Combining vectors in a subset of list of lists - r

Question

I'd like to get a matrix mat out by combining vectors in a subset of list of lists. Following the way to do the same using a for loop. I am wondering if there is a faster way to do it.
i <- 1 # the subset
mat<- matrix(NA, ncol = p, nrow = n)
for (j in 1 : p) {
mat[, j] <- list_of_list[[j]][[i]]$the_vector
}
EDIT:
I am after the vectors indexed/subseted by 'i' at any given time. Also, the list_of_list has objects other than the_vector as well.
EDIT 2:
Adding a working example below.
lst <- list()
list_of_list <- list()
lst[[1]] <- list(a="a", c="b1", the_vector = 1:5)
lst[[2]] <- list(a="b", c="b2", the_vector = 1:5+1)
lst[[3]] <- list(a="c", c="b3", the_vector = 1:5+2)
list_of_list[[1]] <- lst
lst[[1]] <- list(a="a", c="b1", the_vector = 1:5*0)
lst[[2]] <- list(a="b", c="b2", the_vector = 1:5*1)
lst[[3]] <- list(a="c", c="b3", the_vector = 1:5*22)
list_of_list[[2]] <- lst
i <- 1 # the subset
p <- 2 # length of the list of list
n <- 5 # length of the vector
mat<- matrix(NA, ncol = p, nrow = n)
for (j in 1 : p) {
mat[, j] <- list_of_list[[j]][[i]]$the_vector
}

Answer 1

You may try the sapply() function:
i <- 1L
mat <- sapply(list_of_list, function(.x) .x[[i]]$the_vector)
mat
[,1] [,2]
[1,] 1 0
[2,] 2 0
[3,] 3 0
[4,] 4 0
[5,] 5 0
I have not benchmarked the code to make sure this is faster in terms of execution speed but it definitely requires fewer key strokes.
sapply() applies a function over a list or vector and is a kind of implied for loop.

Answer 2

You can just unlist your list then reshape it as a matrix:
matrix(unlist(list(list(1,2,3,4),list(5,6,7,8),list(9,10,11,12))), nrow=3, byrow = T)
[,1] [,2] [,3] [,4]
[1,] 1 2 3 4
[2,] 5 6 7 8
[3,] 9 10 11 12

Answer 3

I am not sure if you are looking for something like this. It will give you a list of 3 matrices corresponding to vector from list_of_list's child lists.
mapply(list_of_list[[1]],list_of_list[[2]],
FUN = function(x,y){t(mapply(x$the_vector,y$the_vector,
FUN = function(u,v){matrix(c(u,v),ncol=2,byrow = F,dimnames = NULL)},
SIMPLIFY = T))},SIMPLIFY = F)
#[[1]]
# [,1] [,2]
#[1,] 1 0
#[2,] 2 0
#[3,] 3 0
#[4,] 4 0
#[5,] 5 0
#[[2]]
# [,1] [,2]
#[1,] 2 1
#[2,] 3 2
#[3,] 4 3
#[4,] 5 4
#[5,] 6 5
#[[3]]
# [,1] [,2]
#[1,] 3 22
#[2,] 4 44
#[3,] 5 66
#[4,] 6 88
#[5,] 7 110

Answer 4

Here is another solution really similar to #TUSHAr but that might maybe more modular:
## Lapply wrapping function that outputs a matrix
lapply.wrapper <- function(i, list_of_list) {
matrix(unlist(lapply(list_of_list, function(X, i) X[[i]]$the_vector, i = i)), ncol = length(list_of_list))
}
## Using the wrapper on the first subset:
lapply.wrapper(1, list_of_list)
# [,1] [,2]
#[1,] 1 0
#[2,] 2 0
#[3,] 3 0
#[4,] 4 0
#[5,] 5 0
## Applying the function to all subsets
sapply(1:length(list_of_list[[1]]), lapply.wrapper, list_of_list, simplify = FALSE)
#[[1]]
# [,1] [,2]
#[1,] 1 0
#[2,] 2 0
#[3,] 3 0
#[4,] 4 0
#[5,] 5 0
#
#[[2]]
# [,1] [,2]
#[1,] 2 1
#[2,] 3 2
#[3,] 4 3
#[4,] 5 4
#[5,] 6 5
#
#[[3]]
# [,1] [,2]
#[1,] 3 22
#[2,] 4 44
#[3,] 5 66
#[4,] 6 88
#[5,] 7 110