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

Bill Dunlap w||||@mwdun|@p @end|ng |rom gm@||@com
Fri Jul 9 19:50:01 CEST 2021

```Try
matrix(init_l, nrow=4, ncol=4,
dimnames=list(c("X1","X2","X3","X4"),c("X1","X2","X3","X4")))
It doesn't give exactly what your code does, but your code introduces an
extra level of "list", which you may not want.

-Bill

On Fri, Jul 9, 2021 at 10:40 AM Laurent Rhelp <LaurentRHelp using free.fr> wrote:

> 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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