How to convert list to matrix in R

Question

I have a list of vectors, for example:

[[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 a list to a matrix where the contents of each vector are associated with the index number of the list 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, and 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)) 

Answer 2

 library(reshape2) lst <- list(c(1:3), c(11:12), c(22)) > melt(lst) value L1 1 1 1 2 2 1 3 3 1 4 11 2 5 12 2 6 22 3 

Answer 3

If l is your source list:

 cbind(rep(seq_along(l), times=sapply(l, length)), unlist(l))