functional_programming_example.rb

city_companies = {
    apple: ["Boston", "Seattle", "San Francisco"],
    uniliver: ["London", "Sussex"],
    microsoft: ["Seattle", "Austin", "Denver"],
    amazon: ["Portland", "Denver", "Washington"],
    starbucks: ["Beijing", "Seattle", "Munich"]
}

tech_giants = [:apple, :microsoft, :amazon]

tech_giants_cities = city_companies
  .select {|company, cities| tech_giants.include?(company)}
  .map {|company, cities| cities}
  .flatten
  .uniq