Booleans and Types

Besides numbers and texts, Lua has one more simple kind of value: a boolean (named after the mathematician George Boole). It has only two values, true and false. It's the answer to a yes-or-no question: is it raining? Is the barn open?

local isRaining = false
local barnOpen = true
print(isRaining)
print("Barn open:", barnOpen)

This prints:

false
Barn open:	true

In the next lesson, booleans will decide what your program does, for example whether Emma waters the field. For now, watch out for three things:

• true and false are keywords, so they're written without quotes. "true" in quotes is a text that only looks like a boolean.
• They're written in lowercase. True with a capital letter is just a name, and since there's no variable called True, its value is nil.
• Like nil, a boolean can't be joined to a text with ..: "Raining: " .. isRaining ends with the error attempt to concatenate a boolean value (local 'isRaining'). Print it with a comma, or turn it into a text first, as you'll learn in the next exercise.

Every value has a type: a number, a text, a boolean… The function type tells you which one, as a text:

print(type(120))
print(type(2.5))
print(type("Emma"))
print(type(true))
print(type(nil))

This prints:

number
number
string
boolean
nil

type says number for integers and floats alike; to tell them apart, use math.type. Later in the course, you'll meet more types, such as table and function (try print(type(print))).

In Lua, the type belongs to the value, not to the variable. The same variable can hold a number now and a text later:

local harvest = 12
print(type(harvest))
harvest = "twelve"
print(type(harvest))

This prints:

number
string

Lua allows it, but it confuses anyone who reads the program. Keep one kind of value in each variable.