[Rd] 'is.integer' (PR#13671)

[Martin Maechler]

>>>>> "SM" == Stavros Macrakis <macrakis at alum.mit.edu>
>>>>>     on Wed, 22 Apr 2009 16:30:36 -0400 writes:

    SM> Dear R experts,
    SM> You are being a bit harsh on this user. 

No! (see below)

    SM> He simply doesn't understand
    SM> the distinction between "object of type integer" and "integer-valued
    SM> object", which is actually fairly subtle.

yes, probably for the vast majority of today's R users.

*However*, Mauricio submitted a *formal* bug report against R
and there are many caveats against doing that "light-heartedly".
Note that he also said

 >> I would really appreciate if you could clarify if this is
 >> really a bug or not. 

and that this is exactly one of situation where one should post
a question to R-help (or maybe R-devel) but *NOT* submit a
formal bug report.

Regards,
Martin


    SM> On Wed, Apr 22, 2009 at 1:45 PM,  <hzambran.newsgroups at gmail.com> wrote:
    >> This is a very simple function that seems not to be working, according to the
    >> definition given by '?is.integer'.
    SM> ...
    >>> is.integer(1)
    >> [1] FALSE
    >> 
    >> and 1 is obviously an integer value.

    SM> The is.integer function is correctly documented to check whether
    SM> objects are of *type* integer, thus:

    SM> is.integer( 1L ) => TRUE

    SM> In R, objects of type integer are only created with literals of the
    SM> form 999L; as the output of some functions when the input is integral
    SM> (e.g. sort, unique, rev, ...); as the output of some functions which
    SM> return index values or differences of index values (which, grep, rle,
    SM> ...); and the output of a few other functions in certain cases (seq).

    SM> Most numbers in R are floating-point numbers (type double), and
    SM> determining whether their value is integral is rather subtle.

    SM> For example, consider the vector 1+1000^-(1:6).  In floating-point
    SM> arithmetic, the first 5 values are distinguishable from the integer 1,
    SM> but the 6th is not, though of course the *mathematical* number
    SM> 1+1000^-6 is not integral.  Now consider 1e40, which has the property
    SM> that floor(x)==x==ceiling(x), which you might think characterizes an
    SM> integer;  but it also has the property that x+1 == x.  Similarly for
    SM> 1/3 * 1e40.

    SM> In other words, it is really a rather subtle question whether a
    SM> floating-point number "represents" an integer....

    SM> -s

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