Nil and variables

Data, or the lack thereof

Humans have different ways of representing a lack of data. If there are no sheep to count then we have zero sheep. If there are no words on a page then the page is blank. In a computer we may represent the number of sheep as 0 or the missing words on a page as an empty "". These are still data though... a number and a string. In software when you want to represent a lack of data we have:

nil

Sometimes called null or undefined data in other languages. It's seemingly useless. You can't use operators on nil.

nil + nil

This will print an error like it did when you tried doing arithmetic on strings. Let's take a look at variables and we'll discover the purpose of nil.

Variables

Sometimes you want to write out data, but you want that data to be easy to change. Variables let you give data a name to reference. Here's an example to try:

name = "Mandy"
"hello my name is " .. name

Since you told it what name is, it knows what value to add to the string "hello my name is ". If you type:

name

...and hit ENTER, it will print out the value that belongs to this variable to remind you. The = (equal) sign tells Lua that you want to assign a value to the given name/variable. You can change the value of a variable and get different results:

name = "Jeff"
"hello my name is " .. name

Assignment isn't the same as it is in Algebra. You can change the value of a variable multiple times. We can tell name that it equals itself with some additional information concatenated to it:

name = "abc"
name = name .. "def"
name

You can assign any type of data to a variable, including numbers:

name = "Jeff"
age = 16
"hello my name is " .. name .. " and I am " .. age .. "."

You can change numbers after assignment too:

age = 16
age = age * 2
"my age doubled is " .. age

So, what if you type in a made up variable name?

noname

You will see it has nil, or no data yet. If you try to use nil in your string operation you will get an error:

"hello my name is " .. nil

[string "return "hello my name is " .. nil"]:1: attempt to concatenate a nil value

"hello my name is " .. noname

[string "return "hello my name is " .. noname"]:1: attempt to concatenate global 'noname' (a nil value)

Try assigning a value to a variable name:

best_color = "purple"

then assigning that variable data to another:

worst_color = best_color
worst_color

You'll see that both variables now have the value "purple".

Variables can have names made up of letters, numbers and underscores (_). Variable names cannot begin with a number though, otherwise it will think you're trying to type in number data. Here's some examples of valid variables:

my_dog = "Poe"
myDog = "Zia"
DOG3 = "Ember"

Exercises

• Try out different variable names. Try a few invalid variables names too just to see what the error message looks like. It's important to see error messages and understand them. They help you understand how a program breaks so you can fix it.