How to Flatten an Array in Ruby: Simple Guide

In Ruby, you can flatten a nested array using the flatten method, which returns a new array with all nested arrays merged into one. Use array.flatten to get a single-level array from any depth of nested arrays.

Syntax

The flatten method is called on an array object to convert nested arrays into a single flat array.

• array.flatten: Returns a new flattened array without modifying the original.
• array.flatten!: Flattens the array in place, modifying the original array.

ruby

array = [1, [2, 3], [4, [5, 6]]]
flat_array = array.flatten
# flat_array is [1, 2, 3, 4, 5, 6]

Example

This example shows how to flatten a nested array using flatten and flatten!. The first returns a new array, the second modifies the original.

ruby

nested = [1, [2, [3, 4]], 5]
flat = nested.flatten
puts "Flattened (new array): #{flat.inspect}"
nested.flatten!
puts "Flattened (in place): #{nested.inspect}"

Output

Flattened (new array): [1, 2, 3, 4, 5]
Flattened (in place): [1, 2, 3, 4, 5]

Common Pitfalls

One common mistake is expecting flatten to modify the original array without using the bang version flatten!. Also, flattening very large or deeply nested arrays can affect performance.

ruby

arr = [1, [2, 3]]
arr.flatten
puts arr.inspect  # Output: [1, [2, 3]] (original unchanged)

arr.flatten!
puts arr.inspect  # Output: [1, 2, 3] (original modified)

Output

[1, [2, 3]]
[1, 2, 3]

Quick Reference

Method	Description
flatten	Returns a new flattened array, original unchanged
flatten!	Flattens the array in place, modifies original
flatten(level)	Flattens array up to specified depth level

Key Takeaways

Use flatten to get a new flat array without changing the original.

Use flatten! to flatten the array in place and modify the original.

You can specify a depth level with flatten(level) to flatten partially.

Remember flatten does not change the original array unless you use the bang version.

Flattening deeply nested arrays can impact performance, so use wisely.