car.rb

#! /usr/local/bin/ruby

class Car

  # excetion raised when not enough fuel left to drive given distance
  class NotEnoughFuel < RuntimeError
  end

  # constructor with one parameter
  def initialize vendor
    @vendor = vendor # remember the vendor
    @fuel = 50 # initial amount of fuel
  end

  # create getter method for fuel
  attr_reader :fuel

  # manually create getter method for vendor
  def vendor
    @vendor
  end

  def empty?
    @fuel == 0 # returns true if @fuel is 0
  end

  def fill!
    @fuel = 50
  end

  # Drives _miles_ miles and returns the needed fuel.
  def drive miles
    # this car drives 10 miles per fuel unit
    needed_fuel = miles / 10.0
    raise NotEnoughFuel if @fuel < needed_fuel
    @fuel -= needed_fuel
    puts "Car just drove #{miles} miles and needed #{needed_fuel} fuel."
    return needed_fuel # 'return' is optional here
  end
end

# if this file was executed on its own (not loaded as a library)
if $0 == __FILE__

  my_car = Car.new "Ford"
  my_car.drive 55
  puts "Fuel left: #{my_car.fuel}"

  begin
    # do forever
    loop do
      # will raise after a few runs...
      my_car.drive 38
    end
  rescue Car::NotEnoughFuel
    puts "Oh shit! Not enough fuel!"
  end

  puts "The end."
end

irb_session.txt

# Run the command "irb" in the Terminal app and do the following.
# ">>" lines were typed.
# "=>" lines are return values.

>> require 'car'
=> true
>> brum = Car.new 'Fiat'
=> #<Car:0x1011ce010 @fuel=50, @vendor="Fiat">
>> brum.drive 100
Car just drove 100 miles and needed 10.0 fuel.
=> 10.0
>> brum.fuel
=> 40.0
>> brum.empty?
=> false
>> brum.drive 100
Car just drove 100 miles and needed 10.0 fuel.
=> 10.0
>> brum.fuel
=> 30.0
>> brum.drive 300
Car just drove 300 miles and needed 30.0 fuel.
=> 30.0
>> brum.empty?
=> true
>> brum.fill!
=> 50
>> brum.fuel
=> 50
>> class Car
>>   def horn
>>     puts "Roooooaaarrrrrr!"
>>     end
>>   end
=> nil
>> brum.horn
Roooooaaarrrrrr!
=> nil
>> module RoadRunner
>>   def meep
>>     puts "Meep meep!"
>>     end
>>   end
=> nil
>> class Car
>>   include RoadRunner
>>   end
=> Car
>> brum.meep
Meep meep!
=> nil

output.txt

paddor@mcpaddor ~ ∴ ruby car.rb 
Car just drove 55 miles and needed 5.5 fuel.
Fuel left: 44.5
Car just drove 38 miles and needed 3.8 fuel.
Car just drove 38 miles and needed 3.8 fuel.
Car just drove 38 miles and needed 3.8 fuel.
Car just drove 38 miles and needed 3.8 fuel.
Car just drove 38 miles and needed 3.8 fuel.
Car just drove 38 miles and needed 3.8 fuel.
Car just drove 38 miles and needed 3.8 fuel.
Car just drove 38 miles and needed 3.8 fuel.
Car just drove 38 miles and needed 3.8 fuel.
Car just drove 38 miles and needed 3.8 fuel.
Car just drove 38 miles and needed 3.8 fuel.
Oh shit! Not enough fuel!
The end.