Patch to allow negative argument in head() and tail()
Le Mardi 18 Juillet 2006 04:42, Martin Maechler a ?crit?:
"Vincent" == Vincent Goulet <vincent.goulet at act.ulaval.ca>
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.
Indeed, I did not pay that much attention to the second paragraph. For me, this usage of head() and tail() is, at first, completely unintuitive since I more used to, say, "start from the beginning (head) of the vector and drop the first n elements" than "return the end of the vector except the first n elements". But I must agree your convention does make sense!
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.
So I should rephrase the message as "n should be a vector of length 1", then.
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}
Actually, this will no longer be needed since head() will always return, well, the head of an object, hence no need to relabel the lines of a matrix.
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' ?
I'll give it a go in the next couple of days. Bye! Vincent
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>
Vincent Goulet, Associate Professor ?cole d'actuariat Universit? Laval, Qu?bec Vincent.Goulet at act.ulaval.ca http://vgoulet.act.ulaval.ca