Lua Functions Defining a function


Example

function add(a, b)
    return a + b
end
-- creates a function called add, which returns the sum of it's two arguments

Let's look at the syntax. First, we see a function keyword. Well, that's pretty descriptive. Next we see the add identifier; the name. We then see the arguments (a, b) these can be anything, and they are local. Only inside the function body can we access them. Let's skip to the end, we see... well, the end! And all that's in between is the function body; the code that's ran when it is called. The return keyword is what makes the function actually give some useful output. Without it, the function returns nothing, which is equivalent to returning nil. This can of course be useful for things that interact with IO, for example:

function printHello(name)
    print("Hello, " .. name .. "!");
end 

In that function, we did not use the return statement.

Functions can also return values conditionally, meaning that a function has the choice of returning nothing (nil) or a value. This is demonstrated in the following example.

function add(a, b)
    if (a + b <= 100) then
        return a + b -- Returns a value
    else
        print("This function doesn't return values over 100!") -- Returns nil
    end
end

It is also possible for a function to return multiple values seperated by commas, as shown:

function doOperations(a, b)
    return a+b, a-b, a*b
end

added, subbed, multiplied = doOperations(4,2)

Functions can also be declared local

do
    local function add(a, b) return a+b end
    print(add(1,2)) --> prints 3
end
print(add(2, 2)) --> exits with error, because 'add' is not defined here

They can be saved in tables too:

tab = {function(a,b) return a+b end}
(tab[1])(1, 2) --> returns 3