Functions

In Lua you define a function in the following way:

function hello()
    print("Hello!")
end

hello()

A function, as usual, can have parameters:

function hello(name)
    print("Hello, " .. name)
end

hello("Sam")

To return a value from a function you use return statement:

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

print(sum(10, 20))

A function can return multiple values with comma-separated list:

function person()
    return "Sam", "samblog.com", 100
end

name, website, wallet = person()

print(name)
print(website)
print(wallet)

A function can have multiple return statements as well:

function older_person(name1, age1, name2, age2)
    if age1 > age2 then
        return name1
    elseif age2 > age1 then
        return name2
    else
        return "Noone"
    end
end

print(older_person("Peter", 25, "Alice", 20) .. " is older")

There are also anonymous functions which you can store in variables and pass as function arguments:

function for_each(list, f)
    for n = 1, #list do
        f(list[n])
    end
end

names = {"Peter", "Alice", "Michael", "Tiffany"}

for_each(names, function(name)
    print("Hello, " .. name)
end)

sum = function(a, b)
    return a + b
end

print(sum(1, 2))

There is a simple syntax for variable length parameters list:

function sum(...)
    result = 0
    for i, v in ipairs({...}) do
        result = result + v
    end
    return result
end

print(sum(10, 20, 30))

You can think of ... as a simple replacement of the actual parameters list. For example, you can write something like this:

function sum(...)
    v1, v2, v3 = ...
    return v1 + v2 + v3
end

print(sum(10, 20, 30))