Variables & Assignment

tip

Variables and Assignments are very generic terms in programming, if they are not familiar I recommend checking some resources.

Don't overcomplicate things though, variables are names, assignment is the act of point at things with those names.

In Lua we have two types of Variables, Global and Local

Global

Global Variables are declared with a simple assignment statement, no keywords needed.

helloWorldMessage = "Hello World"
print(helloWorldMessage) -->> Hello World

It is also possible to just reference them without declaration, but you get a nil return value

print(nothingHere) -->> nil

You can also assign nil to a variable which has the effect of deleting the value via de-reference.

De-referenced values are cleaned up by the automatic memory management provided by Lua.

helloWorldMessage = "Hello World"
print(helloWorldMessage) -->> Hello World
helloWorldMessage = nil
print(helloWorldMessage) -->> nil

To give an example of this in action lets create two files.

First we create a file with 1 Global variable PI with the value of 3.14159 assigned to it

--- constants.lua
PI = 3.14159

If we try to reference it we cannot, we will get nil as a result, however once we reference it after the constants.lua is imported it works as expected.

---printPi.lua
-- Reference Before Import
print(PI) -->> nil
-- Import Constants
dofile("constants.lua")
-- Reference after import
print(PI) -->> 3.14159

Local

Local variables are declared using the local key word

local halfPi = PI/2
print(halfPi)

Local variables are slightly different to global in that they are only accessible from within the scope.

We will bring our two files back but let's add local before the variable PI and a print statement after its declaration.

--- constants.lua
local PI = 3.14159
print(PI) -->> 3.14159

When we now try and access the variable PI after the import we still get a nil value

---printPi.lua
-- Reference Before Import
print(PI) -->> nil
-- Import Constants
dofile("constants.lua")
-- Reference after import
print(PI) -->> nil

This is because local variable is not accessible outside the file.

If we run this program we get the following output in the console, the first print comes from the printPi.lua at line 3, the second comes from constants.lua at line 2, the third comes from printPi.lua at line 9

lua.exe helloWorld.lua
nil
3.14159
nil

A Note on Scope

Scope is quite a complex topic, but it's critical to understand at least the fundamental concept before declaring a single variable.

At its core something is scoped when it exists only for a specific block of code.

The first scope you encounter is the file itself. Then any functions/control structures (for loops etc)

Consider this example

--<< Top Level Scope Starts
print(pi) -->> nil
print(localPi) -->> nil
function definePI() --<< Function Scope Starts
pi = 3.14
local localPi = 3.14
print(pi) -->> 3.14
print(localPi) -->> 3.14
end --<< Function Scope Ends
definePI()
print(pi) -->> 3.14
print(localPi) -->> nil
--<< Top Level Scope Ends

The variable localPi is limited to the scope of the function definePi, when we leave the function the variable no longer exists and is not accessible, it is said to be scoped, the Variable pi however is NOT and we can access it after calling the function.

warning

It may not be obvious but there is a glaring issue here, often languages when defining a variable like we do with pi = 3.14 limit this to only be accessible inside the function, meaning in large scripts you don't get random collisions, that's not the case here, if for example we had defined pi as 3.14159 at the start, then it was replaced inside a function by accident to 3.14 we have just lost a lot of precision!

LESSON: USE local VARIABLES UNLESS YOU EXPLICITLY WANT global ONES

References

Edit this page