# [R] How to create a matrix from a list without a for loop

Laurent Rhelp L@urentRHe|p @end|ng |rom |ree@|r
Fri Jul 9 19:40:08 CEST 2021

```Dear R-Help-list,

I have a list init_l containing 16 dataframes and I want to create a
matrix 4 x 4 from this list with a dataframe in every cell of the
matrix. I succeeded to do that but my loop is very uggly (cf. below).
Could somebody help me to write nice R code to do this loop ?

Thank you very much

Laurent

##
## mock data, 16 dataframes in a list
##
init_l <- lapply( seq(1,16) , function(x) {
data.frame( V1 = rnorm(3),
V2 = rnorm(3),
V3 = rnorm(3)
)
})

##
## lists matrix creation with n = 4 columns and n = 4 rows
##
n <- 4
## an example of row to create the matrix with lists in the cells
one_row <- rbind( rep( list(rep(list(1),3)) , n) )
mymat <- do.call( "rbind" , rep( list(one_row) , n) )

##
## The UGGLY loop I would like to improve:
##

## populate the matrix
k <- 1
for( i in 1:n){
for( j in 1:n){
mymat[i,j][[1]] <- list( init_l[[ k ]] )
k <- k+1
}
}

colnames(mymat) <- c("X1", "X2", "X3", "X3")
rownames(mymat) <- c("X1", "X2", "X3", "X4")

mymat

# X1     X2     X3     X3
# X1 List,1 List,1 List,1 List,1
# X2 List,1 List,1 List,1 List,1
# X3 List,1 List,1 List,1 List,1
# X4 List,1 List,1 List,1 List,1

#
# verification, it works
#
mymat[2,2]
init_l[[6]]

##
init_l[[6]]

library(tidyverse)
mymat.t <- as.tibble(mymat)
mymat.t
unnest(mymat.t[2,2],cols="X2")[[1]][[1]]

mymat.df <- as.data.frame(mymat)
mymat.df[2,2][[1]][[1]]

thx

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```