11. Variables and types

Chapter 7 wrote text directly inside print calls. That works, but any information needed in more than one place has to be retyped every time. Variables fix that. A variable is a name that stands in for a value, so you write the value once and reuse it.

Declaring a variable

The most common shape of a variable declaration in Lua:

local name = "Keiko"
print(name)

Four things are going on in that first line:

• local is a keyword meaning create a new variable that lives inside this file. Always use it. The reason is in the box below.
• name is the variable's name. You pick it.
• = is the assignment operator. It puts the value on the right into the variable on the left. It does not mean "equal to" the way it does in maths.
• "Keiko" is the value being stored.

Now name stands in for the string "Keiko", so print(name) prints Keiko.

Changing a variable

A variable's value can change. Use = again, without local:

local score = 0
print(score)   -- prints 0
score = 50
print(score)   -- prints 50

local belongs only on the first mention of a name. Repeating it makes a separate variable that happens to share the name, which is almost never what you want.

Types

In Lua a value also has a type, not just data. The four types you need right now:

• string — text, written between quotes: "hello", 'a', "" (the empty string).
• number — any number, with or without a decimal point: 0, -7, 3.14.
• boolean — exactly two values: true and false. No quotes.
• nil — the absence of any value at all.

type() is a built-in function that returns a value's type as a string:

print(type("hello"))   -- string
print(type(7))         -- number
print(type(true))      -- boolean
print(type(nil))       -- nil

Variable names

You pick variable names, but Lua has rules about which are allowed:

• They can contain letters, digits, and the underscore _.
• They cannot start with a digit. level1 is fine; 1level is not.
• They are case sensitive. Score, score, and SCORE are three different names.
• They cannot be one of Lua's reserved words: and, break, do, else, elseif, end, false, for, function, goto, if, in, local, nil, not, or, repeat, return, then, true, until, while. You will meet most of these later.

Beyond the rules, follow these habits:

• Choose meaningful names. player_name beats n. hit_points beats hp once code gets long, though hp is fine for short scripts.
• Lower case with underscores is the usual Lua style: max_score, enemy_count. CamelCase (maxScore) is also common, especially in Roblox. Pick one and stick to it inside a file.

Printing a variable

print does not need quotes around variable names. Quotes are only for text you type directly:

local level = 7
print("Level:", level)        -- Level:    7
print("Level: " .. level)     -- Level: 7

The first line uses print's tab separator. The second uses .. to glue "Level: " and the value of level into one string.

Multi-assignment

Lua lets you declare several variables and assign several values on one line. Names on the left pair with values on the right by position:

local name, level = "Keiko", 7
print(name, level)   -- Keiko    7

This groups related declarations neatly. It also enables a handy trick — swapping two variables in one step:

local a, b = 1, 2
a, b = b, a
print(a, b)   -- 2  1

Lua reads the whole right side first, then assigns. So a, b = b, a takes the current b and a and puts them in opposite slots.

Homework

Homework starters are in exercises/11/homework/.

Stuck or finished? Open the homework solutions page.