[Rcpp-devel] Missing values

[Hadley Wickham]

On Fri, Nov 16, 2012 at 2:25 AM, Romain Francois
<romain at r-enthusiasts.com> wrote:
> Thanks for exploring these issue. This looks very useful.
>
> I get:
>
>> str( first_log(NA) )
>  logi TRUE
>> str( first_int(NA_integer_) )
>  int NA
>> str( first_num(NA_real_) )
>  num NA
>> str( first_char(NA_character_) )
>  chr "NA"
>
>
> For first_log: a bool can either be true or false. In R logical vectors are
> represented as arrays of ints. When we coerce to bool, we test whether the
> value is not 0. This works for most cases. I guess conversion to bool should
> be avoided.

There's obviously no perfect solution, and that seems to be consistent
with C++'s treatment of NAs in logical expression: evalCpp("NAN ||
FALSE") == TRUE

> We have the is_na template function that can help:
>
>> evalCpp( 'traits::is_na<LGLSXP>( NA_LOGICAL )' )
> [1] TRUE
>> evalCpp( 'traits::is_na<REALSXP>( NA_REAL )' )
> [1] TRUE
>
> And from this I can see we don't have is_na<STRSXP>, will fix this.
>
>> str( evalCpp( 'traits::get_na<REALSXP>()' ) )
>  num NA
>> str( evalCpp( 'traits::get_na<INTSXP>()' ) )
>  int NA
>
> I guess we could come up with a nicer syntax for these, maybe static
> functions in Vector<> so that we could do :
>
> IntegerVector::is_na( )
> NumericVector::get_na( )

Yes, that would be very nice.
>
>> str( evalCpp( 'std::numeric_limits<int>::min()' ) )
>  int NA
>
> This is how NA_integer_ is represented.

Thanks - the danger is that you might do something like:

evalCpp( 'traits::get_na<INTSXP>() + 1' )

I'll make a note.

> As said above, I'll add
>
> ...Vector::is_na
> ...Vector::get_na
>
> to have something more consistent and not as cryptic as traits::is_na<...>(
> ). People should not need to know what REALSXP, INTSXP, LGLSXP, ... mean.

Great, thanks!

Hadley

-- 
RStudio / Rice University
http://had.co.nz/