How do I make a matrix from a list of vectors in R?

Question

Goal: from a list of vectors of equal length, create a matrix where each vector becomes a row.

Example:

> a <- list()
> for (i in 1:10) a[[i]] <- c(i,1:5)
> a
[[1]]
[1] 1 1 2 3 4 5

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

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

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

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

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

[[7]]
[1] 7 1 2 3 4 5

[[8]]
[1] 8 1 2 3 4 5

[[9]]
[1] 9 1 2 3 4 5

[[10]]
[1] 10  1  2  3  4  5

I want:

      [,1] [,2] [,3] [,4] [,5] [,6]
 [1,]    1    1    2    3    4    5
 [2,]    2    1    2    3    4    5
 [3,]    3    1    2    3    4    5
 [4,]    4    1    2    3    4    5
 [5,]    5    1    2    3    4    5
 [6,]    6    1    2    3    4    5
 [7,]    7    1    2    3    4    5
 [8,]    8    1    2    3    4    5
 [9,]    9    1    2    3    4    5
[10,]   10    1    2    3    4    5

Answer 1

One option is to use do.call():

 > do.call(rbind, a)
      [,1] [,2] [,3] [,4] [,5] [,6]
 [1,]    1    1    2    3    4    5
 [2,]    2    1    2    3    4    5
 [3,]    3    1    2    3    4    5
 [4,]    4    1    2    3    4    5
 [5,]    5    1    2    3    4    5
 [6,]    6    1    2    3    4    5
 [7,]    7    1    2    3    4    5
 [8,]    8    1    2    3    4    5
 [9,]    9    1    2    3    4    5
[10,]   10    1    2    3    4    5

Answer 2

Not straightforward, but it works:

> t(sapply(a, unlist))
      [,1] [,2] [,3] [,4] [,5] [,6]
 [1,]    1    1    2    3    4    5
 [2,]    2    1    2    3    4    5
 [3,]    3    1    2    3    4    5
 [4,]    4    1    2    3    4    5
 [5,]    5    1    2    3    4    5
 [6,]    6    1    2    3    4    5
 [7,]    7    1    2    3    4    5
 [8,]    8    1    2    3    4    5
 [9,]    9    1    2    3    4    5
[10,]   10    1    2    3    4    5

Answer 3

A data frame is defined as a "list of variables of the same number of rows with unique row names". So:

data.frame(a)

is what you want if the vectors are of mixed type. If you need a matrix, as.matrix(data.frame(a)) would work, but the answers by @Paolo and @chris_dubois are nicer.

Answer 4

> library(plyr)
> as.matrix(ldply(a))
      V1 V2 V3 V4 V5 V6
 [1,]  1  1  2  3  4  5
 [2,]  2  1  2  3  4  5
 [3,]  3  1  2  3  4  5
 [4,]  4  1  2  3  4  5
 [5,]  5  1  2  3  4  5
 [6,]  6  1  2  3  4  5
 [7,]  7  1  2  3  4  5
 [8,]  8  1  2  3  4  5
 [9,]  9  1  2  3  4  5
[10,] 10  1  2  3  4  5