Recursive remove NULL elements of list of lists

Question

I have a named list of lists like this one:

x <- list(a=1, b=2, c=list(ca=1, cb=2, cc=NULL), d=NULL)

I want to remove all NULL elements, including in the sublists. The expected result do not contain x$d and x$c$cc .

I tried purrr::compact(x) but it only removes from the top list. The question and answers from R: removing NULL elements from a list only applies to the top list too.

I tried rapply(x, purrr::compact) but it do not return a list.

Answer 1

One approach is to use rrapply in the rrapply -package (extension of base rapply ):

library(rrapply)

x <- list(a=1, b=2, c=list(ca=1, cb=2, cc=NULL), d=NULL)

rrapply(x, condition = Negate(is.null), how = "prune")
#> $a
#> [1] 1
#> 
#> $b
#> [1] 2
#> 
#> $c
#> $c$ca
#> [1] 1
#> 
#> $c$cb
#> [1] 2

---

Benchmark timings

Benchmarking the computation time of rrapply against rlist's list.clean function for some large nested lists, I get the following results:

## recursively create nested list with dmax layers and 50% NULL elements
f <- function(len, d, dmax) {
  x <- vector(mode = "list", length = len)
  for(i in seq_along(x)) {
    if(d + 1 < dmax) {
      x[[i]] <- Recall(len, d + 1, dmax)
    } else {
      x[[i]] <- list(1, NULL)
    }
  }
  return(x)
}

## long shallow list (3 layers, total 5e5 nodes)
x_long <- f(len = 500, d = 1, dmax = 3)

microbenchmark::microbenchmark(
  rlist = rlist::list.clean(x_long, recursive = TRUE),
  rrapply = rrapply::rrapply(x_long, condition = Negate(is.null), how = "prune"),
  check = "equal",
  times = 5L
)
#> Unit: milliseconds
#>     expr       min        lq      mean    median        uq       max neval
#>    rlist 2331.4914 2343.3001 2438.9674 2441.3850 2512.3484 2566.3121     5
#>  rrapply  353.7169  393.0646  400.8198  399.7971  417.7235  439.7972     5

## deeply nested list (18 layers, total 2^18 nodes)
x_deep <- f(len = 2, d = 1, dmax = 18)

microbenchmark::microbenchmark(
  rlist = rlist::list.clean(x_deep, recursive = TRUE),
  rrapply = rrapply::rrapply(x_deep, condition = Negate(is.null), how = "prune"),
  check = "equal",
  times = 5L
)
#> Unit: milliseconds
#>     expr       min        lq      mean    median        uq       max neval
#>    rlist 2167.2946 2251.5203 2279.9963 2292.5045 2332.4432 2356.2188     5
#>  rrapply  268.9463  274.7437  325.9585  292.4559  354.1607  439.4857     5

Answer 2

You can use rlist::list.clean(x, recursive = TRUE) . If you study the source code, you see how the recursion is implemented with a simple lapply loop.