> Mick Jordan
> on Wed, 4 Jan 2017 08:15:03 -0800 writes:
> On 1/4/17 1:26 AM, Martin Maechler wrote:
>>> Mick Jordan
>>> on Tue, 3 Jan 2017 07:57:15 -0800 writes:
>> > This is a message for someone familiar with the implementation.
>> > Superficially the R code for seq.default and the C code for seq.int
>> > appear to be semantically very similar. My question is whether, in
fact,
>> > it is intended that behave identically for all inputs.
>>
>> Strictly speaking, "no": As usual, RT?Manual (;-)
>>
>> The help page says in the very first paragraph ('Description'):
>>
>> ‘seq’ is a standard generic with a default method.
>> ‘seq.int’ is a primitive which can be much faster but has a few
restrictions.
>>
>> > I have found two cases so far where they differ, first
>> > that seq.int will coerce a character string to a real (via
>> > Rf_asReal) whereas seq.default appears to coerce it to NA
>> > and then throws an error:
>>
>> >> seq.default("2", "5")
>> > Error in seq.default("2", "5") : 'from' cannot be NA, NaN or infinite
>> >> seq.int("2", "5")
>> > [1] 2 3 4 5
>> >>
>>
>> this may be a bit surprising (if one does _not_ look at the code),
>> indeed, notably because seq.int() is mentioned to have more
>> restrictions than seq() which here calls seq.default().
>> "Surprising" also when considering
>>
>> > "2":"5"
>> [1] 2 3 4 5
>>
>> and the documentation of ':' claims 'from:to' to be the same as
>> rep(from,to) apart from the case of factors.
>>
>> --- I am considering a small change in seq.default()
>> which would make it work for this case, compatibly with ":" and
seq.int().
>>
>>
>> > and second, that the error messages for non-numeric arguments differ:
>>
>> which I find fine... if the functions where meant to be
>> identical, we (the R developers) would be silly to have both,
>> notably as the ".int" suffix has emerged as confusing the
>> majority of useRs (who don't read help pages).
>>
>> Rather it has been meant as saying "internal" (including "fast") also
for other
>> such R functions, but the suffix of course is a potential clash
>> with S3 method naming schemes _and_ the fact that 'int' is used
>> as type name for integer in other languages, notably C.
>>
>> > seq.default(to=quote(b), by=2)
>> > Error in is.finite(to) : default method not implemented for type
'symbol'
>>
>> which I find a very appropriate and helpful message
>>
>> > seq.int(to=quote(b), by=2)
>> > Error in seq.int(to = quote(b), by = 2) :
>> > 'to' cannot be NA, NaN or infinite
>>
>> which is true, as well, and there's no "default method" to be
>> mentioned, but you are right that it would be nicer if the
>> message mentioned 'symbol' as well.
> Thanks for the clarifications. It was surprising that seq.int supported
> more types than seq.default. I was expecting the reverse.
exactly, me too!
> BTW, There are a couple of, admittedly odd, cases, exposed by brute
> force testing, where seq.int will actually return "missing", which I
> presume is not intended, and seq.default behaves differently, vis:
>> seq.default(to=1,by=2)
> [1] 1
>> seq.int(to=1,by=2)
>> > x <- seq.int(to=1,by=2)
>> x
> Error: argument "x" is missing, with no default
> Lines 792 and 799 of seq.c return the incoming argument (as opposed to a
> value based on its coercion to double via asReal) and this can, as in
> the above example, be "missing".
> Thanks
> Mick Jordan
Thanks a lot, Mick -- you are right!
I'm fixing these (the line numbers have greatly changed in the
mean time: Remember we work with "R-devel", i.e., the "trunk" :
always available at
https://svn.r-project.org/R/trunk/src/main/seq.c
Martin Maechler
ETH Zurich
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