Solar System

At a Glance

• Build a model of a solar system to practice working with classes
• Individual, stage 1 project
• Due before class, Wednesday Sept 23rd

Learning Goals

• Create a class to tie together data and methods
• Create a class which manages a collection of other objects
• Write code that interacts with classes

Project Structure

This project is broken up into three waves, each of which consists of a number of steps. You should git commit after you've completed each step!

Wave 1

The learning goal for this wave is to practice working with individual instances of a single class.

Instructions

1. In a file called planet.rb, create a class called Planet. Each instance of this class will keep track of information about a single planet.

   Pay attention to the details of the class name Planet:

   • Class names always start with a capital letter
   • Class names are usually a noun or noun-phrase
   • Because each instance is only one planet, we use a singular noun (Planet instead of Planets)

2. Add a constructor to your Planet class. Your constructor should take at least these 5 parameters:

   • name
   • color
   • mass_kg
   • distance_from_sun_km
   • fun_fact

   Each parameter should be saved in an instance variable with the same name (e.g. @color for color). These instance variables should be readable from outside the class, but not writable.

   Once you're done, you should be able to write code like this:

   # Load Planet into pry:
   # $ pry -r ./planet.rb
   earth = Planet.new('Earth', 'blue-green', 5.972e24, 1.496e8, 'Only planet known to support life')
   
   puts earth.name
   # => Earth
   puts earth.fun_fact
   # => Only planet known to support life
   
   earth.color = 'pink'
   # => NoMethodError: undefined method `color=' for #<Planet:0x00007fcfba04c130>
   # => Did you mean?  color

3. Add another file, main.rb. This file should require_relative planet.rb, and contain one method called main that will exercise your code. This method should create two different instances of Planet and print out some of their attributes.

   main.rb should have this structure:

   # main.rb
   require_relative 'planet'
   
   def main
     # ... do stuff with planets ...
   end
   
   main

   Note that we invoke main as the last line of your program.

4. Add an instance method to Planet called summary. This method should return (not puts) a string containing a nicely-formatted description of the planet. Exercise your summary method in the main method.

   Question: Why do we puts in main but not in Planet#summary?

5. OPTIONAL: Add error checking to your constructor.

   • Both mass_kg and distance_from_sun_km must be numbers that are greater than zero.
   • What should your program do if they aren't?
   • How will you make sure this behavior works?

6. OPTIONAL: Add minitest tests for Planet.

Wave 2

In this wave you will build a second class, SolarSystem, which is responsible for keeping track of a collection of instances of Planet.

Instructions

1. In a new file called solar_system.rb, create a new class called SolarSystem.

   • The constructor should take one parameter, star_name, and save it in an instance variable.
   • Each SolarSystem should have an instance variable called @planets, which will store an array of planets. When the SolarSystem is created, @planets should be set to an empty array.
   • Both @star_name and @planets should be readable but not writable.

2. Create a method SolarSystem#add_planet, which will take an instance of Planet as a parameter and add it to the list of planets.

3. Create a method SolarSystem#list_planets, which will return (not puts) a string containing a list of all the planets in the system. The string should be formatted in this style:

   Planets orbiting <star name>
   1.  Mercury
   2.  Venus
   3.  Earth
   4.  Mars
   5.  Jupiter

4. Update your driver code in main.rb to create an instance of SolarSystem, add all your Planets to it, and then print the list. Here is an example with one Planet:

   solar_system = SolarSystem.new('Sol')
   
   earth = Planet.new('Earth', ...)
   solar_system.add_planet(earth)
   
   list = solar_system.list_planets
   puts list
   # => Planets orbiting Sol
   # => 1.  Earth

   NOTE: When you first run your driver code, you may get an error like this:

   NameError: uninitialized constant SolarSystem
   

   What does this error mean? What do you need to do to fix it?

5. Create a method SolarSystem#find_planet_by_name, that takes the name of a planet as a string, and returns the corresponding instance of Planet. The lookup should be case-insensitive, so that Earth, earth and eArTh all return the same planet.

   Update your driver code to exercise this method:

   found_planet = solar_system.find_planet_by_name('Earth')
   
   # found_planet is an instance of class Planet
   puts found_planet
   # => #<Planet:0x00007fe7c2868ee8>
   
   puts found_planet.summary
   # => Earth is a blue-green planet ...

   Questions for you to consider as you write this method:

   • What should your method do if there is no planet with the given name?
   • What should your method do if there are multiple planets with the given name?
   • Is there a built-in Ruby enumerable method that could help you here?

6. OPTIONAL: Create a method, SolarSystem#distance_between, that takes two planet names as parameters and returns the distance between them.

   You can assume that all the planets are lined up in a straight line.

7. OPTIONAL: Add minitest tests for SolarSystem.

Wave 3

In this wave, you will build a command line interface (CLI) to allow a user to interact with your classes. This will replace the driver code currently in the main method of main.rb.

Instructions

1. Restructure main to do the following:

   1. Create a SolarSystem and add some Planets
   2. Enter a control loop that repeatedly asks the user what to do next. The two options at this point are list planets and exit.

   Remember that you've already implemented listing planets - you should be able to call that method rather than recreating the logic here.

2. Add a planet details option to the control loop. When the user picks this option, the program should should call a separate method that will:

   1. Ask them for the name of the planet they wish to learn about
   2. Display details for that planet

   Again, you should reuse a method you've already built to handle formatting the planet information.

3. Add an add planet option to the control loop. When the user picks this option, the program should call a separate method that will:

   1. Ask the user for details about the planet
   2. Create a new instance of Planet with that info
   3. Add it to the SolarSystem

   The new planet should now appear in the list of planets, and the user should be able to see its details.

4. OPTIONAL: Add error handling to the control loop. What happens if the user enters a bad command, asks for details on a planet that doesn't exist, or enters an bad value for a new planet's mass?

5. OPTIONAL: Implement the optional distance_between method from wave 2, then add an option to the control loop to expose it to the user.

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