[Rd] Negative argument for head() and tail()
Charles C. Berry
cberry at tajo.ucsd.edu
Thu Apr 21 00:49:26 CEST 2005
I'd like to second Vincent's appeal.
Before 'head' and 'tail' were added to package:utils, I had
versions that allowed negative subscripts.
And I found the code rendered by them readable and less subject to
mental or typographical errors.
I think all that is needed for the versions in package::utils is:
if (n < 0 ) n <- length(x) + n
if (n < 0 ) n <- nrow(x) + n
as the first line of the body of each method except head.function.
On Wed, 20 Apr 2005, Vincent Goulet wrote:
> Le 20 Avril 2005 16:05, A.J. Rossini a écrit :
>> x[-length(x)] ?
> I feel idiot from that one, but hold on to my point. ;-)
> Better example, then: how about a compact way to drop, say, the last 3
> elements of a vector? I think
>> tail(x, -3)
> would be nicer --- and more readable --- than
>> x[0:3 -length(x)]
> or some other convoluted way to achieve the same result.
> (BTW, dropping the last element of a vector would be 'tail(x, -1)., not
> 'tail(x, -length(x))' as stated in my original post. The latter would drop
> all the elements.)
>> On 4/20/05, Vincent Goulet <vincent.goulet at act.ulaval.ca> wrote:
>>> Dear R developers,
>>> I'm a former APL programmer. In that language, the "take" (up arrow) and
>>> "drop" (down arrow) operators were extensively used to, well, take and
>>> drop elements of vectors. Functions head() and tail() are equivalents in
>>> R for the "take" operator, but nothing seems to mimic the "drop"
>>> operator. I think it would be useful.
>>> For example, is there any simpler way to extract all elements of a vector
>>> but the last one than doing
>>>> x[1:(length(x) - 1)] ?
>>> An equivalent of "drop" would make this easy.
>>> Now, I think this could be easily implemented with the existing functions
>>> by allowing negative arguments to head() and tail(). For example,
>>>> head(x, -1)
>>> would drop the first element of a vector and
>>>> tail(x, -length(x))
>>> the last one.
>>> Is there any interest for such an extension?
>>> I would be very willing to contribute a patch, although I don't have much
>>> experience in doing so.
>>> Best regards,
>>> Vincent Goulet, Associate Professor
>>> École d'actuariat
>>> Université Laval, Québec
>>> Vincent.Goulet at act.ulaval.ca http://vgoulet.act.ulaval.ca
>>> R-devel at stat.math.ethz.ch mailing list
> Vincent Goulet, Professeur agrégé
> École d'actuariat
> Université Laval, Québec
> Vincent.Goulet at act.ulaval.ca http://vgoulet.act.ulaval.ca
> R-devel at stat.math.ethz.ch mailing list
Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu UC San Diego
http://biostat.ucsd.edu/~cberry/ La Jolla, San Diego 92093-0717
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