How to convert a list into a matrix in R

Question

I do have a list of vectors, like this:

[[1]]
[1]  1  2  7  9 10 13 14 15 20

[[2]]
[1] 3 4 5 6

[[3]]
[1]  8 11 12

[[4]]
[1] 16 17 18 19

[[5]]
[1] 21 22 23

I want to convert the list to a matrix where the contents of each vector are associated with the list index number in double brackets.

Example:

       [,1] [,2]
 [1,]   1    1
 [2,]   1    2
 [3,]   1    7
 [4,]   1    9
 [5,]   1   10
 [6,]   1   13
 [7,]   1   14
 [8,]   1   15
 [9,]   1   20
[10,]   2    3
[11,]   2    4
[12,]   2    5
[13,]   2    6
[14,]   3    8
[15,]   3   11
[16,]   3   12
[17,]   4   16
[18,]   4   17
[19,]   4   18
[20,]   4   19
[21,]   5   21
[22,]   5   22
[23,]   5   23

Answer 1

You can use unlist to get the values in the list, then use sapply to get the number of values in each element of the list.

# Generate the list
a <- list(1:10, 20:30, 40:45)
# Find the number of elements
num.el <- sapply(a, length)
# Generate the matrix
res <- cbind(unlist(a), rep(1:length(a), num.el))