Re: [Rd] "warning: assignment discards qualifiers from pointer target type"

From: oliver <oliver_at_first.in-berlin.de>
Date: Thu, 09 Jun 2011 15:28:18 +0200

On Thu, Jun 09, 2011 at 07:43:20AM -0400, Duncan Murdoch wrote:
> On 11-06-09 7:27 AM, oliver wrote:
> >On Wed, Jun 08, 2011 at 08:35:34PM -0400, Simon Urbanek wrote:
> >>
> >>On Jun 8, 2011, at 8:32 PM, oliver wrote:
> >>
> >>>On Thu, Jun 09, 2011 at 02:17:31AM +0200, oliver wrote:
> >>>[...]
> >>>>OK, I looked at this now.
> >>>>
> >>>>LENGTH() checks the length of the vector.
> >>>>
> >>>>Good to know this.
> >>>>
> >>>>So the problem of a vector of length 0 can be with any arguments of type SEXP,
> >>>>hence I will need to check ANY arg on it's length.
> >>>>
> >>>>This is vital to stability under any situation.
> >>>>
> >>>>Thanks for this valuable hint!
> >>>>
> >>>>I will add checks for all my SEXP-args.
> >>>[...]
> >>>
> >>>Hey, LENGTH() does not work with String-vectors! :(
> >>>
> >>
> >>Of course it does ...
> >>
> >>
> >
> >It does not so on my R 2.10.1 installation.
> >
> >
> >In the R-Shell I get:
> >
> > ==============================
> > > length(c())
> > [1] 0
> > >
> > ==============================
> >
> >So c() is vec of length 0.
> >
> >When I feed my readjpeg() with c() as filename arg,
> >
> >testing with:
> >====================================================
> > if( LENGTH( filename_sexp )< 1 )
> > {
> > error("LENGTH( filename_sexp )< 1");
> > //error("filename can't be vector of length 0");
> > }
> > else
> > {
> > error("LENGTH( filename_sexp ) is not< 1");
> > }
> >====================================================
> >
> >
> >I got:
> > Error in readjpeg(filename = c()) : LENGTH( filename_sexp ) is not< 1
> >
> >
> >You can explain why?
>
> c() doesn't create a STRSXP, it is NULL, which is a NILSXP.
> LENGTH() doesn't work on that object. (I'd recommend using length()
> rather than LENGTH(); it's a function, not a macro, and it does give
> the expected answer.)
[...]

Interestingly, c() as value for an integer value can be testes correctly with LENGTH().

So the question arises: is c() always creating NILSXP, or is it NILSXP only for STRSXP, and 0 otherwise?

NILSXP, as it might be close to a NULL might be interpreted as 0 length by LENGTH...

Nevertheless it's a littlebid confusing for people who don't know the internals of R.

Will try length() soon.

Ciao,

   Oliver

P.S.: To be picky: NOT knowing the internals and nevertheless using the available

      functions/macros is called encapsulation in OO or the difference between
      interface and implementation... which is good style of programming.

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