How to combine elements from a data list in R?

Question

For instance I have a data list (may have many elements but here only 2):

> datalist = list(a = matrix(1:10, 5, 2), b = matrix(11:20, 5, 2))
> datalist
$a
     [,1] [,2]
[1,]    1    6
[2,]    2    7
[3,]    3    8
[4,]    4    9
[5,]    5   10

$b
     [,1] [,2]
[1,]   11   16
[2,]   12   17
[3,]   13   18
[4,]   14   19
[5,]   15   20

I want to combine those two elements a & b into a data frame, say

> dataframe
      [,1] [,2]
 [1,]    1    6
 [2,]    2    7
 [3,]    3    8
 [4,]    4    9
 [5,]    5   10
 [6,]   11   16
 [7,]   12   17
 [8,]   13   18
 [9,]   14   19
[10,]   15   20

That is, there are many elements in a data list. All of them have the same columns. And I want to combine them into a data frame, such as using "rbind()" function in R. How to do it? Thanks!

Answer 1

I think you want do.call(rbind, ...) .

do.call(rbind, datalist)
#      [,1] [,2]
# [1,]    1    6
# [2,]    2    7
# [3,]    3    8
# [4,]    4    9
# [5,]    5   10
# [6,]   11   16
# [7,]   12   17
# [8,]   13   18
# [9,]   14   19
#[10,]   15   20

Update Sept 16, 2015: A more efficient method for large lists is

data.table::rbindlist(lapply(datalist, as.data.frame)) 

Answer 2

Easy peasy with plyr :

library(plyr)

datalist = list(a = matrix(1:10, 5, 2), b = matrix(11:20, 5, 2))

dat <- ldply(datalist)

print(dat)
##    .id  1  2
## 1    a  1  6
## 2    a  2  7
## 3    a  3  8
## 4    a  4  9
## 5    a  5 10
## 6    b 11 16
## 7    b 12 17
## 8    b 13 18
## 9    b 14 19
## 10   b 15 20

Drop the .id column if you want/have to.