[R] [:]

[Charilaos Skiadas]

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
>>
>> ______________________________________________
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>> PLEASE do read the posting guide http://www.R-project.org/posting- 
>> guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>
> -- 
> Csardi Gabor <csardi at rmki.kfki.hu>    MTA RMKI, ELTE TTK
>