Restructure the list of lists-CodePudding

Here is an example list of lists of my real data:

df <- c(1, 2, 3)
x <- list(df, df, df)
y <- list(df, df, df)
z <- list(df, df, df)
lista <- list(x, y, z)
listb <- list(x, y, z)
lols <- list(a = lista, b =listb)

lols has a structure like this:

        lols
         |
      ________
     |        |
     a        b
     |        |
   _____    _____
  |  |  |  |  |  |
  x  y  z  x  y  z

I want to restructure lols into the following shape:

          lols
           |
      ___________
     |     |     |  
     x     y     z  
     |     |     |
    ___   ___   ___
   |   | |   | |   |
   a   b a   b a   b

I managed to do it using for loops, but I am not sure if it is correct, and if it works efficiently with very large real data:

newl <- rep(list(list()), length(lols[[1]]))

for (i in seq_along(lols)) {
  for(j in seq_along(lols[[i]])) {
    newl[[j]][[i]] <- lols[[i]][[j]]
  }
}

Is there a faster way to do it, as for loop is believed very slow in R?

In my code the list names are dropped, how can I keep the names?

EDIT: A microbenchmark based on mine and the accepted answer and purrr::transpose() as suggested by the comment

fun1 <- function(ls) {
  newl <- rep(list(list()), length(ls[[1]]))
  for (i in seq_along(ls)) {
    for (j in seq_along(ls[[i]])) {
      newl[[j]][[i]] <- ls[[i]][[j]]
    }
  }
  
  return(newl)
}

fun2 <- function(ls) {
  nm <- el(lapply(ls, names))
  newl <- lapply(nm, \(i) lapply(ls, '[[', i)) |> setNames(nm)
}

fun3 <- function(ls) {
  purrr::transpose(ls)
}

microbenchmark::microbenchmark(fun1(loaded), fun2(loaded), fun3(loaded), times = 1000)

#> Unit: microseconds
#>          expr    min      lq      mean  median      uq     max neval
#>  fun1(loaded) 7631.3 8029.35 8877.8146 8296.65 8946.65 37443.3  1000
#>  fun2(loaded)   66.6   81.60  118.0540  113.75  135.00   923.9  1000
#>  fun3(loaded)    2.9    3.90   16.0451   15.60   27.80    70.7  1000

CodePudding user response：

Better name your list elements, then you may loop over the lists' first layer names.

nm <- el(lapply(lols, names))
newl <- lapply(nm, \(i) lapply(lols, '[[', i)) |> setNames(nm)

Gives

str(newl)
# List of 3
# $ x:List of 2
# ..$ a:List of 3
# .. ..$ k: num [1:3] 1 2 3
# .. ..$ l: num [1:3] 1 2 3
# .. ..$ m: num [1:3] 1 2 3
# ..$ b:List of 3
# .. ..$ k: num [1:3] 1 2 3
# .. ..$ l: num [1:3] 1 2 3
# .. ..$ m: num [1:3] 1 2 3
# $ y:List of 2
# ..$ a:List of 3
# .. ..$ k: num [1:3] 1 2 3
# .. ..$ l: num [1:3] 1 2 3
# .. ..$ m: num [1:3] 1 2 3
# ..$ b:List of 3
# .. ..$ k: num [1:3] 1 2 3
# .. ..$ l: num [1:3] 1 2 3
# .. ..$ m: num [1:3] 1 2 3
# $ z:List of 2
# ..$ a:List of 3
# .. ..$ k: num [1:3] 1 2 3
# .. ..$ l: num [1:3] 1 2 3
# .. ..$ m: num [1:3] 1 2 3
# ..$ b:List of 3
# .. ..$ k: num [1:3] 1 2 3
# .. ..$ l: num [1:3] 1 2 3
# .. ..$ m: num [1:3] 1 2 3

Or also without renaming, try

lapply(1:3, \(i) lapply(lols, '[[', i)) |> setNames(c('x', 'y', 'z'))

Data:

lols <- list(a = list(x = list(k = c(1, 2, 3), l = c(1, 2, 3), m = c(1, 
2, 3)), y = list(k = c(1, 2, 3), l = c(1, 2, 3), m = c(1, 2, 
3)), z = list(k = c(1, 2, 3), l = c(1, 2, 3), m = c(1, 2, 3))), 
    b = list(x = list(k = c(1, 2, 3), l = c(1, 2, 3), m = c(1, 
    2, 3)), y = list(k = c(1, 2, 3), l = c(1, 2, 3), m = c(1, 
    2, 3)), z = list(k = c(1, 2, 3), l = c(1, 2, 3), m = c(1, 
    2, 3))))