yangsu

a = [5, 3, 4, 1]
a.sort # => [1, 3, 4, 5]
a.any? {|i| i > 6} # => false
a.any? {|i| i > 4} # => true
a.all? {|i| i > 4} # => false
a.all? {|i| i > 0} # => true
a.collect {|i| i * 2} # => [10, 6, 8, 2]
a.select {|i| i % 2 == 0 } # even => [4]
a.select {|i| i % 2 == 1 } # odd => [5, 3, 1]
a.max # => 5

yangsu

a = [5, 3, 4, 1]
a.inject(0) {|sum, i| sum + i}
# => 13
a.inject {|sum, i| sum + i}
# => 13
a.inject {|product, i| product * i}
# = 560
a.inject(0) do |sum, i|
 puts "sum: #{sum} i: #{i} sum + i: #{sum + i}"
 sum + i

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class Department < ActiveRecord::Base
  has_many :employees
  has_one :manager
end

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class NilClass
  def blank?
    true
  end
end

class String
  def blank?
    self.size == 0
  end

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class Roman
  def self.method_missing name, *args
    roman = name.to_s
    roman.gsub!("IV", "IIII")
    roman.gsub!("IX", "VIIII")
    roman.gsub!("XL", "XXXX")
    roman.gsub!("XC", "LXXXX")
    (
      roman.count("I") +
      roman.count("V") * 5 +

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class Person
  attr_accessor :name
  def self.can_speak  # This is a class method! Notice the self.
    define_method 'speak' do  # an instance method
      puts "I can talk, my name is #{@name}!"
    end
  end

  def initialize(name)
    @name = name

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module Person
  attr_accessor :name
  def self.included(base) # included is invoked whenever a module is included; base is implicit
    base.extend ClassMethods  # extend will add the methods defined in ClassMethods as class methods
  end
  module ClassMethods
    def can_speak
      include InstanceMethods # This includes all the instance methods
    end
  end

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class Tree
  # Define instance variables children and node_name
  # and the associated getters and setters
  attr_accessor :children, :node_name

  # Define a constructor with 2 parameters, with the second having a default value
  def initialize(name, children=[])
    @children = children
    @node_name = name
  end

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1 + 2 // 2
1e2 * 2.5 // 250
Number.MAX_VALUE / Number.MIN_VALUE // Infinity
Infinity - Infinity // NaN
-Infinity + NaN // NaN
Math.log(Math.pow(Math.E, 200)) // 200

yangsu

"hello".charAt(0) // h
'cat'.charAt(1) == 'cat'[1] // true
"hello, world".replace("hello", "goodbye") // goodbye, world
"hello".toUpperCase() // HELLO
'Duke PL Course'.substring(5,7) // 'PL'
"1, 2, 3".split(', ') // ['1', '2', '3']
'Duke PL Course'.indexOf('Courses') // -1