Lua Scripting

Introduction

In this chapter we will talk about scripting in Lua, the tools required, and go over some techniques which you will probably find useful.

Code Editors

A code editor with code highlighting is sufficient for writing scripts in Lua. Code highlighting gives different colours to different words and characters depending on what they mean. This allows you to spot mistakes.

function ctf.post(team,msg)
    if not ctf.team(team) then
        return false
    end
    if not ctf.team(team).log then
        ctf.team(team).log = {}
    end

    table.insert(ctf.team(team).log,1,msg)
    ctf.save()

    return true
end

For example, keywords in the above snippet are highlighted such as if, then, end, and return. table.insert is a function which comes with Lua by default.

Here is a list of common editors well suited for Lua. Other editors are available, of course.

Coding in Lua

Program Flow

Programs are a series of commands that run one after another. We call these commands “statements.” Program flow is how these statements are executed. Different types of flow allow you to skip or jump over sets of commands. There are three main types of flow:

  • Sequence: Just run one statement after another, no skipping.
  • Selection: Skip over sequences depending on conditions.
  • Iteration: Repeating, looping. Keep running the same statements until a condition is met.

So, what do statements in Lua look like?

local a = 2     -- Set 'a' to 2
local b = 2     -- Set 'b' to 2
local result = a + b -- Set 'result' to a + b, which is 4
a = a + 10
print("Sum is "..result)

Whoa, what happened there?

a, b, and result are variables. Local variables are declared by using the local keyword, and then given an initial value. Local will be discussed in a bit, as it’s part of a very important concept called scope.

The = means assignment, so result = a + b means set “result” to a + b. Variable names can be longer than one character unlike in mathematics, as seen with the “result” variable. It’s also worth noting that Lua is case-sensitive; A is a different variable to a.

Variable Types

A variable will be only one of the following types and can change type after an assignment. It’s good practice to make sure a variable is only ever nil or a single non-nil type.

Type Description Example
Nil Not initialised. The variable is empty, it has no value local A, D = nil
Number A whole or decimal number. local A = 4
String A piece of text local D = "one two three"
Boolean True or False local is_true = false, local E = (1 == 1)
Table Lists Explained below
Function Can run. May require inputs and may return a value local result = func(1, 2, 3)

Arithmetic Operators

Not an exhaustive list. Doesn’t contain every possible operator.

Symbol Purpose Example
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”

Selection

The most basic selection is the if statement. It looks like this:

local random_number = math.random(1, 100) -- Between 1 and 100.
if random_number > 50 then
    print("Woohoo!")
else
    print("No!")
end

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 ‘>’?

Logical Operators

Symbol Purpose Example
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:

if not A and B then
    print("Yay!")
end

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.

local A = 5
local is_equal = (A == 5)
if is_equal then
    print("Is equal!")
end

Programming

Programming is the action of taking 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, and isn’t a serious programming language.

Local and Global Scope

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:

-- Accessible from within this script file
local one = 1

function myfunc()
    -- Accessible from within this function
    local two = one + one

    if two == one then
        -- Accessible from within this if statement
        local three = one + two
    end
end

Whereas global variables can be accessed from anywhere in the script file, and from any other mod.

my_global_variable = "blah"

function one()
    my_global_variable = "three"
end

print(my_global_variable) -- Output: "blah"
one()
print(my_global_variable) -- Output: "three"

Locals should be used as much as possible

Lua is global by default (unlike most other programming languages). Local variables must be identified as such.

function one()
    foo = "bar"
end

function two()
    print(dump(foo))  -- Output: "bar"
end

one()
two()

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 about this:

Assignment to undeclared global 'foo' inside function at init.lua:2

To correct this, use “local”:

function one()
    local foo = "bar"
end

function two()
    print(dump(foo))  -- Output: nil
end

one()
two()

Remember that nil means not initialised. 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, and should be made local, as other mods could have functions of the same name.

local function foo(bar)
    return bar * 2
end

API tables should be used to allow other mods to call the functions, like so:

mymod = {}

function mymod.foo(bar)
    return "foo" .. bar
end

-- In another mod, or script:
mymod.foo("foobar")

Including other Lua Scripts

The recommended way to include other Lua scripts in a mod is to use dofile.

dofile(minetest.get_modpath("modname") .. "/script.lua")

“local” variables declared outside of any functions in a script file will be local to that script. A script can return a value, which is useful for sharing private locals:

-- script.lua
return "Hello world!"

-- init.lua
local ret = dofile(minetest.get_modpath("modname") .. "/script.lua")
print(ret) -- Hello world!

Later chapters will discuss how to split up the code of a mod in a lot of detail. However, the simplistic approach for now is to have different files for different types of things - nodes.lua, crafts.lua, craftitems.lua, etc.

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