How to Filter Array in Ruby: Simple Guide with Examples

In Ruby, you can filter an array using the select method, which returns a new array with elements that meet a condition inside a block. Use array.select { |item| condition } to keep only the elements where the condition is true.

Syntax

The select method is called on an array and takes a block with a condition. It returns a new array containing only the elements for which the block returns true.

• array: The original array you want to filter.
• select: The method that filters elements.
• { |item| condition }: The block where item is each element, and condition is what you check.

ruby

filtered_array = array.select { |item| condition }

Example

This example filters an array of numbers to keep only even numbers.

ruby

numbers = [1, 2, 3, 4, 5, 6]
even_numbers = numbers.select { |num| num.even? }
puts even_numbers.inspect

Output

[2, 4, 6]

Common Pitfalls

One common mistake is using select without a block or with a block that always returns true or false. Another is modifying the original array expecting it to change, but select returns a new array instead.

Also, confusing select with reject can cause wrong results; reject keeps elements where the condition is false.

ruby

numbers = [1, 2, 3, 4]

# Wrong: no block given
# filtered = numbers.select

# Wrong: block always false, returns empty array
filtered = numbers.select { |n| false }
puts filtered.inspect  # Output: []

# Right: filter even numbers
filtered = numbers.select { |n| n.even? }
puts filtered.inspect  # Output: [2, 4]

Output

[]
[2, 4]

Quick Reference

Use select to keep elements matching a condition.

Use reject to remove elements matching a condition.

Remember, both return new arrays and do not change the original.

Method	Purpose	Returns
select	Keeps elements where block is true	New filtered array
reject	Keeps elements where block is false	New filtered array
select!	Filters array in place (modifies original)	Modified original array or nil if no changes

Key Takeaways

Use select with a block to filter arrays by a condition.

select returns a new array and does not change the original.

Make sure the block returns true only for elements you want to keep.

Use reject to filter out elements matching a condition.

Use select! if you want to modify the original array.