lecture-04/index.html at gh-pages · coreruby/lecture-04 · GitHub

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<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8" />
    <title>Enumerable</title>
    <link href="css/reveal.css" rel="stylesheet" />
    <link href="css/simple.css" rel="stylesheet" />
    <link href="css/googlecode.css" rel="stylesheet" />
  </head>
  <body>
    <div class="reveal">
      <div class="slides"><section class="center" data-background="#000">
  <h1 class="white">
    Enumerable
  </h1>
</section>
<section>
  <h2>
    Gotchas
  </h2>
  <pre><code class="ruby">assert_equal {one: 1, two: 2}, [[:one, 1], [:two, 2]].to_hash </code></pre>
</section>
<section>
  <h2>
    Gotchas
  </h2>
  <pre><code class="ruby">syntax error, unexpected ':', expecting '}' (SyntaxError)
   assert_equal {one: 1, two: 2}, [[:one, 1], [:two, 2]].to_hash
                     ^ </code></pre>
</section>
<section>
  <h2>
    Gotchas
  </h2>
  <pre><code class="ruby">assert_equal({one: 1, two: 2}, [[:one, 1], [:two, 2]].to_hash) </code></pre>
</section>
<section>
  <h2>
    Gotchas
  </h2>
  <pre><code class="ruby">def index_by(*block)
  # ...
end </code></pre>
</section>
<section>
  <h2>
    Gotchas
  </h2>
  <pre><code class="ruby">def index_by(&block)
  # ...
end </code></pre>
</section>
<section>
  <h2>
    Gotchas
  </h2>
  <pre><code class="ruby">class Array
  def to_hash
    hash = {}
    map { |pair| hash[pair[0]] = pair[1] }
    hash
  end
end </code></pre>
</section>
<section class="center" data-background="#000">
  <p class="white">
    This is still Enumerable#each
  </p>
</section>
<section class="center" data-background="#000">
  <h2 class="white">
    Questions
  </h2>
</section>
<section class="center">
  <p>
    What is a block?
  </p>
</section>
<section class="center">
  <p>
    How can I pass an argument from a method to the block?
  </p>
</section>
<section class="center">
  <p>
    What is a proc?
  </p>
</section>
<section class="center">
  <p>
    What is a lambda?
  </p>
</section>
<section class="center">
  <p>
    What is the difference between proc and lambda?
  </p>
</section>
<section class="center" data-background="#000">
  <h2 class="white">
    Challenge
  </h2>
</section>
<section class="class">
  <p>
    Write me a method that yields its block 5 times
  </p>
</section>
<section class="class">
  <p>
    Write me a method that yields the numbers: 1..5
  </p>
</section>
<section class="class">
  <p>
    Write me a method that yields the numbers: 1..n
  </p>
</section>
<section class="class">
  <p>
    Write me a method that yields the numbers: 1..n and their squares
  </p>
</section>
<section class="class">
  <p>
    Write me a method that yields the numbers: 1..n and their squares and their cubes
  </p>
</section>
<section class="center" data-background="#000">
  <h2 class="white">
    Enumerable
  </h2>
</section>
<section>
  <h2>
    Enumerable#each
  </h2>
  <p>
    The heart of it all
  </p>
</section>
<section>
  <h2>
    Enumerable#each
  </h2>
  <p>
    Every other method in Enumerable is implemented in the terms of #each
  </p>
</section>
<section>
  <p>
    A powerful thing
  </p>
</section>
<section>
  <p>
    All this machinery is moved by a single method
  </p>
</section>
<section>
  <h2>
    Enumerable#each
  </h2>
  <p>
    Executes its block for each element in the collection
  </p>
</section>
<section>
  <h2>
    Enumerable#each
  </h2>
  <p>
    Nothing less, nothing more
  </p>
</section>
<section>
  <h2>
    Enumerable#each
  </h2>
  <pre><code class="ruby">['Vitalii', 'Stamat', 'Larodi'].each do |person|
  send_email(person)
end </code></pre>
</section>
<section>
  <h2>
    Enumerable#each
  </h2>
  <pre><code class="ruby">{'Genadi' => 'Samokovarov', 'Radoslav' => 'Georgiev'}.each do |first_name, last_name|
  send_email(first_name, last_name)
end </code></pre>
</section>
<section>
  <h2>
    Enumerable#each
  </h2>
  <p>
    We (usually) don't use the result of each
  </p>
</section>
<section class="center" data-background="#000">
  <h2 class="white">
    Side Effect
  </h2>
</section>
<section>
  <h2>
    Side Effect
  </h2>
  <p>
    An expression, that isn't used for its result is a side effect
  </p>
</section>
<section>
  <h2>
    Question
  </h2>
  <p>
    Is this a side effect?
  </p>
  <pre><code class="ruby">{'Genadi' => 'Samokovarov', 'Radoslav' => 'Georgiev'}.each do |first_name, last_name|
  send_email(first_name, last_name)
end </code></pre>
