How to Make a Matrix from a List of Vectors in R

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

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

Efficiently create a matrix from list of named vectors

You can try rbindlist function from the data.table package. See the example.

require(data.table)
require(dplyr)

all_names <- unique(do.call(paste0, replicate(5, sample(LETTERS, 1e4, TRUE), FALSE)))
anyDuplicated(all_names)
n <- 5e3

vec_a <- rnorm(n)
names(vec_a) <- sample(all_names, n)

vec_b <- rnorm(n)
names(vec_b) <- sample(all_names, n)

length(union(names(vec_a), names(vec_b)))

system.time(tmp1 <- bind_rows(vec_a, vec_b))

system.time(tmp2 <- rbindlist(list(as.list(vec_a), as.list(vec_b)), fill = T))

all.equal(tmp1, tmp2)

The output:

> system.time(tmp1 <- bind_rows(vec_a, vec_b))
   user  system elapsed 
   3.20    0.00    3.21 

> system.time(tmp2 <- rbindlist(list(as.list(vec_a), as.list(vec_b)), fill = T))
   user  system elapsed 
   0.01    0.00    0.01 

> all.equal(tmp1, tmp2)
[1] TRUE

Make a list of 2x2 matrices from a list of vectors

How about this? Here, x is taking the place of your SC_sum.

x <- split(seq_len(16), gl(4, 4))
x <- lapply(x, `names<-`, letters[1:4])
x

$`1`
a b c d 
1 2 3 4 

$`2`
a b c d 
5 6 7 8 

$`3`
 a  b  c  d 
 9 10 11 12 

$`4`
 a  b  c  d 
13 14 15 16

m <- matrix(unlist(x), nrow = length(x[[1]]), ncol = length(x), 
            dimnames = list(names(x[[1]]), NULL))
m

  [,1] [,2] [,3] [,4]
a    1    5    9   13
b    2    6   10   14
c    3    7   11   15
d    4    8   12   16

l <- apply(m, 1, matrix, ncol = 2, simplify = FALSE)
l

$a
     [,1] [,2]
[1,]    1    9
[2,]    5   13

$b
     [,1] [,2]
[1,]    2   10
[2,]    6   14

$c
     [,1] [,2]
[1,]    3   11
[2,]    7   15

$d
     [,1] [,2]
[1,]    4   12
[2,]    8   16

If you really want to use lapply and not apply, then you can do this:

index <- seq_along(x[[1]])
names(index) <- names(x[[1]])
l <- lapply(index, function(i) matrix(vapply(x, `[[`, 0, i), ncol = 2))

But since your data are fundamentally rectangular, constructing and operating on a matrix is (I think) much more intuitive.

Convert list of vectors into matrix

Have a look at ?do.call and ?cbind, e.g:

## create an example list with 3 vectors
l <- list(c(1:3), c(4:6), c(7:9))

## columnwise binding of all vectors in the list `l`
do.call(cbind, l)

Or you could use a simple for loop:

for (i in seq(along=l)) {
   n <- length(l[[i]])
   b[seq(n), i] <- l[[i]]
}

How to create a matrix of lists in R?

This builds that matrix although the print method does not display it in the manner you imagined:

 matrix( list(c(1,2,4), c(NULL), c(1,2), c(3,4,5,6), c(1), c(1,3)), 2,3)
 #---------
     [,1]      [,2]      [,3]     
[1,] Numeric,3 Numeric,2 1        
[2,] NULL      Numeric,4 Numeric,2

Inspect the first element:

> Mlist <- matrix( list(c(1,2,4), c(NULL), c(1,2), c(3,4,5,6), c(1), c(1,3)), 2,3)
> Mlist[1,1]
[[1]]
[1] 1 2 4

> is.matrix(Mlist)
[1] TRUE
> class( Mlist[1,1] )
[1] "list"

Demonstration of creating "matrix of lists" from a list:

> will.become.a.matrix <- list(c(1,2,4), c(NULL), c(1,2), c(3,4,5,6), c(1), c(1,3))
> is.matrix(will.become.a.matrix)
[1] FALSE
> dim(will.become.a.matrix) <- c(2,3)
> is.matrix(will.become.a.matrix)
[1] TRUE
> dim(will.become.a.matrix)
[1] 2 3
> class(will.become.a.matrix[1,1])
[1] "list"

Further requested demonstration:

 A<- list(); F=list() E=list()
 A[1]<-c(3) ;  F[[1]]<-numeric(0);  E[[1]]<-numeric(0)
 A[2]<-c(1) ;  F[2]<-c(1)   ;        E[2]<-c(1)
 A[3]<-c(1) ;  F[3]<-c(2)  ;         E[[3]]<-numeric(0)
 A[[4]]<-list(1,3) ;F[[4]]<-numeric(0) ; E[[4]]<-numeric(0)
 A[5]<-c(4) ; F[5]<-c(4)       ;    E[5]<-c(4)
 Mlist= c(A,F,E)
 M <- matrix(Mlist, length(A), 3)
#=====================================
> M
     [,1]   [,2]      [,3]     
[1,] 3      Numeric,0 Numeric,0
[2,] 1      1         1        
[3,] 1      2         Numeric,0
[4,] List,2 Numeric,0 Numeric,0
[5,] 4      4         4

You asked (in comments) "....is there a way to define number of column and rows , but not the element itself because they are unknown?"

Answered (initially in comments)

b<-matrix(rep(list(), 6),nrow = 2, ncol =3) 
#.... then replace the NULL items with values. 
# Need to use "[[": for assignment (which your 'Update 1' did not 
# ....and your Update2 only did for some but not all of the assignments.)

b[[1]] <- c(1,2,3,4)

Convert a matrix to a list of column-vectors

In the interests of skinning the cat, treat the array as a vector as if it had no dim attribute:

 split(x, rep(1:ncol(x), each = nrow(x)))

How to extract a vector from a list of vectors via matrix in R

You could use paste:

X[paste0("x", newX[newX != 0])]

$`x4`
[1] 5 6 7

$x7
[1] 3 4 5 6

$x9
[1] 44 56  7

$x10
[1] 34  5  4

paste0("x", newX[newX != 0] will create the character vector "x4", "x7", "x9", "x10" which you can use for indexing the list.