Welcome!

Homework Discussion

How was last week's homework? Do you have any questions or concepts that you'd like to discuss?

grandma.rb

chatting = true
byes = 0
while chatting
  puts "Ask Grandma a question:"
  said = gets.chomp!
  if said == "BYE"
    byes += 1
    if byes > 2
      chatting = false
    else
      puts "** Grandma is ignoring you **"
    end
  elsif said == said.upcase
    year = 1930 + Random.rand(20)
    puts "NO, NOT SINCE #{year}!"
  else
    puts "HUH?!  SPEAK UP, SONNY!"
  end
end
puts "Thanks for chatting with Grandma"
          

leap.rb

puts "Enter a starting year in the format YYYY"
starting = gets.chomp!.to_i
puts "Enter an ending year in the format YYYY"
ending = gets.chomp!.to_i
puts "LEAP YEARS between #{starting} and #{ending}"

starting.upto(ending) do |year|
  divisible_by_4 = (year % 4) == 0
  not_divisible_by_100 = (year % 100) != 0
  divisible_by_400 = (year % 400) == 0
  is_leap_year = (divisible_by_4 && not_divisible_by_100)
                   || divisible_by_400
  if is_leap_year
    puts "#{year}"
  end
end
          

Review

• Conditions - if, elsif, else
• Loops - while, for
• Arrays - [1,2,3] or %w(one two three)
• Hashes - {"one" => 1, "two" => 2}

What we will cover today

• Methods
• Objects
• Object Oriented Programming

Methods

A method is a name for a chunk of code

Methods are often used to define reusable behavior.

Let's look at a method we've already used:

1.even?
=> false

"fred".capitalize
=> "Fred"
          

If you have some code you want to reuse or organize, methods will help.

Let's Develop It!

Find the methods available on String and Integer

Do they have any similar methods?

"abc".next
=> "abd"

"abz".next
=> "aca"

1.next
=> 2

-1.next
=> 0
          

Write a Method

Let's write a method together

def means define

subtract is the name of the method

x, y are parameters

x - y is the body of the method

x - y is also the return value

def subtract x, y
  x - y
end
        

Let's Develop It!

Let's write a method that takes in a number and tells us if it is even.

Hint: if number % 2 == 0, then it is even

def method_name(parameter)
  # method implementation
end
          

Method Return Value

Every Ruby method returns something.

Usually, it's the last statement in the method.

It could be the value sent to return if that came first.

def subtract(x,y)
  x - y
  'another thing'
end

return_value = subtract(5, 2)
puts return_value

def subtract(x,y)
  return x - y
  'another thing'
end

return_value = subtract(5, 2)
puts return_value
          

Arguments vs. Parameters

Technically speaking, arguments are passed and parameters are declared.

Note that the variable names don't have to match!

In this code, 5 is an argument and x is a parameter

def subtract(x,y)
  x - y
end

subtract(5, 2)
          

Splat Arguments

def greet(greeting, *names)
  names.each do |name|
    puts "#{greeting}, #{name}!"
  end
end

>> greet("Hello", "Alice", "Bob", "Charlie")
Hello, Alice!
Hello, Bob!
Hello, Charlie!
          

Default Values

def eat(food = "chicken")
  puts "Yum, #{food}!"
end

>> eat
Yum, chicken!

>> eat "bananas"
Yum, bananas!
          

Default Hash Parameter

def add_to_x_and_y(amount, vals)
  x = vals[:x]
  y = vals[:y]
  x + y + amount
end

add_to_x_and_y(2, {:x => 1, :y => 2})

# same as...
add_to_x_and_y 2, :x => 1, :y => 2

# same as...
add_to_x_and_y 2, x: 1, y: 2

# same as...
add_to_x_and_y 2, y: 2, x: 1
          

Named Parameters

To pass variable parameters, or to pass named parameters, you can use an options hash

bake("Wheat")
bake("Sourdough", :flour => "sour")
bake("Pumpernickel", :creamer => "butter")

def bake(name, options = {})
  flour = options[:flour] || "rye"
  creamer = options[:creamer] || "cream"
  puts "baking a nice #{flour} loaf with #{creamer}"
end
          

Methods & Variables

Local vars are only available in the methods where they were defined.

# Let's extract the length of the padded string into
# a local variable
def message(our_string)
  length = 80
  puts our_string.center(length, "-")
end

def different_message(some_string)
  puts our_string.center(length, "=")
end
          

Objects

An Object is the building block of a program.

Objects do things and have properties.

Object: Number
What can it do?
- Add
- Subtract
- Divide
- Multiply

What are some properties of your Number?
- Length
- Base
- Even
          

Object Class

Objects can be grouped by Class.

Classes define a template for objects. They describe what a certain type of object can do and what properties they have in common.

There are many kinds of objects, including String, Number, Array, Hash, Time, ...

To create an object we use .new

array = []
array = Array.new
          

now array refers to an object instance

Object Methods

Methods can be defined on objects.

array = Object.new

def array.delete
  puts "deleted the array"
end

array.methods(false)
          

Instance Variables

Represent object state

Names start with an @

Only visible inside the object

cookie = Object.new

def cookie.add_chips(num_chips)
  @chips = num_chips
end

def cookie.yummy?
  @chips > 100
end

cookie.add_chips(500)
cookie.yummy?   #=> true
          

Object Oriented Programming

• Object Oriented Programming is one of many programming approaches: procedural, event-driven, functional, declarative...
• These days, most programming is done in an OO language: Java, C#, C++.
• All this means is that programs are organized around objects (data & methods).
• You might choose OOP because Objects make things easier to understand and act as a natural way to modularize code. When a large code base is composed of many small Objects, it is easier to maintain and change.

OOP & Easy Understanding

Objects often take on names from their domain which makes them easy to conceptualize.

For example, without seeing my code, you can probably guess what a Bike object can do (or not do)

OOP & Encapsulation

One thing that makes code easier to maintain is Encapsulation which is one of the fundamentals of OOP.

Encapsulation is a nice way of saying 'put all my properties/behaviors in my capsule where other objects can't touch them'.

By assigning certain responsibilities to objects, code is more purposeful and less likely to be changed by accident.

OOP & Inheritance

Just as you inherited traits from your mom, your objects may inherit traits from other objects.

Inheritance is when you use one class as the basis for another.

You might use Inheritance if two objects have the same properties and methods. This way you don't need to write the same thing twice.

class MyString < String
end

my_string = MyString.new
my_string.upcase
          

OOP & Polymorphism

• Similar to inherited traits and behaviors, there are also polymorphic traits and behaviors.
• Polymorphic means many forms.
• Within OOP, this relates to different objects being able to respond to the same message in different ways.

[1, "abc"].each {|object| puts object.next}
          

Questions?

Homework

Practice: Do some of the exercises at the end of Chapter 8 of Learn to Program.

Prep: Read Chapter 9 of Learn To Program- don't try to do the exercises at the end yet, though.