Booleans

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

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

Step 1

Here are some expressions that return true or false:
Type this in irb, hitting Enter after each line:
IRB
15 < 5
15 > 5
15 >= 5
10 == 12

Step 2

Notice we use a double equals sign to check if things are equal. It's a common mistake to use a single equals sign.
Type this in irb, hitting Enter after each line:
IRB
a = 'apple'
b = 'banana'
a == b
puts a + b
a = b
puts a + b
Surprise!

Step 3

For 'not equals', try these:
Type this in irb, hitting Enter after each line:
IRB
a = 'apple'
b = 'banana'
a != b
The exclamation point means the opposite of
Type this in irb, hitting Enter after each line:
IRB
!true
!false
!(a == b)
In !(a == b), Ruby first evaluated a == b, then gave the opposite.
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.

Step 4

We can check more than one condition with and and or . && and || (two pipes) is another notation for and and or.

We do something like this when we Google for 'microsoft and cambridge and not seattle'

Let's type some code into IRB. First, let's define variables:
Type this in irb, hitting Enter after each line:
IRB
yes = true
no = false
Now experiment. Boolean rule 1: AND means everything must be true. For example, true combined with true is true:
Type this in irb, hitting Enter after each line:
IRB
yes and yes
yes && yes
true combined with false fails the test because and means everything must be true:
Type this in irb, hitting Enter after each line:
IRB
yes and no
no and yes
no and no
Boolean rule 2: or says at least one must be true:
Type this in irb, hitting Enter after each line:
IRB
yes or no
yes || no
yes or yes

Step 5

By convention, methods in Ruby that return booleans end with a question mark.
Type this in irb, hitting Enter after each line:
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?

Explanation

In code we ask a lot of questions. Boolean logic gives us tools to express the questions.

Next Step:

Go on to Conditionals

Back to Numbers And Arithmetic