[Rd] Using lengthgets() correctly (was SET_LENGTH)

[Prof Brian Ripley]

I assume this is not true of your actual example, but the sample code
allocates a character matrix and calls INTEGER() on it.  Ouch!

On Sun, 17 Apr 2005, Prof Brian Ripley wrote:

> If you use R not the S-compatibility macros you will be less likely to 
> confuse yourself.  The definition is (analogous to realloc)
>
> Rdefines.h:#define SET_LENGTH(x, n)  (x = lengthgets(x, n))
>
> so x is changed and needs to be reprotected.  [I don't know that
> SET_LENGTH *is* part of the API: it is not defined in `Writing R 
> Extensions'.]
>
> Your error will potentially occur at the first garbage collection.  Try using 
> REPROTECT.
>
> Note though that what you are doing is very inefficient, and re-setting the 
> length without changing the dim is also an error.  What R's internal examples 
> do is to allocate a modest vector, double in size when needed, call 
> lengthgets to shorten at the end, then set the dim attribute.
>
> On Sat, 16 Apr 2005, Larry Tordsen wrote:
>
>> Hello.
>> 
>> I'm trying to read a delimited file.  I'm a little new
>> to the R api.  In the program, I do something like
>> this...
>> 
>> /*****************************/
>> SEXP retval;
>> PROTECT(retval = allocMatrix(STRSXP,1,names.dim));
>> int r_eltctr = 0;
>> while(!myfile.eof())
>> {
>>  SET_LENGTH(retval,r_eltctr+num_of_fields);
>>  for(int i = 0;numofcolumns; ++i)
>>    {
>>      INTEGER(retval)[r_eltctr++] = 1;/*some
>> value...*/
>>    }
>>  ++r_eltctr;
>> }
>> UNPROTECT(1);
>> /***************************************/
>> 
>> There is a segmentation fault when SET_LENGTH() is
>> called several iterations into the program.  It seems
>> like it should not be a memory issue, unless a
>> constraint is set by R.  The vector only has about
>> 3500 elements at the time it crashes.
>> 
>> Does anyone have any ideas what the problem could be?
>
> The problem seems to be the assumption that R (and not the user) was at 
> fault.
>
> -- 
> 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
>
> ______________________________________________
> R-devel at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>

-- 
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