On Mon, Sep 21, 2015 at 11:20 AM, Simon Urbanek
<simon.urba...@r-project.org> wrote:
>
> On Sep 20, 2015, at 3:06 PM, Henrik Bengtsson <henrik.bengts...@ucsf.edu>
> wrote:
>
>> Is there a missing value constant defined for R_xlen_t, cf. NA_INTEGER
>> (== R_NaInt == INT_MIN) for int(eger)? If not, is it correct to
>> assume that missing values should be taken care/tested for before
>> coercing from int or double?
>>
>
> R_xlen_t is type of the vector length (see XLENGTH()) and as such never holds
> a missing value (since there is no such thing as a missing length). It is
> *not* a native type for R vectors and therefore there is no official
> representation of NAs in R_xlen_t.
>
> Although native R vectors can be used as indices, the way it typically works
> is that the code first checks for NAs in the R vector and only then converts
> to R_xlen_t, so the NA value is never stored in R_xlen_t even for indexing.
>
> --- cut here, content below is less relevant ---
>
> That said, when converting packages from "legacy" .Call code before long
> vector support which used asInteger() to convert an index I tend to use this
> utility for convenience:
>
> static R_INLINE R_xlen_t asLength(SEXP x, R_xlen_t NA) {
> double d;
> if (TYPEOF(x) == INTSXP && LENGTH(x) > 0) {
> int res = INTEGER(x)[0];
> return (res == NA_INTEGER) ? NA : ((R_xlen_t) res);
> }
> d = asReal(x);
> return (R_finite(d)) ? ((R_xlen_t) d) : NA;
> }
>
> Note that this explicitly allows the caller to specify NA representation
> since it depends on the use - often it's simply 0, other times -1 will do
> since typically anything negative is equally bad. As noted above, this is not
> what R itself does, so it's more of a convenience to simplify conversion of
> legacy code.
Thank you Simon,
all this helped clarify it for me. It's in line with what I
suspected, but it is really useful to hear it from the "officials".
Cheers,
Henrik
>
> Cheers,
> Simon
>
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