Question

I have a list as follows:

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

There are multiple ID repeats, and each one has a unique value. I want to turn this into something as follows:

id |  list
----------
 4    c(600, 899)
 7    c(19)
13    c(4930, 300)
 :    :

Is there a vectorized method of accomplishing this?

---

EDIT: Extending the first question, is there a simple way to do the same thing for a generic MxN matrix? I.e., turning this:

 id | value1  value2
 -------------------
  4     600        a
  4     899        b
  7      19        d
 13    4930        e
 13     300        a
  :       :        :

into this:

id |  list
----------
 4    list(c(600, 899),c('a','b'))
 7    list(c(19),c('b'))
13    list(c(4930, 300),c('e','a'))
 :    :

Thanks!

Answer 1

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"))