[R] [:]

[Gabor Csardi]

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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-- 
Csardi Gabor <csardi at rmki.kfki.hu>    MTA RMKI, ELTE TTK