[R] how to use list index to get vector

[Luke]

Hi Gabor, thanks a lot. I understand it now. But Andrew's method is
easier for me to understand. Can I extend my question? I have a data
file, every line has such format:
2:102 5:85 ...
The number before colon is data entry index, the number after colon is
data entry value, and other data entries are zero. I use scan to read
them in R and need to separate the index and value to different
vectors. For example:

> foo
[1] "2:102" "5:85"
> foo2 <- strsplit(foo, split=":")
> foo2
[[1]]
[1] "2"   "102"

[[2]]
[1] "5"  "85"

> myIndex <- as.numeric(unlist(lapply(foo2, function(x) x[1])))
or
> myIndex <- as.numeric(foo2, "[", 1)
> myIndex
[1] "2" "5"

Is this the simplest way to get the index or value vector?

-Luke

On 5/16/05, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
> Its the indexing function written in ordinary function form.  That is,
> foo[1:2] can be written as "["(foo, 1:2)
> 
> On 5/16/05, Luke <jyzz88 at gmail.com> wrote:
> > Yes, it works. Althought I can understand the help page of sapply,  I
> > don't know why it works.  What is "["?
> >
> > -Luke
> >
> > On 5/16/05, Gabor Grothendieck <ggrothendieck at gmail.com> wrote:
> > > On 5/16/05, Luke <jyzz88 at gmail.com> wrote:
> > > > I have a simple question, but I couldn't find the answer in R manuals.
> > > >
> > > > Assume I have a list:
> > > > > foo <- list()
> > > > > foo[[1]] <- c(1, 2, 3)
> > > > > foo[[2]] <- c(11,22,33)
> > > > > foo[[3]] <- c(111,222,333)
> > > > > foo
> > > > [[1]]
> > > > [1] 1 2 3
> > > >
> > > > [[2]]
> > > > [1] 11 22 33
> > > >
> > > > [[3]]
> > > > [1] 111 222 333
> > > >
> > > > How to use list index to get a vector of, say, the first elements of
> > > > list elements?
> > > > That is, how to get a vector c(1, 11, 111) from foo?
> > > >
> > > > foo[[]][1] doesn't work.
> > > >
> > >
> > > Try this:
> > >
> > > sapply(foo, "[", 1)
> > >
> >
>