Using functions

Most programming languages come with some variables already defined for us. Lua has many, so let's type one in and hit ENTER to see what the value is:

string.reverse

=> function: 0x2381b60

Oh my. So "function" is another data type, but what is 0x2381b60? It's just telling you where in the computer's memory that function exists, just in case you wanted to know. Functions work very differently than numbers in strings. Essentially functions are pre-defined instructions that tell the program how to do different things. They take data and return back different data. Let's see how to give this function data:

string.reverse("hello")

=> olleh

At the end of the function's variable name, string.reverse, we type a set of parenthesis, string.reverse(), and put inside the parenthesis some data we want changed (string.reverse("hello")). Making the function run is often called invoking the function. Having a function that reverses text in a string for us can be useful, and we can capture the return value (the results) of the function using a variable. Try it out:

greeting = "hello, how are you?"
backwards_greeting = string.reverse(greeting)
backwards_greeting

=> ?uoy era woh ,olleh

It should be obvious from the name what that function's purpose is. How about this one?

string.upper("hello, how are you?")

Now try capturing that value by assigning it to a variable:

greeting = "hello, how are you?"
shouting_greeting = string.upper(greeting)
crazy_greeting = string.reverse(shouting_greeting)

We can get crazier. How about invoking a function when invoking another function??

string.reverse(string.upper("hey"))

What's happening here is the string is being uppercased by string.upper but then the value from string.upper is being reversed by string.reverse as soon as it is done. It's just like in arithmetic when you have nested parenthesis. The inner-most parenthesis are resolved before doing the outer-most parenthesis.

Let's try one more function. This function has two parameters, meaning it accepts two pieces of data which it requires to work properly.

math.max(7, 10)

When giving more than one piece of data to a function, you need to put a comma (,) between the parameters

These are great functions, but wouldn't it be great if we could make our own? We'll give it a shot in just a few pages.

Exercises

• See if you can figure out what math.max does. Give it different numbers and examine the result.
• There is another function called math.min that also takes two numbers. What does it return?