Lua - If/Else

Overview

Estimated time: 20–25 minutes

Conditional statements allow your program to make decisions and execute different code paths based on conditions. This tutorial covers if, elseif, and else statements with practical examples.

Learning Objectives

• Master basic if statements and conditions
• Use elseif for multiple conditions
• Implement else clauses for default cases
• Create nested conditional statements
• Apply best practices for conditional logic

Prerequisites

• Understanding of Lua operators (especially relational and logical)
• Knowledge of boolean values and truthiness

Basic If Statement

The simplest form of conditional statement:

local age = 18

if age >= 18 then
    print("You are an adult")
end

print("Program continues...")

-- Another example
local temperature = 25

if temperature > 30 then
    print("It's hot outside!")
end

if temperature < 10 then
    print("It's cold outside!")
end

Expected Output:

You are an adult
Program continues...

If-Else Statement

Provide an alternative action when the condition is false:

local score = 75

if score >= 60 then
    print("You passed!")
else
    print("You failed!")
end

-- Voting eligibility
local age = 16

if age >= 18 then
    print("You can vote")
else
    print("You cannot vote yet. You need to wait " .. (18 - age) .. " more years.")
end

-- Weather advice
local is_raining = true

if is_raining then
    print("Take an umbrella!")
else
    print("Enjoy the sunshine!")
end

Expected Output:

You passed!
You cannot vote yet. You need to wait 2 more years.
Take an umbrella!

If-Elseif-Else Chain

Handle multiple conditions with elseif:

local grade = 85

if grade >= 90 then
    print("Grade: A (Excellent!)")
elseif grade >= 80 then
    print("Grade: B (Good job!)")
elseif grade >= 70 then
    print("Grade: C (Satisfactory)")
elseif grade >= 60 then
    print("Grade: D (Needs improvement)")
else
    print("Grade: F (Failed)")
end

-- Day of the week
local day = 3

if day == 1 then
    print("Monday - Start of the work week")
elseif day == 2 then
    print("Tuesday - Getting into the groove")
elseif day == 3 then
    print("Wednesday - Hump day!")
elseif day == 4 then
    print("Thursday - Almost there")
elseif day == 5 then
    print("Friday - TGIF!")
elseif day == 6 or day == 7 then
    print("Weekend - Time to relax!")
else
    print("Invalid day number")
end

Expected Output:

Grade: B (Good job!)
Wednesday - Hump day!

Complex Conditions

Use logical operators to create complex conditions:

local age = 25
local has_license = true
local has_car = false

-- Multiple conditions with AND
if age >= 18 and has_license then
    print("You can legally drive")
    
    if has_car then
        print("You can drive your own car")
    else
        print("You need to borrow or rent a car")
    end
else
    print("You cannot drive legally")
end

-- Multiple conditions with OR
local is_weekend = true
local is_holiday = false

if is_weekend or is_holiday then
    print("No work today!")
else
    print("Time to go to work")
end

-- Complex condition
local temperature = 22
local is_sunny = true
local wind_speed = 5

if temperature >= 20 and temperature <= 30 and is_sunny and wind_speed < 10 then
    print("Perfect day for a picnic!")
else
    print("Maybe stay indoors today")
end

Expected Output:

You can legally drive
You need to borrow or rent a car
No work today!
Perfect day for a picnic!

Nested If Statements

Place if statements inside other if statements:

local user_type = "premium"
local account_active = true
local balance = 150

if account_active then
    print("Account is active")
    
    if user_type == "premium" then
        print("Premium user detected")
        
        if balance >= 100 then
            print("Sufficient balance for premium features")
            print("Access granted to all features")
        else
            print("Low balance - some features may be restricted")
        end
    elseif user_type == "standard" then
        print("Standard user")
        
        if balance >= 50 then
            print("Access granted to standard features")
        else
            print("Please add funds to your account")
        end
    else
        print("Unknown user type")
    end
else
    print("Account is inactive - please contact support")
end

Expected Output:

Account is active
Premium user detected
Sufficient balance for premium features
Access granted to all features

Truthiness in Conditions

Remember: only nil and false are falsy in Lua:

-- Testing various values
local values = {true, false, nil, 0, "", "hello", {}, -1}

for i, value in ipairs(values) do
    if value then
        print("Value " .. i .. " (" .. tostring(value) .. ") is truthy")
    else
        print("Value " .. i .. " (" .. tostring(value) .. ") is falsy")
    end
end

