[ALTREP] What is the meaning of the return value of Is_sorted and No_NA function?

Hi Jiefei,

The meanings of the return values for sortedness can be found in
RInternals.h, and are as follows:

/* ALTREP sorting support */
enum {SORTED_DECR_NA_1ST = -2,
      SORTED_DECR = -1,
      UNKNOWN_SORTEDNESS = INT_MIN, /*INT_MIN is NA_INTEGER! */
      SORTED_INCR = 1,
      SORTED_INCR_NA_1ST = 2,
      KNOWN_UNSORTED = 0};

The default value there is NA_INTEGER (ie INT_MIN), indicating that there
is no sortedness information.

Currently, *_NO_NA  effectively return a boolean, (even though the actual
return value is int). This can be seen in the method we provide for compact
sequences in altclasses.c:


static int compact_intseq_No_NA(SEXP x)
{
#ifdef COMPACT_INTSEQ_MUTABLE
    /* If the vector has been expanded it may have been modified. */
    if (COMPACT_SEQ_EXPANDED(x) != R_NilValue)
return FALSE;
#endif
    return TRUE;
}

(FALSE is a macro for 0, TRUE is a macro for 1).

Think of the meaning of the return value to No_NA methods as the object's
answer to the following question

"Are you sure there are zero NAs in your data?"

When it is sure of that, it  says "yes" (returning 1, ie TRUE). When it
either is sure there are NAs *OR* doesn't have any information about
whether there are NAs, it says "no" (returning 0, ie FALSE).

Also please note, it is possible there may be another API point in the
future which asks the object *how many NAs it has.??* If that
materializes, No_NA would just  consume the answer to thatto get the
binarized version, but again there is nothing like that in there now.

Hope that helps.

Best,
~G

On Wed, Sep 11, 2019 at 12:04 AM Wang Jiefei <szwjf08 at gmail.com> wrote:

Hi,



I would like to figure out the meaning of the return value of these two
functions. Here are the default definitions I find from R source code:



static int altreal_Is_sorted_default(SEXP x) { return UNKNOWN_SORTEDNESS;
}

static int altreal_No_NA_default(SEXP x) { return 0; }

I guess the macro *UNKNOWN_SORTEDNESS *in *Is_sorted* and 0 in *No_NA
*simply means
unknown sorted/NA status of the vector, so R will loop over the vector and
find the answer. However, what should we return in these functions to
indicate whether the vector has been sorted/ contains NA? My initial guess
is 0/1 but since *NA_NA *uses 0 as its default value so it will be
ambiguous. Are there any macros to define yes/no return values for these
functions? I would appreciate any thought here.



Best,

Jiefei

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