Hi Peng,
If I undertood your point, try this:
x<-runif(10)
y<-runif(10)
z<-runif(10)
w<-runif(10)
myDF<-data.frame(cbind(x,y,z,w))
myDF
myDF[,c("w","z")]
Happy new year
miltinho
On Thu, Dec 31, 2009 at 5:15 PM, Peng Yu <pengyu...@gmail.com> wrote:
> I don't see where describes the implementation of '[]'.
>
> For example, if x is a matrix or a data.frame, how the lookup of
> 'colname1' is x[, 'colname1'] executed. Does R perform a lookup in the
> a hash of the colnames? Is the reference O(1) or O(n), where n is the
> second dim of x?
>
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