How to Implement Stack in Ruby: Simple Guide with Examples

In Ruby, you can implement a stack using an array with push and pop methods to add and remove elements. Alternatively, create a custom Stack class that encapsulates these operations for clearer code and better control.

Syntax

A stack in Ruby can be implemented using an array with these main methods:

• push(element): Adds an element to the top of the stack.
• pop: Removes and returns the top element from the stack.
• peek or last: Returns the top element without removing it.

Alternatively, you can define a Stack class with these methods to organize stack behavior.

ruby

class Stack
  def initialize
    @elements = []
  end

  def push(element)
    @elements.push(element)
  end

  def pop
    @elements.pop
  end

  def peek
    @elements.last
  end

  def empty?
    @elements.empty?
  end
end

Example

This example shows how to use the Stack class to add, remove, and check elements in the stack.

ruby

class Stack
  def initialize
    @elements = []
  end

  def push(element)
    @elements.push(element)
  end

  def pop
    @elements.pop
  end

  def peek
    @elements.last
  end

  def empty?
    @elements.empty?
  end
end

stack = Stack.new
stack.push(10)
stack.push(20)
stack.push(30)
puts "Top element: #{stack.peek}"
puts "Removed element: #{stack.pop}"
puts "Top element after pop: #{stack.peek}"
puts "Is stack empty? #{stack.empty?}"

Output

Top element: 30
Removed element: 30
Top element after pop: 20
Is stack empty? false

Common Pitfalls

Common mistakes when implementing a stack in Ruby include:

• Using shift instead of pop, which removes from the front, not the top.
• Not checking if the stack is empty before popping, which can cause errors.
• Modifying the internal array directly from outside the class, breaking encapsulation.

Always use pop to remove the top element and check empty? before popping.

ruby

stack = []
# Wrong: removes first element, not top
stack.push(1)
stack.push(2)
stack.shift # removes 1, not 2

# Right:
stack.push(1)
stack.push(2)
stack.pop # removes 2

# Check empty before pop
stack = []
if !stack.empty?
  stack.pop
else
  puts "Stack is empty, cannot pop"
end

Output

Stack is empty, cannot pop

Quick Reference

Stack operations in Ruby using arrays:

Operation	Method	Description
Add element	push(element)	Adds element to the top of the stack
Remove element	pop	Removes and returns the top element
View top	last or peek	Returns the top element without removing
Check empty	empty?	Returns true if stack has no elements

Key Takeaways

Use Ruby arrays with push and pop to implement a stack easily.

Encapsulate stack behavior in a class for cleaner and safer code.

Always check if the stack is empty before popping to avoid errors.

Avoid using shift as it removes from the bottom, not the top.

Use peek (last) to see the top element without removing it.