[Rd] .Call and data frames

[Hin-Tak Leung]

Hin-Tak Leung wrote:
> I think you want
>         else if (TYPEOF(colData) == STRSXP)
> ... instead.
> 
> I don't know if this will convert from factors to string's,
> but somewhere it probably involves something like this:
>      PROTECT(colData = coerceVector(colData, STRSXP));

FWIW, a factor consists of all these things internally:
(1) TYPEOF(colData)  is INTSXP
(2) attr(colData, "levels") exists and is a STRSXP type
(string representation for the levels).
(3) class(colData) = "factor"

if coerVector() doesn't do it, you can test for (3) in your C code,
and use the integer vector in (1) to index into the string vector in (2)
to regenerate the string vector manually.

Not all of this is correct, just an idea:

class = getAttrib(colData, R_ClassSymbol);
...
if (.../* do some test on class */...)
{
     levels = getAttrib(colData, "levels");
     PROTECT(back_to_str = allocVector(STRSXP, LENGTH(colData));
     for(int i = 0; i < LENGTH=(colData) ; i++)
     {
       SET_STRING_ELT(back_to_str, i,
           mkChar(STRING_ELT(levels, INTEGER(colData)[i])));
     }
}

> 
> Dominick Samperi wrote:
>> Hello,
>>
>> I'm trying to fetch a data frame through the C API,
>> and have no problem doing this when all columns
>> are numbers, but when there is a column of
>> strings I have a problem. On the C-side the
>> function looks like:
>> SEXP myfunc(SEXP df),
>> and it is called with a dataframe from
>> the R side with:
>>
>> .Call("myfunc", somedataframe)
>>
>> On the C side (actually C++ side) I use code
>> like this:
>> SEXP colnames = getAttrib(df, R_NamesSymbol)
>> cname  = string(CHAR(STRING_ELT(colnames,i))
>> SEXP coldata = VECTOR_ELT(df,i) (data for i-th column)
>> if(isReal(colData))
>>     x = REAL(colData)[j];
>> else if(isInteger(colData))
>>     i = INTEGER(colData)[j];
>> else if(isString(colData))
>>     s = CHAR(STRING_ELT(colData,j))
>>
>> The problem is that the last test (isString) never passes,
>> even when I pass in a frame for which one or more cols
>> contain character strings. When the column contains
>> strings the isVector(colData) test passes, but no matter
>> how I try to fetch the string data I get a seg fault. That
>> is, forcing CHAR(STRING_ELT(colData,j)) will
>> fault, and so will VECTOR_ELT(colData,0), even
>> though colData passes the isVector test.
>>
>> Any ideas?
>> Thanks,
>> ds
>>
>> ______________________________________________
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>> https://stat.ethz.ch/mailman/listinfo/r-devel
> 
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