Lua - First Program

Overview

Estimated time: 15–20 minutes

In this tutorial, you'll write and run your first Lua programs. We'll start with the classic "Hello, World!" and gradually introduce basic concepts like variables, functions, and user input.

Learning Objectives

• Write and run your first Lua program
• Understand basic Lua syntax
• Learn about the print function
• Explore variables and basic operations
• Create simple interactive programs

Prerequisites

• Lua installed on your system
• Text editor or IDE ready
• Basic command line knowledge

Hello, World!

Let's start with the traditional first program. Create a file called hello.lua:

print("Hello, World!")

Run it from the command line:

lua hello.lua

Expected Output:

Hello, World!

Understanding the Code

• print() is a built-in function that outputs text to the console
• Strings are enclosed in double quotes "..." or single quotes '...'
• Each statement in Lua can end with a semicolon, but it's optional

Multiple Outputs

The print function can take multiple arguments:

print("Hello", "World", "from", "Lua!")
print("Numbers:", 1, 2, 3)
print("Mixed:", "The answer is", 42)

Expected Output:

Hello	World	from	Lua!
Numbers:	1	2	3
Mixed:	The answer is	42

Note that print separates multiple arguments with tabs.

Working with Variables

Variables in Lua don't need type declarations:

-- This is a comment
local name = "Alice"
local age = 25
local height = 5.6

print("Name:", name)
print("Age:", age)
print("Height:", height, "feet")

Expected Output:

Name:	Alice
Age:	25
Height:	5.6	feet

Local vs Global Variables

Always use local for variable declarations:

-- Good practice: local variables
local x = 10
local y = 20

-- Global variable (avoid unless necessary)
z = 30

print("Local x:", x)
print("Local y:", y)
print("Global z:", z)

Expected Output:

Local x:	10
Local y:	20
Global z:	30

Basic Arithmetic

Lua supports standard arithmetic operations:

local a = 10
local b = 3

print("Addition:", a + b)
print("Subtraction:", a - b)
print("Multiplication:", a * b)
print("Division:", a / b)
print("Floor division:", a // b)
print("Modulo:", a % b)
print("Exponentiation:", a ^ b)

Expected Output:

Addition:	13
Subtraction:	7
Multiplication:	30
Division:	3.3333333333333
Floor division:	3
Modulo:	1
Exponentiation:	1000

String Operations

Working with text in Lua:

local first_name = "John"
local last_name = "Doe"

-- String concatenation with ..
local full_name = first_name .. " " .. last_name
print("Full name:", full_name)

-- String length
print("Length of full name:", #full_name)

-- Multiple lines
local message = "This is line 1\n" ..
                "This is line 2\n" ..
                "This is line 3"
print(message)

Expected Output:

Full name:	John Doe
Length of full name:	8
This is line 1
This is line 2
This is line 3

Simple Functions

Creating and using functions:

-- Function definition
local function greet(name)
    return "Hello, " .. name .. "!"
end

-- Function call
local message = greet("World")
print(message)

-- Function with multiple parameters
local function add_numbers(x, y)
    return x + y
end

local result = add_numbers(5, 7)
print("5 + 7 =", result)

Expected Output:

Hello, World!
5 + 7 =	12

User Input

Creating interactive programs with io.read():

print("What's your name?")
local name = io.read()

print("Hello, " .. name .. "!")

print("Enter a number:")
local input = io.read()
local number = tonumber(input)

if number then
    print("Your number squared is:", number ^ 2)
else
    print("That's not a valid number!")
end

Example Run:

What's your name?
Alice
Hello, Alice!
Enter a number:
5
Your number squared is:	25

Complete Example Program

Let's put it all together in a simple calculator:

-- Simple Calculator
print("=== Simple Lua Calculator ===")

-- Get first number
print("Enter first number:")
local input1 = io.read()
local num1 = tonumber(input1)

-- Get second number
print("Enter second number:")
local input2 = io.read()
local num2 = tonumber(input2)

-- Check if inputs are valid
if not num1 or not num2 then
    print("Error: Please enter valid numbers!")
    return
end

-- Perform calculations
print("\nResults:")
print(num1 .. " + " .. num2 .. " = " .. (num1 + num2))
print(num1 .. " - " .. num2 .. " = " .. (num1 - num2))
print(num1 .. " * " .. num2 .. " = " .. (num1 * num2))

if num2 ~= 0 then
    print(num1 .. " / " .. num2 .. " = " .. (num1 / num2))
else
    print("Cannot divide by zero!")
end

print("\nThanks for using the calculator!")

Example Run:

=== Simple Lua Calculator ===
Enter first number:
12
Enter second number:
4

Results:
12 + 4 = 16
12 - 4 = 8
12 * 4 = 48
12 / 4 = 3

Thanks for using the calculator!

Running Programs Different Ways

1. As Script Files

lua program.lua

2. Interactive Mode

$ lua
> print("Hello from REPL!")
Hello from REPL!
> x = 5 * 7
> print(x)
35
> os.exit()

3. Loading Files in Interactive Mode

$ lua
> dofile("hello.lua")
Hello, World!

Common Pitfalls

• Forgetting local: Always use local for variables to avoid global pollution
• String concatenation: Use .. not + for joining strings
• Input validation: Always check if tonumber() returns a valid number
• File extension: Lua files should end with .lua

Checks for Understanding

1. What function is used to output text in Lua?
2. How do you concatenate strings in Lua?
3. What's the difference between local x = 5 and x = 5?
4. How do you get the length of a string?
5. What function converts a string to a number?

Exercises

1. Write a program that calculates the area of a rectangle using user input
2. Create a program that converts temperatures between Celsius and Fahrenheit
3. Make a program that asks for your birth year and calculates your age

Next Steps

Great job! You've written your first Lua programs and learned basic syntax. Next, we'll explore comments and code documentation to make your programs more readable and maintainable.