How to Reject Elements from an Array in Ruby

In Ruby, you can use the reject method on an array to remove elements that meet a certain condition. It returns a new array excluding those elements for which the block returns true. Use reject! if you want to modify the original array in place.

Syntax

The reject method is called on an array and takes a block that defines the condition for rejection. Elements for which the block returns true are excluded from the returned array.

• array.reject { |element| condition } - returns a new array without rejected elements.
• array.reject! { |element| condition } - modifies the original array by removing rejected elements.

ruby

array.reject { |element| condition }

Example

This example shows how to remove all even numbers from an array using reject. The original array remains unchanged, and a new filtered array is returned.

ruby

numbers = [1, 2, 3, 4, 5, 6]
filtered = numbers.reject { |num| num.even? }
puts "Original array: #{numbers}"
puts "Filtered array (no even numbers): #{filtered}"

Output

Original array: [1, 2, 3, 4, 5, 6]
Filtered array (no even numbers): [1, 3, 5]

Common Pitfalls

A common mistake is expecting reject to change the original array. It returns a new array instead. To modify the original array, use reject!. Also, forgetting to provide a block will cause an error.

ruby

arr = [1, 2, 3, 4]
arr.reject { |x| x > 2 }
puts arr.inspect  # Original array unchanged

arr.reject! { |x| x > 2 }
puts arr.inspect  # Original array modified

Output

[1, 2, 3, 4]
[1, 2]

Quick Reference

Method	Description
reject { |e| condition }	Returns a new array excluding elements where condition is true
reject! { |e| condition }	Removes elements from the original array where condition is true
select { |e| condition }	Opposite of reject; keeps elements where condition is true

Key Takeaways

Use reject to get a new array excluding elements matching a condition.

Use reject! to modify the original array by removing matching elements.

Always provide a block with a condition to reject or reject!.

Remember reject does not change the original array unless you use reject!.

For the opposite effect, use select to keep elements matching a condition.