Functions in Lua

-- Function definition
function greet()
    print("Hello, World!")
end

-- Function call
greet()  -- Output: Hello, World!

-- Another example
function say_goodbye()
    print("Goodbye!")
    print("See you next time.")
end

say_goodbye()

function greet_user(name)
    print("Hello, " .. name .. "!")
end

greet_user("Alice")   -- Hello, Alice!
greet_user("Bob")     -- Hello, Bob!

-- Multiple parameters
function calculate_sum(a, b, c)
    local result = a + b + c
    print("Sum:", result)
end

calculate_sum(5, 10, 15)  -- Sum: 30

function add(a, b)
    return a + b
end

local result = add(10, 5)
print(result)  -- 15

-- Multiple return values
function get_person_info()
    return "Alice", 30, "Engineer"
end

local name, age, profession = get_person_info()
print(name, age, profession)  -- Alice 30 Engineer

-- Traditional syntax
function multiply(a, b)
    return a * b
end

-- Variable syntax (equivalent)
local divide = function(a, b)
    return a / b
end

print(multiply(4, 5))  -- 20
print(divide(10, 2))    -- 5

local function secret_operation(x)
    return x * 2 + 10
end

local result = secret_operation(5)  -- Works: 20
-- secret_operation(result)  -- Error if called from outside scope

function global_greet(name)
    return "Hello, " .. name
end

-- Can be called from anywhere in the program
print(global_greet("World"))  -- Hello, World

function apply_operation(x, y, operation)
    return operation(x, y)
end

function add(a, b) return a + b end
function multiply(a, b) return a * b end

local sum = apply_operation(5, 3, add)        -- 8
local product = apply_operation(5, 3, multiply)  -- 15

print("Sum:", sum)
print("Product:", product)

-- Using an anonymous function directly
local numbers = {1, 2, 3, 4, 5}

local doubled = {}
for i, num in ipairs(numbers) do
    doubled[i] = (function(x) return x * 2 end)(num)
end

print(table.concat(doubled, ", "))  -- 2, 4, 6, 8, 10

function create_counter()
    local count = 0
    return function()
        count = count + 1
        return count
    end
end

local counter1 = create_counter()
local counter2 = create_counter()

print(counter1())  -- 1
print(counter1())  -- 2
print(counter2())  -- 1 (independent counter)

-- Math operations
local calculator = {
    add = function(a, b) return a + b end,
    subtract = function(a, b) return a - b end,
    multiply = function(a, b) return a * b end,
    divide = function(a, b) 
        if b ~= 0 then
            return a / b
        else
            error("Division by zero!")
        end
    end
}

-- Using calculator
print(calculator.add(10, 5))      -- 15
print(calculator.multiply(3, 7))  -- 21

function format_name(first, last, title)
    local full_name = first .. " " .. last
    if title then
        return title .. " " .. full_name
    end
    return full_name
end

print(format_name("John", "Doe", "Dr."))  -- Dr. John Doe
print(format_name("Jane", "Smith"))        -- Jane Smith

local function is_positive_number(x)
    return type(x) == "number" and x > 0
end

local function validate_age(age)
    if not is_positive_number(age) then
        return false, "Age must be a positive number"
    end
    if age > 120 then
        return false, "Age seems unrealistic"
    end
    return true, "Valid age"
end

local is_valid, message = validate_age(25)
print(is_valid, message)  -- true Valid age

local is_valid2, message2 = validate_age(-5)
print(is_valid2, message2)  -- false Age must be a positive number

function process_data(data, processors)
    local result = data
    for i, processor in ipairs(processors) do
        result = processor(result)
    end
    return result
end

local function normalize(text)
    return string.lower(string.gsub(text, "%s+", " "))
end

local function capitalize(text)
    return string.upper(string.sub(text, 1, 1)) .. string.sub(text, 2)
end

local text = "  HELLO   WORLD  "
local processors = {
    function(t) return string.gsub(t, "^%s+", "") end,  -- trim start
    function(t) return string.gsub(t, "%s+$", "") end,  -- trim end
    normalize,
    capitalize
}

local result = process_data(text, processors)
print(result)  -- Hello world