Recursively modify a list

modify_tree() allows you to recursively modify a list, supplying functions that either modify each leaf or each node (or both).

Usage

modify_tree(
  x,
  ...,
  leaf = identity,
  is_node = NULL,
  pre = identity,
  post = identity
)

Arguments

x

A list.

...

Reserved for future use. Must be empty

leaf

A function applied to each leaf.

is_node

A predicate function that determines whether an element is a node (by returning TRUE) or a leaf (by returning FALSE). The default value, NULL, treats simple lists as nodes and everything else (including richer objects like data frames and linear models) as leaves, using vctrs::vec_is_list(). To recurse into all objects built on lists use is.list().

pre, post

Functions applied to each node. pre is applied on the way "down", i.e. before the leaves are transformed with leaf, while post is applied on the way "up", i.e. after the leaves are transformed.

See also

Other modify variants: map_depth(), modify()

Examples

x <- list(list(a = 2:1, c = list(b1 = 2), b = list(c2 = 3, c1 = 4)))
x |> str()
#> List of 1
#>  $ :List of 3
#>   ..$ a: int [1:2] 2 1
#>   ..$ c:List of 1
#>   .. ..$ b1: num 2
#>   ..$ b:List of 2
#>   .. ..$ c2: num 3
#>   .. ..$ c1: num 4

# Transform each leaf
x |> modify_tree(leaf = \(x) x + 100) |>  str()
#> List of 1
#>  $ :List of 3
#>   ..$ a: num [1:2] 102 101
#>   ..$ c:List of 1
#>   .. ..$ b1: num 102
#>   ..$ b:List of 2
#>   .. ..$ c2: num 103
#>   .. ..$ c1: num 104

# Recursively sort the nodes
sort_named <- function(x) {
  nms <- names(x)
  if (!is.null(nms)) {
    x[order(nms)]
  } else {
    x
   }
}
x |> modify_tree(post = sort_named) |> str()
#> List of 1
#>  $ :List of 3
#>   ..$ a: int [1:2] 2 1
#>   ..$ b:List of 2
#>   .. ..$ c1: num 4
#>   .. ..$ c2: num 3
#>   ..$ c:List of 1
#>   .. ..$ b1: num 2