r - Named List To/From Data.Frame

Question

I'm looking for a quick way to get back and forth between a list of the following format:

$`a`
  [1] 1 2 3
$`b`
  [1] 4 5 6

to/from a data.frame of the following format:

   name x
 1    a 1
 2    a 2
 3    a 3
 4    b 4
 5    b 5
 6    b 6

(Don't really care what the names of the columns are, in this case.)

Here's the data frame used above in R-format:

df <- data.frame(name=c(rep("a",3),rep("b",3)), x=c(1:3,4:6))

Again, I'm looking for two separate operations: one to convert the above data.frame to a list, and another to convert it back to a data.frame.

question from:https://stackoverflow.com/questions/10432993/named-list-to-from-data-frame

Answer 1

Use stack and unstack in base R:

x <- data.frame(a=1:3, b=4:6)

x
  a b
1 1 4
2 2 5
3 3 6

Use stack to from wide to tall, i.e. stack the vectors on top of one another.

y <- stack(x)
y
  values ind
1      1   a
2      2   a
3      3   a
4      4   b
5      5   b
6      6   b

Use unstack to do the reverse.

unstack(y)
  a b
1 1 4
2 2 5
3 3 6

---

If your data structure is more complicated than you described, stack and unstack may no longer be suitable. In that case you'll have to use reshape in base R, or melt and dcast in package reshape2.