booleans.step

message "George Boole was an English mathematician who specialized in logic,
especially logic rules involving true and false.  The Boolean datatype
is named in his honor.

In code, as in life, we base a lot of decisions on whether something is true or
false. *\"If it is raining, then I will bring an umbrella; otherwise I will wear
sunglasses.\"*  In the conditionals section we'll make decisions. First we need to look at true and false."

goals do
  goal "Meet True and False"
  goal "Compare numbers and strings"
  goal "Evaluate 'and', 'or', and 'not' logic"
  goal "Understand methods ending with question marks (predicates)"
end


step do
  message 'Here are some expressions that return `true` or `false`:'
  irb <<-IRB
    15 < 5
    15 > 5
    15 >= 5
    10 == 12
  IRB
end


step do
  message 'Notice we use a double equals sign to check if things are equal.  It\'s a common mistake to use a single equals sign.'
  irb <<-IRB
    a = 'apple'
    b = 'banana'
    a == b
    puts a + b
    a = b
    puts a + b
  IRB
  message "Surprise!"
end

step do
  message "For 'not equals', try these:"
  irb <<-IRB
    a = 'apple'
    b = 'banana'
    a != b
  IRB
  message "The exclamation point means **the opposite of**"
  irb <<-IRB
    !true
    !false
    !(a == b)
  IRB
  message "In `!(a == b)`,  Ruby first evaluated `a == b`, then gave the opposite."
  message "It also means **not true** .  In conditionals, we'll see things like


        if not sunny
            puts \"Bring an umbrella!\"

We can also say

        if sunny == false
            puts \"Bring an umbrella!\"
but \"if not sunny\" is a little more natural sounding.  It's also a little safer
- that double equals is easy to mistype as a single equals."
end

step do
    message "We can check more than one condition with `and` and `or` . `&&` and `||` (two pipes) is another notation for `and` and `or`."
    message "We do something like this when we Google for 'microsoft and cambridge and not seattle'"
    message "Let's type some code into IRB. First, let's define variables:"
    irb <<-IRB
      yes = true
      no = false
    IRB

    message <<-CONTENT
      Now experiment. Boolean rule 1:  AND means everything must be true. For example, `true`
      combined with `true` is `true`:
    CONTENT
    irb <<-IRB
      yes and yes
      yes && yes
    IRB

    message "`true` combined with `false` fails the test because `and` means everything must be true:"
    irb <<-IRB
      yes and no
      no and yes
      no and no
    IRB

    message "Boolean rule 2: `or` says at least one must be true:"
    irb <<-IRB
      yes or no
      yes || no
      yes or yes
    IRB
end


step do
  message 'By convention, methods in Ruby that return booleans end with a question mark.'
  irb <<-IRB
    'sandwich'.end_with?('h')
    'sandwich'.end_with?('z')
    [1,2,3].include?(2)
    [1,2,3].include?(9)
    'is my string'.empty?
    ''.empty?
    'is this nil'.nil?
    nil.nil?
  IRB
end


explanation do
  message "In code we ask a lot of questions. Boolean logic gives us tools to express the questions."
end

further_reading do
    message "Some languages offer wiggle room about what evaluates to true or false. Ruby has very little. See [What's Truthy and Falsey in Ruby?](https://gist.github.com/jfarmer/2647362)"
    message "[What's Truthy and Falsey in Ruby?](https://gist.github.com/jfarmer/2647362) has a more detailed walkthrough of booleans."
    message "Ruby documentation for [true](http://www.ruby-doc.org/core-2.1.0/TrueClass.html) and [false]](http://www.ruby-doc.org/core-2.1.0/TrueClass.html)"
end

next_step "conditionals"