-- Practical example: checking if a table has elements
local fruits = {"apple", "banana"}
local empty_fruits = {}

if #fruits > 0 then
    print("We have fruits:", fruits[1])
else
    print("No fruits available")
end

if #empty_fruits > 0 then
    print("We have fruits in empty_fruits")
else
    print("empty_fruits is empty")
end

Expected Output:

Value 1 (true) is truthy
Value 2 (false) is falsy
Value 3 (nil) is falsy
We have fruits:	apple
empty_fruits is empty

Practical Examples

User Authentication System

local function authenticate_user(username, password, is_admin)
    if not username or username == "" then
        return "Error: Username is required"
    end
    
    if not password or password == "" then
        return "Error: Password is required"
    end
    
    if #password < 8 then
        return "Error: Password must be at least 8 characters"
    end
    
    -- Simulate authentication
    if username == "admin" and password == "admin123" then
        if is_admin then
            return "Welcome, Administrator!"
        else
            return "Welcome, " .. username .. "!"
        end
    elseif username == "user" and password == "password123" then
        return "Welcome, " .. username .. "!"
    else
        return "Error: Invalid credentials"
    end
end

-- Test the authentication
print(authenticate_user("", "password123", false))
print(authenticate_user("user", "123", false))
print(authenticate_user("user", "password123", false))
print(authenticate_user("admin", "admin123", true))

Temperature Converter

local function convert_temperature(temp, from_unit, to_unit)
    if not temp or not from_unit or not to_unit then
        return "Error: Missing parameters"
    end
    
    -- Normalize units to lowercase
    from_unit = string.lower(from_unit)
    to_unit = string.lower(to_unit)
    
    local celsius
    
    -- Convert to Celsius first
    if from_unit == "celsius" or from_unit == "c" then
        celsius = temp
    elseif from_unit == "fahrenheit" or from_unit == "f" then
        celsius = (temp - 32) * 5 / 9
    elseif from_unit == "kelvin" or from_unit == "k" then
        celsius = temp - 273.15
    else
        return "Error: Unknown source unit"
    end
    
    -- Convert from Celsius to target unit
    if to_unit == "celsius" or to_unit == "c" then
        return celsius
    elseif to_unit == "fahrenheit" or to_unit == "f" then
        return celsius * 9 / 5 + 32
    elseif to_unit == "kelvin" or to_unit == "k" then
        return celsius + 273.15
    else
        return "Error: Unknown target unit"
    end
end

-- Test conversions
print("32°F to Celsius:", convert_temperature(32, "f", "c"))
print("0°C to Fahrenheit:", convert_temperature(0, "c", "f"))
print("273.15K to Celsius:", convert_temperature(273.15, "k", "c"))
print("Invalid conversion:", convert_temperature(100, "x", "c"))

Game Logic Example

local function check_game_status(player_health, enemy_health, player_level, has_special_item)
    if player_health <= 0 then
        return "Game Over - You died!"
    end
    
    if enemy_health <= 0 then
        return "Victory - Enemy defeated!"
    end
    
    if player_health < 20 then
        if has_special_item then
            return "Low health - Use your special item!"
        else
            return "Critical health - Find a health potion!"
        end
    end
    
    if enemy_health < 30 and player_level >= 5 then
        return "Enemy is weak - Use your special attack!"
    end
    
    if player_health > 80 and enemy_health > 80 then
        return "Both fighters are strong - This will be a long battle!"
    end
    
    return "Battle continues..."
end

-- Test game scenarios
print(check_game_status(0, 50, 3, false))
print(check_game_status(100, 0, 5, true))
print(check_game_status(15, 60, 2, true))
print(check_game_status(85, 25, 6, false))

Expected Output:

Error: Username is required
Error: Password must be at least 8 characters
Welcome, user!
Welcome, Administrator!
32°F to Celsius:	0
0°C to Fahrenheit:	32
273.15K to Celsius:	0
Invalid conversion:	Error: Unknown source unit
Game Over - You died!
Victory - Enemy defeated!
Low health - Use your special item!
Enemy is weak - Use your special attack!

Common Pitfalls

• Missing then: Always include then after the condition
• Assignment vs comparison: Use == for comparison, not =
• Truthiness confusion: Remember that 0 and "" are truthy in Lua
• Missing end: Each if must have a corresponding end
• Complex conditions: Use parentheses to make complex conditions clearer

Checks for Understanding

1. What keyword is required after an if condition?
2. How do you check if a number is between 10 and 20 (inclusive)?
3. What values are considered falsy in Lua?
4. What's the difference between if and elseif?
5. How do you end an if statement?

Next Steps

Now that you can make decisions in your programs, you're ready to learn about loops, which allow you to repeat code blocks multiple times.