Array#flatten

arr.flatten(depth=nil) -> array

.flatten returns a new array with all nested arrays recursively collapsed into a single flat array.

Syntax

array.flatten      # flatten all levels
array.flatten(1)   # flatten only one level deep
array.flatten(2)   # flatten two levels deep

Parameters

Parameter	Type	Default	Description
depth	Integer or nil	nil	How many levels to flatten. nil or negative flattens all levels.

Examples

Basic usage

nested = [1, [2, 3], [4, [5, 6]]]

nested.flatten
# => [1, 2, 3, 4, 5, 6]

Without arguments, .flatten recursively flattens all nested arrays.

Controlling depth

nested = [1, [2, [3, [4]]]]

nested.flatten(0)
# => [1, [2, [3, [4]]]]   # no flattening

nested.flatten(1)
# => [1, 2, [3, [4]]]    # flatten one level

nested.flatten(2)
# => [1, 2, 3, [4]]      # flatten two levels

Passing an integer controls how many levels of nesting to collapse.

Common Patterns

Converting tree structures to lists

tree = [1, [2, [3, [4, [5]]]]]
tree.flatten
# => [1, 2, 3, 4, 5]

Flattening after mapping

matrix = [[1, 2], [3, 4], [5, 6]]

matrix.map { |row| row.map { |n| n * 2 } }.flatten
# => [2, 4, 6, 8, 10, 12]

Handling mixed data

mixed = [1, [2, 3], {a: 1}, [4, [5]]]
mixed.flatten
# => [1, 2, 3, {:a=>1}, 4, 5]

See Also

• Array#compact
• Array#flatten!
• Array#intersection