In this chapter we will talk about scripting in Lua, the tools required, and go over some techniques which you will probably find useful.
A text editor with code highlighting is sufficient for writing scripts in Lua. Code highlighting gives different colors to different words and characters depending on what they mean. This allows you to spot mistakes.
For example, keywords in the above snippet are highlighted such as if, then, end, return. table.insert is a function which comes with Lua by default.
Other editors are available, of course.
IDEs allow you to debug code like a native application. These are harder to set up than just a text editor.
One such IDE is Eclipse with the Koneki Lua plugin:
Programs are a series of commands that run one after another. We call these commands “statements.”
Program flow is important, it allows you to direct or skip over statements. There are three main types of flow:
So, what do statements in Lua look like?
Woah, what happened there? a, b, and result are variables. They’re like what you get in mathematics, A = w * h. The equals signs are assignments, so “result” is set to a + b. Variable names can be longer than one character unlike in mathematics, as seen with the “result” variable. Lua is case sensitive. A is a different variable than a.
The word “local” before they are first used means that they have local scope, I’ll discuss that shortly.
|Integer||Whole number||local A = 4|
|Float||Decimal||local B = 3.2, local C = 5 / 2|
|String||A piece of text||local D = “one two three”|
|Boolean||True or False||local is_true = false, local E = (1 == 1)|
|Function||Can run. May require inputs and may return a value||local result = func(1, 2, 3)|
Not an exhaustive list. Doesn’t contain every possible type.
|A + B||Addition||2 + 2 = 4|
|A - B||Subtraction||2 - 10 = -8|
|A * B||Multiplication||2 * 2 = 4|
|A / B||Division||100 / 50 = 2|
|A ^ B||Powers||2 ^ 2 = 22 = 4|
|A .. B||Join strings||“foo” .. “bar” = “foobar”|
A string in programming terms is a piece of text.
Not an exhaustive list. Doesn’t contain every possible operator.
The most basic selection is the if statement. It looks like this:
That example generates a random number between 1 and 100. It then prints “Woohoo!” if that number is bigger than 50, otherwise it prints “No!”. What else can you get apart from ‘>’?
|A == B||Equals||1 == 1 (true), 1 == 2 (false)|
|A ~= B||Doesn’t equal||1 ~= 1 (false), 1 ~= 2 (true)|
|A > B||Greater than||5 > 2 (true), 1 > 2 (false), 1 > 1 (false)|
|A < B||Less than||1 < 3 (true), 3 < 1 (false), 1 < 1 (false)|
|A >= B||Greater than or equals||5 >= 5 (true), 5 >= 3 (true), 5 >= 6 (false)|
|A <= B||Less than or equals||3 <= 6 (true), 3 <= 3 (true)|
|A and B||And (both must be correct)||(2 > 1) and (1 == 1) (true), (2 > 3) and (1 == 1) (false)|
|A or B||either or. One or both must be true.||(2 > 1) or (1 == 2) (true), (2 > 4) or (1 == 3) (false)|
|not A||not true||not (1 == 2) (true), not (1 == 1) (false)|
That doesn’t contain every possible operator, and you can combine operators like this:
Which prints “Yay!” if A is false and B is true.
Logical and arithmetic operators work exactly the same, they both accept inputs and return a value which can be stored.
Programming is the action of talking a problem, such as sorting a list of items, and then turning it into steps that a computer can understand.
Teaching you the logical process of programming is beyond the scope of this book; however, the following websites are quite useful in developing this:
Codecademy is one of the best resources for learning to ‘code’, it provides an interactive tutorial experience.
Scratch is a good resource when starting from absolute basics,
learning the problem solving techniques required to program.
Scratch is designed to teach children how to program, it isn’t a serious programming language.
Whether a variable is local or global determines where it can be written to or read to. A local variable is only accessible from where it is defined. Here are some examples:
Whereas global variables can be accessed from anywhere in the script file, and from any other mod.
Lua is global by default (unlike most other programming languages). Local variables must be identified as such.
dump() is a function that can turn any variable into a string so the programmer can see what it is. The foo variable will be printed as “bar”, including the quotes which show it is a string.
This is sloppy coding, and Minetest will in fact warn you about this:
[WARNING] Assigment to undeclared global 'foo' inside function at init.lua:2
To correct this, use “local”:
Nil means not initalised. The variable hasn’t been assigned a value yet, doesn’t exist, or has been uninitialised. (ie: set to nil)
The same goes for functions. Functions are variables of a special type. You should make functions as local as much as possible, as other mods could have functions of the same name.
If you want your functions to be accessible from other scripts or mods, it is recommended that you add them all into a table with the same name as the mod:
You can include Lua scripts from your mod or another mod like this:
“local” variables declared outside of any functions in a script file will be local to that script. You won’t be able to access them from any other scripts.
As for how you divide code up into files, it doesn’t matter that much. The most important thing is that your code is easy to read and edit. You won’t need to use it for smaller projects.