How to manipulate NULL elements in a nested list?

Question

I have a nested list containing NULL elements, and I\'d like to replace those with something else. For example:

l <- list(
  NULL,
  1,
  list(
    2,
           


        

Answer 1

This can also be done with rrapply() in the rrapply-package. Below are a few different ways we could replace the NULL elements in a nested list by NA values:

library(rrapply)

l <- list(
    NULL,
    1,
    list(
        2,
        NULL,
        list(
            3,
            NULL
        )
    )
)

## replace NULL by NA using only f
rrapply(l, f = function(x) if(is.null(x)) NA else x, how = "replace")
#> [[1]]
#> [1] NA
#> 
#> [[2]]
#> [1] 1
#> 
#> [[3]]
#> [[3]][[1]]
#> [1] 2
#> 
#> [[3]][[2]]
#> [1] NA
#> 
#> [[3]][[3]]
#> [[3]][[3]][[1]]
#> [1] 3
#> 
#> [[3]][[3]][[2]]
#> [1] NA

## replace NULL by NA using condition argument
rrapply(l, condition = is.null, f = function(x) NA, how = "replace")
#> [[1]]
#> [1] NA
#> 
#> [[2]]
#> [1] 1
#> 
#> [[3]]
#> [[3]][[1]]
#> [1] 2
#> 
#> [[3]][[2]]
#> [1] NA
#> 
#> [[3]][[3]]
#> [[3]][[3]][[1]]
#> [1] 3
#> 
#> [[3]][[3]][[2]]
#> [1] NA

## replace NULL by NA using condition and deflt arguments 
rrapply(l, condition = Negate(is.null), deflt = NA, how = "list")
#> [[1]]
#> [1] NA
#> 
#> [[2]]
#> [1] 1
#> 
#> [[3]]
#> [[3]][[1]]
#> [1] 2
#> 
#> [[3]][[2]]
#> [1] NA
#> 
#> [[3]][[3]]
#> [[3]][[3]][[1]]
#> [1] 3
#> 
#> [[3]][[3]][[2]]
#> [1] NA

We can also prune the NULL elements from the list altogether by setting how = "prune":

## keep only non-NULL elements
rrapply(l, condition = Negate(is.null), how = "prune")
#> [[1]]
#> [1] 1
#> 
#> [[2]]
#> [[2]][[1]]
#> [1] 2
#> 
#> [[2]][[2]]
#> [[2]][[2]][[1]]
#> [1] 3