Expression.
Am I doing something wrong here, or is it ok, but I have somehow
to say the extracting macros/functions, that it is really intended
to throw away information and that a warning is not necessary?
The result of calling CHAR should be a "const char *". You are not allowed
to touch the string it points to.
Note too that isString() exists for this purpose, and there is no
check in that code that LENGTH(filename_sexp) > 0 (or == 1). In the R
sources you will often see things like
if(!isString(sfile) || LENGTH(sfile) < 1)
error("invalid '%s' argument", "description");
Then, reading on,
file = translateChar(STRING_ELT(sfile, 0));
for you cannot (in general) assume that the character vector passed is
in the native encoding.
--
Brian D. Ripley, rip...@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
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