how to use list index to get vector

From: 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)