" #{smth" \n \t" in double quotes

"tatiana".length
=> 7

s.empty?

s.include?("foo")

&&  || !

c.to_s      (str)
==============REVERSE================
"abc".reverse           #cba
============EMPTY/if/else============
 def string_message(str = '')
>>   if str.empty?
>>     "It's an empty string!"
>>   else
>>     "The string is nonempty."
>>   end
>> end
=> :string_message
>> puts string_message("foobar")
The string is nonempty.
===========RETURN=============
>> def string_message(str = '')
>>   return "It's an empty string!" if str.empty?
>>   return "The string is nonempty."
>> end

==================ARRAY=======================
"foo bar     baz".split     # Split a string into a three-element array.
=> ["foo", "bar", "baz"]

"fooxbarxbaz".split('x')
=> ["foo", "bar", "baz"]

>> a = [42, 8, 17]
=> [42, 8, 17]
>> a[0]               # Ruby uses square brackets for array access.
=> 42
>> a[1]
=> 8
>> a[2]
=> 17
>> a[-1]              # Indices can even be negative!
=> 17

>> a                  # Just a reminder of what 'a' is
=> [42, 8, 17]
>> a.first
=> 42
>> a.second
=> 8
>> a.last
=> 17
>> a.last == a[-1]    # Comparison using ==
=> true

>> a
=> [42, 8, 17]
>> a.empty?
=> false
>> a.include?(42)
=> true
>> a.sort
=> [8, 17, 42]
>> a.reverse
=> [17, 8, 42]
>> a.shuffle
=> [17, 42, 8]
>> a
=> [42, 8, 17]

>> a.sort!                                  sort! - chanche the arr
=> [8, 17, 42]
>> a
=> [8, 17, 42]

a.push(6)                  # Pushing 6 onto an array
=> [42, 8, 17, 6]
>> a << 7                     # Pushing 7 onto an array
=> [42, 8, 17, 6, 7]
>> a << "foo" << "bar"        # Chaining array pushes
=> [42, 8, 17, 6, 7, "foo", "bar"]

>> a.join                       # Join on nothing.
=> "4281767foobar"
>> a.join(', ')                 # Join on comma-space.
=> "42, 8, 17, 6, 7, foo, bar"

>> (0..9).to_a            # Use parentheses to call to_a on the range.
=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

=============Ranges========================
>> a = %w[foo bar baz quux]         # Use %w to make a string array.
=> ["foo", "bar", "baz", "quux"]
>> a[0..2]
=> ["foo", "bar", "baz"]

a[2..(a.length-1)]               # Explicitly use the array's length.
=> [2, 3, 4, 5, 6, 7, 8, 9]

==================================
3.times { puts "Betelgeuse!" }   # 3.times takes a block with no variables.
"Betelgeuse!"
"Betelgeuse!"
"Betelgeuse!"
=> 3
>> (1..5).map { |i| i**2 }          # The ** notation is for 'power'.
=> [1, 4, 9, 16, 25]
>> %w[a b c]                        # Recall that %w makes string arrays.
=> ["a", "b", "c"]
>> %w[a b c].map { |char| char.upcase }
=> ["A", "B", "C"]
>> %w[A B C].map { |char| char.downcase }
=> ["a", "b", "c"]
 -------------or----------------
%w[A B C].map(&:downcase)
=> ["a", "b", "c"]

('a'..'z').to_a.shuffle[0..7].join
==============================Hash============================
user = { "first_name" => "Michael", "last_name" => "Hartl" }
=> {"last_name"=>"Hartl", "first_name"=>"Michael"}

>> "name".split('')
=> ["n", "a", "m", "e"]
>> :name.split('')
NoMethodError: undefined method `split' for :name:Symbol
>> "foobar".reverse
=> "raboof"
>> :foobar.reverse
NoMethodError: undefined method `reverse' for :foobar:Symbol

>> user = { :name => "Michael Hartl", :email => "michael@example.com" }
=> {:name=>"Michael Hartl", :email=>"michael@example.com"}

>> h1 = { :name => "Michael Hartl", :email => "michael@example.com" }
=> {:name=>"Michael Hartl", :email=>"michael@example.com"}
>> h2 = { name: "Michael Hartl", email: "michael@example.com" }
=> {:name=>"Michael Hartl", :email=>"michael@example.com"}
>> h1 == h2
=> true

>> params[:user] = { name: "Michael Hartl", email: "mhartl@example.com" }
=> {:name=>"Michael Hartl", :email=>"mhartl@example.com"}
>> params
=> {:user=>{:name=>"Michael Hartl", :email=>"mhartl@example.com"}}
>>  params[:user][:email]
=> "mhartl@example.com"

>> params = {}        # Define a hash called 'params' (short for 'parameters').       !!!!!
=> {}
>> params[:user] = { name: "Michael Hartl", email: "mhartl@example.com" }
=> {:name=>"Michael Hartl", :email=>"mhartl@example.com"}
>> params
=> {:user=>{:name=>"Michael Hartl", :email=>"mhartl@example.com"}}
>>  params[:user][:email]
=> "mhartl@example.com"
-----------------------------------------------------------
person1 = {:first => 'Al',    :last => 'Bundy'}
person2 = {:first => 'Peggy', :last => 'Bundy'}
person3 = {:first => 'Kelly', :last => 'Bundy'}
params = {
 :father => person1,
 :mother => person2,
 :child  => person3
}
params[:father][:first] #=> 'Al'
=============CSS==================
stylesheet_link_tag 'application', { media: 'all',
                                     'data-turbolinks-track': 'reload' }
stylesheet_link_tag 'application', media: 'all',
                                   'data-turbolinks-track': 'reload'


=================================Validation===============================================
>> %w[foo bar baz]
=> ["foo", "bar", "baz"]
>> addresses = %w[USER@foo.COM THE_US-ER@foo.bar.org first.last@foo.jp]
=> ["USER@foo.COM", "THE_US-ER@foo.bar.org", "first.last@foo.jp"]
>> addresses.each do |address|
?>   puts address
>> end
USER@foo.COM
THE_US-ER@foo.bar.org
first.last@foo.jp