# 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.