Reshape matrix into a list of lists

Question

I have a list as follows:

 id | value
 ----------
  4     600
  4     899
  7      19
 13    4930
 13     300
  :       :

There are multiple ID

Answer 1

I'd just split() the data:

d <- read.table(text = "id  value
  4     600
  4     899
  7      19
 13    4930
 13     300", header=T)

split(d$value, d$id)
$`4`
[1] 600 899

$`7`
[1] 19

$`13`
[1] 4930  300

Answer 2

You could also use tapply if you want to stick with base functions:

tapply(dat$value,dat$id,c)
$`4`
[1] 600 899

$`7`
[1] 19

$`13`
[1] 4930  300

Edit:

For your edited problem, I would go with split and lapply:

x <- lapply(split(dat[2:3],dat$id),c,use.names=F)

dput(x)
structure(list(`4` = list(c(600, 899), c("a", "b")), `7` = list(
19, "d"), `13` = list(c(4930, 300), c("e", "a"))), .Names = c("4", "7", "13"))

Answer 3

The functions in package plyr should be of help here.

In the following example I assume your data is in the form of a data.frame - even if it really is a list, as you say, it should be straight-forward to convert to a data.frame:

dat <-   data.frame(
    id = c(4, 4, 7, 13, 13),
    value = c(600, 899, 19, 4930, 300)
)

library(plyr)
dlply(dat, .(id), function(x)x$value)

The result is a list as you specified:

$`4`
[1] 600 899

$`7`
[1] 19

$`13`
[1] 4930  300

attr(,"split_type")
[1] "data.frame"
attr(,"split_labels")
  id
1  4
2  7
3 13