planet.rb

# public enum Planet {
#     MERCURY (3.303e+23, 2.4397e6),
#     VENUS   (4.869e+24, 6.0518e6),
#     EARTH   (5.976e+24, 6.37814e6),
#     MARS    (6.421e+23, 3.3972e6),
#     JUPITER (1.9e+27,   7.1492e7),
#     SATURN  (5.688e+26, 6.0268e7),
#     URANUS  (8.686e+25, 2.5559e7),
#     NEPTUNE (1.024e+26, 2.4746e7);
#
#     private final double mass;   // in kilograms
#     private final double radius; // in meters
#     Planet(double mass, double radius) {
#         this.mass = mass;
#         this.radius = radius;
#     }
#     private double mass()   { return mass; }
#     private double radius() { return radius; }
#
#     // universal gravitational constant  (m3 kg-1 s-2)
#     public static final double G = 6.67300E-11;
#
#     double surfaceGravity() {
#         return G * mass / (radius * radius);
#     }
#     double surfaceWeight(double otherMass) {
#         return otherMass * surfaceGravity();
#     }
#     public static void main(String[] args) {
#         if (args.length != 1) {
#             System.err.println("Usage:  java Planet <earth_weight>");
#             System.exit(-1);
#         }
#         double earthWeight = Double.parseDouble(args[0]);
#         double mass = earthWeight/EARTH.surfaceGravity();
#         for (Planet p : Planet.values())
#            System.out.printf("Your weight on %s is %f%n",
#                              p, p.surfaceWeight(mass));
#     }
# }

class Planet

  G = 6.67300E-11;

  attr_reader :name, :mass, :radius

  def initialize(name, mass, radius)
    @name = name
    @mass = mass
    @radius = radius
    (@@planets ||= []) << self
  end

  def surface_gravity
    G * @mass / (@radius * @radius);
  end

  def surface_weight(other_mass)
    other_mass * surface_gravity
  end

  def self.all
    @@planets
  end
  def self.each(&block)
    @@planets.each(&block)
  end
  def self.[](name)
    @@planets.find { |pl| pl.name == name }
  end
end

Planet.new('Mercury', 3.303e+23, 2.4397e6)
Planet.new('Venus', 4.869e+24, 6.0518e6)
Planet.new('Earth', 5.976e+24, 6.37814e6)
Planet.new('Mars', 6.421e+23, 3.3972e6)
Planet.new('Jupiter', 1.9e+27, 7.1492e7)
Planet.new('Saturn', 5.688e+26, 6.0268e7)
Planet.new('Uranus', 8.686e+25, 2.5559e7)
Planet.new('Neptune', 1.024e+26, 2.4746e7)

if ARGV.length < 1
  STDERR.puts("Usage: ruby planet.rb <earth_weight>")
end

earth_weight = ARGV[0].to_f
mass = earth_weight / Planet['Earth'].surface_gravity

Planet.each do |planet|
  puts "your weight on #{planet.name} is #{planet.surface_weight(mass)}"
end