modify_tree: Recursively modify a list
In purrr: Functional Programming Tools

modify_tree

R Documentation

Recursively modify a list

Description

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::obj_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.

Examples

x <- list(list(a = 2:1, c = list(b1 = 2), b = list(c2 = 3, c1 = 4)))
x |> str()

# Transform each leaf
x |> modify_tree(leaf = \(x) x + 100) |>  str()

# 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()

purrr documentation built on April 3, 2025, 6:13 p.m.