Groups

Heroes and Monsters

We're going to build a game!

Groups will work together, but elect one typer.

The Hero

Create variables to hold the hero's:

• Name (String)
• Health (FixNum)
• Power (FixNum)
• Items (Array<String>)
• Exp (FixNum)

The Monster

Create variables to hold the monster's:

• Name (String)
• Health (FixNum)
• Power (FixNum)
• Type (String, one of "Dragon", "Goblin", "Salman")

Time: 1 minute

...actually more monsters

Our game needs more monsters.

Make a monsters array with 3.

Time: 1 minute

Attack!

Attacking subtracts the attacker's power from the defendant's health.

Attack method

def method_name(hero, monster)
  ...
end

Time: 1 minute

Attack method

def attack(hero, monster)
  monster[:health] -= hero[:power]
end

Use the Method

Use your method to have the hero attack the first monster.

Use the Method

attack(hero, monsters[0])

Monster attack

Does it need to be different?

Generic Attack

def attack(attacker, defendant)
  defendant[:health] -= attacker[:power]
end

What about EXP?

When a hero attacks, s/he gains EXP.

Modify the function: attacker gains 1 EXP with each attack.

Time: 1 minute

EXP attack

def attack(attacker, defendant)
  defendant[:health] -= attacker[:power]
  attacker[:exp] += 1
end

Monster attack

Does it need to be different? Monsters don't gain exp...

This is getting complicated

...and there's still so much to do!

Code should be easy to change

Code should be DRY*

* Don't repeat yourself

CS solution: Objects

Objects let us group related properties and methods.

• property = variable (i.e. health)
• method = function (i.e. attack)

Key concepts

• Class: a template for an object
• Instance: a particular one of that object

Class vs. Instance

• "Hero" is a class
• "Hercules" is an instance
  • So is "The Bru"

What are other examples?

Instance properties and methods

• Describe what an object is like and can do
• Belong to a particular instance
  • Hercules has a name, health, power, etc.
  • He can also attack
• It doesn't make sense for "Hero" as a concept to have a name

Constructors

• Each class has a constructor, which explains how to make an instance
• A special method with the name initialize

Hero class

class Hero
  def initialize(name, health, power)
    @name = name
    @health = health
    @power = power
    @items = []
    @exp = 0
  end
end

@?

To denote an instance variable, prefix it with an @

Remember, this can be referred to anywhere in the class.

Which of these are instance variables and which are local variables?

1. @name
2. hercules
3. @power
4. items
5. he@lth

Add attack

What should go in the ???

class Hero
  def initialize(name, health, power)
    ...
  end

  # Teach your hero how to attack
  def attack(monster)
    monster[:health] -= ???
  end
end

Modify Hero's attack method to also increment exp.

Time: 1 minute

Make a new Hero

hero = Hero.new("hal", 20, 10)

Class attack

hero.attack(monsters[1])

Monsters, too

As a group, create a class called Monster.

Write a constructor for it, and also an attack method.

Challenge: if the monster's type is "Salman", have the attack do nothing :P

Time: 5 minutes

Key Concepts

• Declare a class with methods:

  class Apartment
  ... # contains initialize method, among others
  end

• Instantiate an instance of a Class with:

  apartment = Apartment.new

• Call methods on the instance just like with other objects

  apartment.add_resident('Bilbo Baggins')

• Use classes to encapsulate shared behavior and data

The code

Without classes: heroes.rb

With classes: heroes_with_classes.rb