The Ruby Way, Third Edition

parenthesis_matcher.rb

# use a stack to validate parenthesis completion
class Stack
  def initialize
    @store = []
  end

  def push(x)
    @store.push(x)
  end

  def pop
    @store.pop
  end

  def peek
    @store.last
  end

  def empty?
    @store.empty?
  end
end

def paren_matcher(str)
  stack = Stack.new
  lsym = "{[(<"
  rsym = "}])>"

  str.each_char do |sym|
    if lsym.include? sym
      stack.push(sym)
    elsif rsym.include?(sym)
      # if stack is empty, assign top to something that
      # will never be in the symbol string
      stack.empty? ? top = "-1" : top = stack.peek
      top = stack.peek
      if lsym.index(top) != rsym.index(sym)
        return false
      else
        stack.pop
      end
      # ignore non-grouped characters...
    end
  end
  puts "finish and return: #{stack.empty?}"

  # Ensure stack is empty
  return stack.empty?
end

parenthesis_matcher_spec.rb

require_relative './parenthesis_matcher.rb'

describe "#paren_matcher" do
  it "returns true with basic parentheses" do
    str = "(a)"
    expect(paren_matcher(str)).to eq true
  end

  it "returns false when final closing paren is missing" do
    str = "(a"
    expect(paren_matcher(str)).to eq false
  end

  it "returns false when first opening paren is missing" do
    str = "a)"
    expect(paren_matcher(str)).to eq false
  end

  it "returns true when multiple embedded parens are valid" do
    str = "[[(a-(b-c))], [[x,y]]]"
    expect(paren_matcher(str)).to eq true
  end

  it "returns true when multiple embedded parens are valid and one set is not" do
    str = "[[(a-(b-c))], [[x,y]]"
    expect(paren_matcher(str)).to eq false
  end

  it "returns true when multiple embedded parens are valid and one set is not" do
    str = "(((a+b))*((c-d)-(e*f))"
    expect(paren_matcher(str)).to eq false
  end
end

Added: Line 35

Resolve issue where algorithm fails if the argument's first symbol is included within rsym. Since the stack is empty, stack.peek returns nil. Thus when top stores the value nil. When we call index and pass in nil, an error occurs.
Solution: If the stack is empty, then store any string value in top that you would not ever see in lsym. In this example, I used -1.

Rspec error:
Failure/Error: if lsym.index(top) != rsym.index(sym)
TypeError: type mismatch: NilClass given