</section>
<section>
  <h2>
    Question
  </h2>
  <p>
    Why?
  </p>
  <pre><code class="ruby">{'Genadi' => 'Samokovarov', 'Radoslav' => 'Georgiev'}.each do |first_name, last_name|
  send_email(first_name, last_name)
end </code></pre>
</section>
<section>
  <h2>
    Question
  </h2>
  <p>
    Is this a side effect?
  </p>
  <pre><code class="ruby">puts "Hello World" </code></pre>
</section>
<section>
  <h2>
    Question
  </h2>
  <p>
    Why?
  </p>
  <pre><code class="ruby">puts "Hello World" </code></pre>
</section>
<section>
  <h2>
    Question
  </h2>
  <p>
    Is this a side effect?
  </p>
  <pre><code class="ruby">squares = 1.upto(10).map { |number| number ** 2 } </code></pre>
</section>
<section>
  <h2>
    Question
  </h2>
  <p>
    Why?
  </p>
  <pre><code class="ruby">squares = 1.upto(10).map { |number| number ** 2 } </code></pre>
</section>
<section class="center" data-background="#000">
  <h2 class="white">
    Enumerable#map
  </h2>
</section>
<section>
  <h2>
    Enumerable#map
  </h2>
  <p>
    Invokes the given block once for each element of self
  </p>
</section>
<section>
  <h2>
    Enumerable#map
  </h2>
  <p>
    Creates a new array containing the values returned by the block.
  </p>
</section>
<section class="center" data-background="#000">
  <pre class="white">[  a1,   a2,    a3,    a4    ...    an ]
   |      |      |      |           |
   |      |      |      |           |
   |      |      |      |           |
   |      |      |      |           |
   |      |      |      |           |
   .      .      .      .           .
[f(a1), f(a2), f(a3), f(a4)  ...  f(an)] </pre>
</section>
<section class="center">
  <pre><code class="ruby" contenteditable="">class Array
  def cubes
    # Your code goes here.
  end
end </code></pre>
</section>
<section class="center">
  <pre><code class="ruby">class Array
  def cubes
    map { |number| number ** 3 }
  end
end </code></pre>
</section>
<section>
  <h2>
    Enumerable#map
  </h2>
  <p>
    How does it relates to each?
  </p>
</section>
<section class="center">
  <pre><code class="ruby" contenteditable="">class Array
  def cubes
    map { |number| number ** 3 }
  end
end </code></pre>
</section>
<section class="center">
  <pre><code class="ruby">class Array
  def cubes
    cubes = []
    each { |number| cubes << number ** 3 }
    cubes
  end
end </code></pre>
</section>
<section>
  <h2>
    Enumerable#map
  </h2>
  <p>
    Let's generalize it
  </p>
</section>
<section class="center">
  <pre><code class="ruby" contenteditable="">module Enumerable
  def map
    # Your code goes here.
  end
end </code></pre>
</section>
<section class="center">
  <pre><code class="ruby">module Enumerable
  def map
    result = []
    each { |element| result << yield(element) }
    result
  end
end </code></pre>
</section>
<section>
  <h2>
    Enumerable#map
  </h2>
  <p>
    Its nothing more than that
  </p>
</section>
<section>
  <h2>
    Enumerable#map
  </h2>
  <p>
    Let's checkout how <a href="https://github.com/rubinius/rubinius/blob/master/kernel/common/enumerable.rb#L45-L57">Rubinius</a> implemented it
  </p>
</section>
<section>
  <h2>
    Enumerable#collect
  </h2>
  <p>
    Its just naming, Enumerable#collect == Enumerable#map
  </p>
</section>
<section class="center" data-background="#000">
  <p>
    Look for this pattern in your code and convert it to Enumerable#map
  </p>
</section>
<section class="center">
  <pre><code class="ruby">def transform(data)
  result = []
  data.each { |element| result << operation(element) }
  result
end </code></pre>
</section>
<section class="center" data-background="#000">
  <h2 class="white">
    Enumerable#select
  </h2>
</section>
<section>
  <h2>
    Enumerable#select
  </h2>
  <p>
    Returns an array containing all elements of the collection for which the given block returns a true value
  </p>
</section>
<section>
  <h2>
    Enumerable#select
  </h2>
  <p>
    We talked about it as filter
  </p>
</section>
<section>
  <h2>
    Enumerable#select
  </h2>
  <p>
    Its just naming, the idea is the same
  </p>
</section>
<section class="center" data-background="#000">
  <pre class="white">[ a1,          a2,          a3          ... an          ]
  |            |            |               |
  |            |            |               |
  |            |            |               |
  |            |            |               |
  |            |            |               |
  .            .            .               .

