Alexy,
note that negative indices have a different meaning in R, namely
they indicate elements to remove. v[-(1:i)] would mean all but the
elements from 1 to i (make sure i is non-negative first). So if you
want:
All elements up to i: v[1:i]
All elements from i: v[-1:(i-1)]
You can actually wrap these in functions likely.
If you know *how many* elements from the end you want, you can use
"tail":
x<-1:10
tail(x,1) # => 10
Haris Skiadas
Department of Mathematics and Computer Science
Hanover College
On Nov 24, 2007, at 7:28 AM, Gabor Csardi wrote:
> Please someone correct me if i'm wrong, but i think this is impossible
> with the current R language. ':' is an ordinary function (see
> get(":")) just like "[", so v[1:3] is actually the composition of two
> functions, it is the same as "["(":"(1,3)). The ":" has no idea about
> whether it'll be embedded into a "[" function of not, it certainly
> cannot know the length of the vector i'll be indexed with.
>
> While this has the drawback you mentioned, it also has some
> advantages, when someone writes something like
>
> lapply(l, "[[", 1)
>
> or
>
> lapply(10:1, ":", 20)
>
> So, I'm afraid you'll have to live with length(v).
>
> Gabor
>
> On Sat, Nov 24, 2007 at 12:49:59PM +0300, Alexy Khrabrov wrote:
>> What are idioms for taking a head or a tail of a vector, either up to
>> an index, or from an index to the end? Also -- is it necessary to
>> use length(v) to refer to the last element? E.g., Python has
>>
>> v[:3] # indices 0,1,2
>> v[3:] # indices 3,4,...
>> v[-1] # the last element of v
>> v[:-1] # all but last
>>
>> Cheers,
>> Alexy
>>
>> ______________________________________________
>> R-help@r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide http://www.R-project.org/posting-
>> guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> --
> Csardi Gabor <[EMAIL PROTECTED]> MTA RMKI, ELTE TTK
>
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