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

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