[ a1 if f(a1), a2 if f(a2), a3 if f(a3) ... an if f(an) ] </pre>
</section>
<section>
  <h2>
    Enumerable#select
  </h2>
  <p>
    The idea is to filter only the elements we need for future operation
  </p>
</section>
<section class="center">
  <pre><code class="ruby" contenteditable="">require 'prime'

class Range
  def primes
    # Your code goes here.
  end
end </code></pre>
</section>
<section class="center">
  <pre><code class="ruby">require 'prime'

class Range
  def primes
    select { |number| number.prime? }
  end
end </code></pre>
</section>
<section>
  <h2>
    Enumerable#select
  </h2>
  <p>
    Let's generalize it
  </p>
</section>
<section class="center">
  <pre><code class="ruby" contenteditable="">module Enumerable
  def select
    # Your code goes here.
  end
end </code></pre>
</section>
<section class="center">
  <pre><code class="ruby">module Enumerable
  def select
    result = []
    each { |element| result << element if yield(element) }
    result
  end
end </code></pre>
</section>
<section>
  <h2>
    Enumerable#select
  </h2>
  <p>
    Let's checkout how <a href="https://github.com/rubinius/rubinius/blob/master/kernel/common/enumerable.rb#L450-L461">Rubinius</a> implemented it
  </p>
</section>
<section class="center" data-background="#000">
  <h2 class="white">
    Enumerable#reject
  </h2>
</section>
<section>
  <h2>
    Enumerable#reject
  </h2>
  <p>
    The opposite of Enumerable#select
  </p>
</section>
<section class="center">
  <pre><code class="ruby" contenteditable="">module Enumerable
  def reject
    # Your code goes here.
  end
end </code></pre>
</section>
<section class="center">
  <pre><code class="ruby">module Enumerable
  def reject
    result = []
    each { |element| result << element unless yield(element) }
    result
  end
end </code></pre>
</section>
<section class="center" data-background="#000">
  <h2 class="white">
    Enumerable#reduce
  </h2>
</section>
<section>
  <h2>
    Enumerable#reduce
  </h2>
  <p>
    Reduces a collection of elements into a single result
  </p>
</section>
<section class="center" data-background="#000">
  <pre class="white">a0: initial value; f: operation

f(a0, a1) </pre>
</section>
<section class="center" data-background="#000">
  <pre class="white">a0: initial value; f: operation

f(f(a0, a1), a2) </pre>
</section>
<section class="center" data-background="#000">
  <pre class="white">a0: initial value; f: operation

f(f(f(a0, a1), a2), a3) </pre>
</section>
<section class="center" data-background="#000">
  <pre class="white">a0: initial value; f: operation

f(f(f(a0, a1), a2), a3) </pre>
</section>
<section class="center" data-background="#000">
  <pre class="white">a0: 0 f: +

(((1 + 2) + 3) + 4) + 5 </pre>
</section>
<section>
  <h2>
    Enumerable#reduce
  </h2>
  <pre><code class="ruby" contenteditable=""># Same using a block and inject
(5..10).inject { |sum, n| sum + n }            #=> 45

# Sum some numbers
(5..10).reduce(:+)                             #=> 45 </code></pre>
</section>
<section>
  <h2>
    Enumerable#reduce
  </h2>
  <pre><code class="ruby" contenteditable=""># Same using a block
(5..10).inject(1) { |product, n| product * n } #=> 151200

