>>>>> "Vincent" == Vincent Goulet <[EMAIL PROTECTED]>
>>>>> on Mon, 17 Jul 2006 15:03:34 -0400 writes:
Vincent> Dear developeRs (and other abuseRs ;-),
Vincent> I would like to contribute a patch against
Vincent> functions head() and tail() of package utils to
Vincent> allow for a negative 'n' argument. This allows to
Vincent> extract all but the first/last 'n'
Vincent> elements/rows/lines of an object, similar to the
Vincent> "drop" operator of APL. [1]
Hmm, if you reread Bill Venables proposal (URL below), you did
something different : In Bill's (and my!) "book",
head would always give the *first* few entries and
tail would always give the *last* few entries.
That's different from APL's drop, but for a good reason:
The words 'head' and 'tail' exactly suggest so.
Vincent> I put the patched head.R and head.Rd files, along with diff files
in
Vincent> http://vgoulet.act.ulaval.ca/pub/R/
Vincent> The differences were obtained against today's version of r-devel
(more
Vincent> specifically revision 30277 of head.R and revision 30915 of
head.Rd).
That's good (to take the "current" sources for the diffs).
Vincent> Some comments:
Vincent> - The current version of head() and tail() will
Vincent> accept a vector of length > 1 for argument 'n' but
Vincent> will silently use the smallest value. This became
Vincent> awkward to reproduce in my versions and did not
Vincent> seem interesting anyway. Instead, I added an error
Vincent> message if length(n) > 1.
that's ok in my view
Vincent> - I used the word "scalar" in the aforementioned
Vincent> error message to mean a vector of length 1. Perhaps
Vincent> is this not the correct R terminology?
indeed, it's rarely used in R terminology; for one reason
because S (and hence R) does not differentiate between length-1
vectors and scalars the way APL does.
Vincent> - I added a 'addrownums = TRUE' argument to head() used when n <
0, similar to
Vincent> tail() with n > 0. This required to write separate methods for
Vincent> classes 'data.frame' and 'matrix'.
seems not unreasonable {I did not yet look at your implementation there}
Vincent> - The 'function' methods are not modified.
Vincent> - In the man page, the 'function' method was not documented in the
usage
Vincent> section. Done now.
ok, though not necessary: The recommended approach is to only
document methods when they have ``surprising arguments'', i.e.,
arguments not in the generic function.
In our case, 'n = 6' is not part of the generic, so strictly
speaking *is* a "surprising argument".
Probably it was not made part of the generic, since it's
imaginable to have objects whose "head" is always of a fixed
given size, and where specifying 'n' does not make sense.
Vincent> - I don't think the patch would break any existing code, except
code using the
Vincent> (undocumented) "feature" mentioned in my first remark, above.
Vincent> I hope you will find my (albeit small) contribution useful.
Yes, but can you change it such that head() does give the "head"
and tail() the "tail" also for negative 'n' ?
Vincent> Best regards,
Bonnes salutations,
Martin
Vincent> -----
Vincent> [1] See previous messages to r-devel by myself
Vincent> <https://stat.ethz.ch/pipermail/r-devel/2005-April/032881.html>
Vincent> and Bill Venables
Vincent> <https://stat.ethz.ch/pipermail/r-devel/2005-May/033081.html>
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