How to Implement Queue in Ruby: Simple Guide and Example

In Ruby, you can implement a queue using an Array by adding elements with push and removing them with shift. For a more structured approach, create a Queue class with methods like enqueue and dequeue to manage the queue behavior.

Syntax

A queue in Ruby can be implemented using an Array. Use push(element) to add items to the end and shift to remove items from the front, following the FIFO (First In, First Out) rule.

Alternatively, define a Queue class with methods:

• enqueue(item): adds an item to the queue
• dequeue: removes and returns the first item
• peek: shows the first item without removing it
• empty?: checks if the queue is empty

ruby

class Queue
  def initialize
    @items = []
  end

  def enqueue(item)
    @items.push(item)
  end

  def dequeue
    @items.shift
  end

  def peek
    @items.first
  end

  def empty?
    @items.empty?
  end
end

Example

This example shows how to create a queue, add items, remove them, and check the queue's state.

ruby

class Queue
  def initialize
    @items = []
  end

  def enqueue(item)
    @items.push(item)
  end

  def dequeue
    @items.shift
  end

  def peek
    @items.first
  end

  def empty?
    @items.empty?
  end
end

queue = Queue.new
queue.enqueue("apple")
queue.enqueue("banana")
queue.enqueue("cherry")

puts queue.dequeue  # Output: apple
puts queue.peek     # Output: banana
puts queue.empty?   # Output: false
queue.dequeue
queue.dequeue
puts queue.empty?   # Output: true

Output

apple
banana
false
true

Common Pitfalls

One common mistake is using pop instead of shift to remove items, which breaks the FIFO order by removing from the end instead of the front.

Another pitfall is modifying the queue array directly from outside the class, which can cause unexpected behavior.

ruby

queue = []
queue.push(1)
queue.push(2)
queue.pop    # Removes 2, not 1 (wrong for queue)

# Correct way:
queue = []
queue.push(1)
queue.push(2)
queue.shift  # Removes 1 (correct FIFO)

Quick Reference

Operation	Method	Description
Add item	push(item)	Adds item to the end of the queue
Remove item	shift	Removes item from the front of the queue
Check first item	first or peek method	Returns the first item without removing
Check empty	empty?	Returns true if queue has no items

Key Takeaways

Use an array with push and shift to implement a queue in Ruby.

Create a Queue class for clearer, reusable queue operations.

Always remove items with shift to maintain FIFO order.

Avoid directly modifying the queue's internal array from outside.

Use methods like peek and empty? to safely inspect the queue.