Operators in Lua

Beginner ~20 min read

Operators are symbols that perform operations on values and variables. Lua provides a comprehensive set of operators for arithmetic, comparison, logic, and string manipulation. Understanding operators is fundamental to writing effective Lua code. Let's explore all the operators Lua has to offer!

Arithmetic Operators

Lua supports all standard arithmetic operations:

Operator	Description	Example	Result
`+`	Addition	`10 + 5`	15
`-`	Subtraction	`10 - 5`	5
`*`	Multiplication	`10 * 5`	50
`/`	Division	`10 / 5`	2
`%`	Modulo (remainder)	`10 % 3`	1
`^`	Exponentiation	`2 ^ 3`	8
`-`	Unary negation	`-10`	-10

Try It Yourself

-- Arithmetic Operators
local a = 10
local b = 3

print("=== Arithmetic Operators ===")
print("a + b =", a + b)   -- Addition: 13
print("a - b =", a - b)   -- Subtraction: 7
print("a * b =", a * b)   -- Multiplication: 30
print("a / b =", a / b)   -- Division: 3.333...
print("a % b =", a % b)   -- Modulo (remainder): 1
print("a ^ b =", a ^ b)   -- Exponentiation: 1000
print("a // b =", a // b) -- Floor division: 3 (Lua 5.3+)

-- Unary minus
print("-a =", -a)         -- Negation: -10

-- Relational Operators
print("\n=== Relational Operators ===")
print("a == b:", a == b)  -- Equal to: false
print("a ~= b:", a ~= b)  -- Not equal to: true
print("a > b:", a > b)    -- Greater than: true
print("a < b:", a < b)    -- Less than: false
print("a >= b:", a >= b)  -- Greater or equal: true
print("a <= b:", a <= b)  -- Less or equal: false

-- Logical Operators
print("\n=== Logical Operators ===")
local x = true
local y = false

print("x and y:", x and y)  -- Logical AND: false
print("x or y:", x or y)    -- Logical OR: true
print("not x:", not x)      -- Logical NOT: false
print("not y:", not y)      -- Logical NOT: true

-- String Concatenation
print("\n=== String Concatenation ===")
local first = "Hello"
local last = "World"
print(first .. " " .. last)  -- "Hello World"

-- Length Operator
print("\n=== Length Operator ===")
local str = "Lua"
local arr = {10, 20, 30, 40}
print("#str =", #str)  -- Length of string: 3
print("#arr =", #arr)  -- Length of array: 4

Output

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Tip: The ^ operator is for exponentiation, not XOR like in some other languages. Lua doesn't have built-in bitwise operators in Lua 5.1, but they were added in Lua 5.2+.

Relational Operators

Relational operators compare two values and return true or false:

Operator	Description	Example	Result
`==`	Equal to	`5 == 5`	true
`~=`	Not equal to	`5 ~= 3`	true
`<`	Less than	`3 < 5`	true
`>`	Greater than	`5 > 3`	true
`<=`	Less than or equal	`3 <= 5`	true
`>=`	Greater than or equal	`5 >= 5`	true

Important: Lua uses ~= for "not equal", not != like many other languages. Also, use == for comparison, not = (which is assignment).

Logical Operators

Logical operators work with boolean values and use short-circuit evaluation:

Operator	Description	Example	Result
`and`	Logical AND	`true and false`	false
`or`	Logical OR	`true or false`	true
`not`	Logical NOT	`not true`	false

Short-Circuit Evaluation

Lua uses short-circuit evaluation for and and or:

and returns the first operand if it's false, otherwise the second
or returns the first operand if it's true, otherwise the second

-- Short-circuit evaluation with 'and' and 'or'

-- 'and' returns first falsy value or last value
print("=== AND operator ===")
print("true and 5:", true and 5)      -- 5
print("false and 5:", false and 5)    -- false
print("nil and 5:", nil and 5)        -- nil
print("5 and 10:", 5 and 10)          -- 10

-- 'or' returns first truthy value or last value
print("\n=== OR operator ===")
print("true or 5:", true or 5)        -- true
print("false or 5:", false or 5)      -- 5
print("nil or 5:", nil or 5)          -- 5
print("5 or 10:", 5 or 10)            -- 5

-- Practical use: Default values
print("\n=== Default Values ===")
local name = nil
local displayName = name or "Guest"
print("Display name:", displayName)   -- "Guest"

local score = 0
local finalScore = score or 100
print("Final score:", finalScore)     -- 0 (not 100! 0 is truthy)

-- Ternary-like expression
print("\n=== Ternary-like ===")
local age = 20
local status = (age >= 18) and "Adult" or "Minor"
print("Status:", status)  -- "Adult"

Output

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Best Practice: Use short-circuit evaluation for default values: local value = input or "default". This is a common Lua idiom!

String Concatenation

Lua uses .. (two dots) for string concatenation:

local greeting = "Hello" .. " " .. "World"
print(greeting)  -- Hello World

local name = "Alice"
local message = "Welcome, " .. name .. "!"
print(message)  -- Welcome, Alice!

Tip: Numbers are automatically converted to strings when concatenated: "Score: " .. 100 works fine!

Length Operator

The # operator returns the length of strings and tables:

local str = "Hello"
print(#str)  -- 5

local arr = {10, 20, 30, 40}
print(#arr)  -- 4

Operator Precedence

When multiple operators appear in an expression, Lua follows this precedence (highest to lowest):

^ (exponentiation)
not, #, - (unary)
*, /, %
+, -
.. (concatenation)
<, >, <=, >=, ~=, ==
and
or

Output

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Best Practice: Use parentheses to make your intentions clear, even if not strictly necessary. (a + b) * c is clearer than

a + b *
                                            c

Practice Exercise

Try solving these operator challenges:

Output

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Summary

In this lesson, you learned:

Arithmetic operators: +, -, *, /, %, ^
Relational operators: ==, ~=, <, >, <=, >=
Logical operators: and, or, not
String concatenation with ..
Length operator #
Operator precedence and short-circuit evaluation

What's Next?

Now that you understand operators, you're ready to learn about strings in Lua. In the next lesson, we'll explore string manipulation, pattern matching, and the powerful string library. Let's continue! 🚀

Previous Variables & Types

Next Strings

Arithmetic Operators

Try It Yourself

Relational Operators

Logical Operators

Short-Circuit Evaluation

String Concatenation

Length Operator

Operator Precedence

Practice Exercise

Summary

What's Next?

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