Flatten

Recursively flattens a nested list into a single flat list containing only the leaf values, preserving left-to-right order.

A bare (class-less) list is recursed into; NULL contributes no leaves; an object of a class registered with register_tree_node() is recursed into through the children its flatten returns; everything else (a classed object, an atomic, a function, ...) is a leaf.

Use build_tree() to capture the nesting structure so it can be restored with unflatten().

Usage

flatten(x, is_leaf = NULL)

Arguments

x

(any)
Object to flatten.

is_leaf

(function | NULL)
Called on every value the walk meets other than NULL, the list x included; a value it returns TRUE for is a leaf, even a list or an object of a registered class, as with JAX's tree_flatten(is_leaf = ). Must return TRUE or FALSE. NULL (default) recurses into every list and registered object.

Value

list() containing the flattened values.

See also

build_tree(), unflatten(), tree_size()

Examples

x <- list(a = 1, b = list(c = 2, d = 3))
flatten(x)
#> [[1]]
#> [1] 1
#> 
#> [[2]]
#> [1] 2
#> 
#> [[3]]
#> [1] 3
#> 

flatten(list(1:3, "hello"))
#> [[1]]
#> [1] 1 2 3
#> 
#> [[2]]
#> [1] "hello"
#> 

# stop at the list `b`
flatten(x, is_leaf = function(v) identical(v, x$b))
#> [[1]]
#> [1] 1
#> 
#> [[2]]
#> [[2]]$c
#> [1] 2
#> 
#> [[2]]$d
#> [1] 3
#> 
#>