# Multiply some numbers
(5..10).reduce(1, :*)                          #=> 151200 </code></pre>
</section>
<section>
  <h2>
    Enumerable#reduce
  </h2>
  <pre><code class="ruby" contenteditable=""># find the longest word
longest = %w(cat sheep bear).inject do |memo, word|
   memo.length > word.length ? memo : word
end
longest </code></pre>
</section>
<section class="center" data-background="#000">
  <h2 class="white">
    Enumerable#include?
  </h2>
</section>
<section>
  <h2>
    Enumerable#include?
  </h2>
  <p>
    Returns true if any member of collection equals a target object
  </p>
</section>
<section>
  <h2>
    Enumerable#include?
  </h2>
  <p>
    Equality is tested using ==
  </p>
</section>
<section class="center">
  <pre><code class="ruby" contenteditable="">module Enumerable
  def include?(obj)
    # Your code goes here.
  end
end </code></pre>
</section>
<section class="center">
  <pre><code class="ruby">module Enumerable
  def include?(obj)
    each { |element| return true if element == obj }
    false
  end
end </code></pre>
</section>
<section class="center" data-background="#000">
  <h2 class="white">
    Enumerable#any?
  </h2>
</section>
<section>
  <h2>
    Enumerable#any?
  </h2>
  <p>
    Passes each element of the collection to the given block.
  </p>
</section>
<section>
  <h2>
    Enumerable#any?
  </h2>
  <p>
    The method returns true if the block ever returns a value other than false or nil.
  </p>
</section>
<section>
  <h2>
    Enumerable#any?
  </h2>
  <p>
    If the block is not given, Ruby adds an implicit block of { |obj| obj }
  </p>
</section>
<section>
  <h2>
    Enumerable#any?
  </h2>
  <p>
    Return true if at least one of the collection members is not false or nil
  </p>
</section>
<section class="center">
  <pre><code class="ruby" contenteditable="">module Enumerable
  def any?
    # Your code goes here.
  end
end </code></pre>
</section>
<section class="center">
  <pre><code class="ruby">module Enumerable
  def any?
    each { |element| return true if yield(element) }
    false
  end
end </code></pre>
</section>
<section class="center" data-background="#000">
  <h2 class="white">
    Enumerable#all?
  </h2>
</section>
<section>
  <h2>
    Enumerable#all?
  </h2>
  <p>
    Passes each element of the collection to the given block.
  </p>
</section>
<section>
  <h2>
    Enumerable#all?
  </h2>
  <p>
    The method returns true if the block never returns false or nil
  </p>
</section>
<section>
  <h2>
    Enumerable#all?
  </h2>
  <p>
    Return true when none of the collection members are false or nil
  </p>
</section>
<section class="center">
  <pre><code class="ruby" contenteditable="">module Enumerable
  def all?
    # Your code goes here.
  end
end </code></pre>
</section>
<section class="center">
  <pre><code class="ruby">module Enumerable
  def all?
    each { |element| return false unless yield(element) }
    true
  end
end </code></pre>
</section>
<section class="center">
  <h2>
    Break, Redo & Next
  </h2>
</section>
<section>
  <h2>
    break
  </h2>
  <p>
    Break terminates the immediately enclosing loop
  </p>
</section>
<section>
  <h2>
    break
  </h2>
  <p>
    Control resumes at the statement following the block
  </p>
</section>
<section>
  <h2>
    redo
  </h2>
  <p>
    Repeats the loop from the start, without reevaluating the condition or fetching the next element
  </p>
</section>
<section>
  <h2>
    next
  </h2>
  <p>
    Skips to the end of the loop, effectively starting the next iteration
  </p>
</section>
<section class="center">
  <p>
    <a href="http://ruby-doc.com/docs/ProgrammingRuby/html/tut_expressions.html#UL">More</a>
  </p>
</section>
<section class="center" data-background="#000">
  <p class="white">
    Implementing #each turns a class into a collection
  </p>
</section>
<section>
  <h2>
    Enumerable
  </h2>
  <pre><code class="ruby" contenteditable="">class Count
  include Enumerable

