Introduction to Stacks in Ruby

Introduction

Greetings! Today, we're unveiling the concept of Stacks in Ruby, a fundamental data structure. Imagine a stack as a pile of dishes: you add a dish to the top (Last In) and take it from the top (First Out). This Last-In, First-Out (LIFO) principle exemplifies the stack. Ruby handles stacks effortlessly with Arrays . This lesson will illuminate the stack data structure, operations, and their Ruby applications. Are you ready to start?

Understanding Stack: A Data Structure on the Rise

A stack is a storage structure that allows Push (addition) and Pop (removal) operations. It's akin to a stack of plates in a cafeteria, where plates are added (pushed) and removed (popped) from the top. No plate can be taken from the middle or the bottom, exemplifying a Last-In, First-Out (LIFO) operation.

Utilizing Arrays as Stacks in Ruby

To create a stack, Ruby employs a built-in data structure known as an Array . For the Push operation, we use push, which adds an element at the end of the array. For the Pop operation, there's the pop method that removes the last element, simulating the removal of the 'top' element in a stack. Here's how it looks: Ruby stack = [] # A new empty stack # Push operations stack.push('John') stack.push('Mary') stack.push('Steve') stack.pop # Pop operation removes 'Steve' puts stack.inspect # Outputs: ["John", "Mary"] In the example provided, we push 'John', 'Mary', and 'Steve' into the stack and then pop 'Steve' from the stack.

Advanced Stack Operations

Stack operations go beyond merely push and pop . For example, to verify if a stack is empty, you can use the empty? method. If it returns true, that means the stack is empty. Conversely, if it returns false, we can infer the stack is not empty. To peek at the top element of the stack without popping it, indexing with -1 is handy. Here's an example: Ruby stack.push('Sam') puts stack.last # Outputs: 'Sam' In this example,'Sam' is added (pushed), and then the topmost stack element, which is'Sam', is peeked.

Practical Stack Applications: Reversing a String

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Practical Stack Applications: Checking Balance of Parentheses

A stack can be utilized to verify if parentheses in an expression are well-matched, i.e., every bracket has a corresponding pair. For example, parentheses in the string "()[{}]" are well-matched, while in the strings "([]()", ")()[]{}", "([)]", and "[{})" they are not.

Let's break down the solution into simple steps:

We start by creating a hash that maps each closing bracket to its corresponding opening bracket and an empty stack. Then, we iterate over each character paren in the string paren_string:

If paren is an opening bracket, it gets appended to the stack.
If paren is a closing bracket and the top element in the stack is the corresponding opening bracket, we remove the top element from the stack.
If neither of the above conditions is met, we return false.

Finally, if the stack is empty (all opening brackets had matching closing brackets), we return true. If there are some unmatched opening brackets left, we return false.

Ruby

def is_paren_balanced(paren_string)
  stack = []
  is_balanced = true
  index = 0
  opening_paren = {')' => '(', ']' => '[', '}' => '{'} # a matching opening parenthesis for every closing one
  
  # Traversing all string characters
  while index < paren_string.length && is_balanced
    paren = paren_string[index]
    if "([{".include?(paren)
      # We met an opening parenthesis, just putting it on stack
      stack.push(paren)
    else
      # We met a closing parenthesis
      if stack.empty?
        # The parenthesis is closing, but there are no items in the stack
        is_balanced = false
      else
        if stack.last != opening_paren[paren]
          # The parenthesis on top of the stack doesn't match
          is_balanced = false
        else
          stack.pop
        end
      end
    end
    index += 1
  end
  
  if !stack.empty?
    # If after traversing all characters, there is something left, it's bad
    is_balanced = false
  end
  is_balanced
end

puts is_paren_balanced("(())") # Outputs: true
puts is_paren_balanced("({[)}") # Outputs: false

def is_paren_balanced(paren_string)
  stack = []
  is_balanced = true
  index = 0
  opening_paren = {')' => '(', ']' => '[', '}' => '{'} # a matching opening parenthesis for every closing one
  
  # Traversing all string characters
  while index < paren_string.length && is_balanced
    paren = paren_string[index]
    if "([{".include?(paren)
      # We met an opening parenthesis, just putting it on stack
      stack.push(paren)
    else
      # We met a closing parenthesis
      if stack.empty?
        # The parenthesis is closing, but there are no items in the stack
        is_balanced = false
      else
        if stack.last != opening_paren[paren]
          # The parenthesis on top of the stack doesn't match
          is_balanced = false
        else
          stack.pop
        end
      end
    end
    index += 1
  end
  
  if !stack.empty?
    # If after traversing all characters, there is something left, it's bad
    is_balanced = false
  end
  is_balanced
end

puts is_paren_balanced("(())") # Outputs: true
puts is_paren_balanced("({[)}") # Outputs: false