[Rd] 'is.integer' (PR#13671)
Martin Maechler
maechler at stat.math.ethz.ch
Thu Apr 23 11:22:06 CEST 2009
>>>>> "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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