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

simplify2array is a base function that is fairly intuitive. However, since R's default is to fill in data by columns first, you will need to transpose the output. (sapply uses simplify2array, as documented in help(sapply).)

> t(simplify2array(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 3

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 4

The built-in matrix function has the nice option to enter data byrow. Combine that with an unlist on your source list will give you a matrix. We also need to specify the number of rows so it can break up the unlisted data. That is:

> matrix(unlist(a), byrow=TRUE, nrow=length(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 5

t(sapply(a, '[', 1:max(sapply(a, length))))

where 'a' is a list. Would work for unequal row size

Answer 6

> 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