Short-Circuit Evaluation

Part of the Fundamentals section of Coddy's Lua journey — lesson 23 of 90.

and and or operators, it uses a clever optimization called short-circuit evaluation. This means Lua stops evaluating as soon as it knows the final result, potentially skipping parts of the expression entirely.

For the and operator, if the first condition is false, Lua immediately knows the entire expression will be false without checking the second condition:

playerAlive = false
hasWeapon = true

canFight = playerAlive and hasWeapon
-- Lua only checks playerAlive (false), never evaluates hasWeapon

Similarly, for the or operator, if the first condition is true, Lua knows the result will be true and skips the second condition:

hasKey = true
hasPassword = false

canEnter = hasKey or hasPassword
-- Lua only checks hasKey (true), never evaluates hasPassword

Cheat sheet

Lua uses short-circuit evaluation for and and or operators, stopping evaluation as soon as the final result is known.

For and operator: if the first condition is false, the second condition is never evaluated:

playerAlive = false
hasWeapon = true

canFight = playerAlive and hasWeapon
-- Only checks playerAlive (false), skips hasWeapon

For or operator: if the first condition is true, the second condition is never evaluated:

hasKey = true
hasPassword = false

canEnter = hasKey or hasPassword
-- Only checks hasKey (true), skips hasPassword

Try it yourself

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Test yourself

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All lessons in Fundamentals

1Introduction

The Language Hello World!Comments

4Operators 2 Relational & Logic

Equality Operators Relational Operators The 'and' Operator The 'or' Operator The 'not' OperatorShort-Circuit EvaluationTruthy and Falsy Values Recap - Simple Logic

7Basic Conditional Logic

The if-then Statement The if-then-else Statement The elseif Statement Recap - Treasure Chest

10Tables Part 1: List-Style

What is a Table?Creating a Table Accessing Elements by Index Modifying Elements by Index The Length Operator '#'table.insert()table.remove()Recap - Manage Party Members

13Project: Simple Potion Shop

Project Setup Adding a Potion

Displaying the Inventory

Calculating Total Stock Value

Finding a Specific Potion

Selling a Potion

3Operators 1 Arithmetic & Conc

Arithmetic Operators Modulo Operator Exponentiation Operator String Concatenation Operator Precedence Recap - Simple Calculations

6Project: Character Stats Disp

Welcome Message Declare Character Stats

Display Basic Stats

Add a Boolean Flag

Final Summary

9Functions Basics

Declaring a Function Calling a Function Functions with Parameters Functions with Multiple Params The 'return' Statement Recap - Area Calculator

12Loops Basics

The while Loop The repeat-until Loop The Numeric for Loop for Loop with a Step The 'break' Statement Recap - Countdown

15Final Challenges

Recap - Player Turn Recap - Simple Database Recap - Sum of Even Numbers