  def initialize(count)
    @count = count
  end

  def each
    @count.times { |iteration| yield iteration }
  end
end </code></pre>
</section>
<section>
  <h2>
    Enumerable
  </h2>
  <pre><code class="ruby" contenteditable="">counter = Counter.new(5)
counter.each { |time| puts time } </code></pre>
</section>
<section>
  <h2>
    Enumerable
  </h2>
  <p>
    Top-level constants that are Enumerable
  </p>
  <p class="fragment">
    Array, Hash, Struct, Range, IO, File, Dir, Enumerator, StringIO
  </p>
</section>
<section>
  <h2>
    Enumerable
  </h2>
  <p>
    Top-level constants that are Enumerable
  </p>
  <pre><code class="ruby" contenteditable="">Object.constants.map { |sym| Object.const_get sym } </code></pre>
</section>
<section>
  <h2>
    Enumerable
  </h2>
  <p>
    Top-level constants that are Enumerable
  </p>
  <pre><code class="ruby" contenteditable="">Object.constants.map { |sym| Object.const_get sym }.select { |const|
  const.is_a? Module and const.ancestors.include? Enumerable
} </code></pre>
</section>
<section class="center" data-background="#000">
  <p class="white">
    Array
  </p>
</section>
<section>
  <h2>
    Array
  </h2>
  <p>
    A mutable collection, backed by an actual array
  </p>
</section>
<section>
  <h2>
    Array#[]
  </h2>
  <pre><code class="ruby" contenteditable="">>> array = []
=> []

>> array[0] == nil
=> true </code></pre>
</section>
<section>
  <h2>
    Array#[]
  </h2>
  <pre><code class="ruby" contenteditable="">>> array = [1, 2]
=> [1, 2]

>> array[0]  == 1
=> true

>> array[-1] == 2
=> true </code></pre>
</section>
<section>
  <h2>
    Array#[]=
  </h2>
  <pre><code class="ruby" contenteditable="">>> array = []
=> []

>> array[2] = 3
=> 3

>> array
=> [nil, nil, 3] </code></pre>
</section>
<section>
  <h2>
    Array#first
  </h2>
  <pre><code class="ruby" contenteditable="">>> array = [1, 2]
=> [1, 2]

>> array.first == array[0] == 1
=> true </code></pre>
</section>
<section>
  <h2>
    Array#last
  </h2>
  <pre><code class="ruby" contenteditable="">>> array = [1, 2, 3]
=> [1, 2, 3]

>> array.last == array[-1] == 3
=> true </code></pre>
</section>
<section>
  <h2>
    Array#uniq
  </h2>
  <p>
    Selects only the unique elements
  </p>
  <pre><code class="ruby" contenteditable="">>> array = [1, 2, 3, 3, 2, 1]
=> [1, 2, 3, 3, 2, 1]

>> array.uniq
=> [1 2, 3] </code></pre>
</section>
<section>
  <h2>
    Array#compact
  </h2>
  <p>
    Selects only the truthy elements
  </p>
  <pre><code class="ruby" contenteditable="">>> array = [0, "", false, nil]
=> [0, "", false, nil]

>> array.compact
=> [0, ""] </code></pre>
</section>
<section class="center" data-background="#000">
  <p class="white">
    Hash
  </p>
</section>
<section>
  <h2>
    Hash
  </h2>
  <p>
    Hashes are used to store key, value pairs
  </p>
</section>
<section>
  <h2>
    Hash
  </h2>
  <p>
    Looking up a key happens in constant time
  </p>
</section>
<section>
  <h2>
    Hash
  </h2>
  <p>
    Which basically means its fast :)
  </p>
</section>
<section>
  <h2>
    Hash
  </h2>
  <p>
    Uses the same literal as the block, kinda
  </p>
  <pre><code class="ruby">{ 'key' => :value, [] => {}, Object.new => Enumerable } </code></pre>
</section>
<section>
  <h2>
    Hash
  </h2>
  <p>
    This creates problems if the first argument of a method is a hash
  </p>
  <pre><code class="ruby">assert_equal {one: 1, two: 2}, [[:one, 1], [:two, 2]].to_hash
syntax error, unexpected ':', expecting '}' (SyntaxError)
   assert_equal {one: 1, two: 2}, [[:one, 1], [:two, 2]].to_hash </code></pre>
</section>
<section>
  <h2>
    Hash
  </h2>
  <p>
    They can store any value as a key or value
  </p>
  <pre><code class="ruby">{ 'key' => :value, [] => {}, Object.new => Enumerable } </code></pre>
</section>
<section>
  <h2>
    Hash
  </h2>
  <p>
    Shortcut syntax for symbol keys
  </p>
  <pre><code class="ruby">{ symbol_as_key: 42 } </code></pre>
</section>
<section>
  <h2>
    Hash#[]
  </h2>
  <p>
    By default the missing keys are nil
  </p>
  <pre><code class="ruby">>> hash = {}
=> {}