minetest_modding_book/_en/basics/lua.md

---
title: Lua Scripting
layout: default
root: ../..
idx: 1.2
description: A basic introduction to Lua, including a guide on global/local scope.
redirect_from: /en/chapters/lua.html
---

## Introduction

In this chapter will talk about scripting in Lua, the tools required
to assist with this, and some techniques which you may find useful.

* [Code Editors](#code-editors)
    * [Integrated Programming Environments](#integrated-programming-environments)
* [Coding in Lua](#coding-in-lua)
    * [Program Flow](#program-flow)
    * [Variable Types](#variable-types)
    * [Arithmetic Operators](#arithmetic-operators)
    * [Selection](#selection)
    * [Logical Operators](#logical-operators)
* [Programming](#programming)
* [Local and Global Scope](#local-and-global-scope)
* [Including other Lua Scripts](#including-other-lua-scripts)

## Code Editors

A code editor with code highlighting is sufficient for writing scripts in Lua.
Code highlighting uses different colours for words and characters
depending on what they represent. This allows you to more easily notice
mistakes and inconsistencies.

For example:

```lua
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
```

Keywords in this code are highlighted including `if`, `then`, `end`, and `return`.
Functions which comes with Lua by default, such as `table.insert` are also highlighted.

Commonly used editors which are well-suited for Lua include:

* Windows: [Notepad++](http://notepad-plus-plus.org/), [Atom](http://atom.io/), [VS Code](https://code.visualstudio.com/)
* Linux: Kate, Gedit, [Atom](http://atom.io/), [VS Code](https://code.visualstudio.com/)
* OSX: [Atom](http://atom.io/), [VS Code](https://code.visualstudio.com/)

Other suitable editors are also available.

### Integrated Programming Environments

Integrated development environments (IDEs) allow you to more easily debug code,
but are often more difficult to set up than a simple text editor.

One such IDE is Eclipse, which can be used with the Koneki Lua plugin.
To use this:

* Install Eclipse and Koneki.
* Create a new Lua project from existing source (specify Minetest's base directory).
* Follow instructions from Koneki wiki on how to "Attach to remote Application" debugging.
* It is suggested to add those lines from wiki at the beginning of builtin.lua.
* Start the debugger. (Set "Break on first line" in debugger configuration to see if it is working).
* Start Minetest.
* Enter the game to startup Lua.

## 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 and different
types of flow allow you to skip or jump over sets of commands.

There are three main types of flow:

* Sequence: This runs one statement after another, with no skipping.
* Selection: This skips over sequences depending on conditions.
* Iteration: Repeating, looping. This runs the same
  statements until a condition is met.

So, what do statements in Lua look like?

```lua
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)
```

In this example `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 later, because it's part of a very important concept called
*scope*.

The `=` sign means *assignment*, so `result = a + b` means set the value of `result` to
the value of `a + b`. Variable names can be longer than one character, 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

Arithmetic operators in Lua include:

| 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 = 2<sup>2</sup> = 4 |
| A .. B | Join strings   | "foo" .. "bar" = "foobar" |

Please note that this is not an exhaustive list; it doesn't contain every possible operator.

### Selection

The most basic method of selection is the if statement. For example:

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

This generates a random number between 1 and 100. It then prints
"Woohoo!" if that number is bigger than 50, and otherwise prints "No!".

### Logical Operators

Logical operators in Lua include:

| 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)                 |

Please note that this doesn't contain every possible operator.

It is also possible to combine operators. For example:

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

This prints "Yay!" if A is false and B is true.

Logical and arithmetic operators work the same way;
they both accept inputs and return a value which can be stored. For example:

```lua
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 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](http://www.codecademy.com/) is one of the best resources for
  learning to write code. It provides an interactive tutorial experience.
* [Scratch](https://scratch.mit.edu) is a good resource, when starting from
  absolute basics, to learn 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 from.
A local variable is only accessible from where it is defined. Here are some examples:

```lua
-- 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
```

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

```lua
my_global_variable = "blah"

function one()
    my_global_variable = "three"
end

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

Local variables should be used whenever possible.

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

For example:

```lua
function one()
    foo = "bar"
end

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

one()
two()
```

The dump() function can turn any variable into a string so the programmer can
see what it is. In this example, the foo variable will be printed as "bar", including the quotes
which show it is a string. This reveals that the foo variable was global, because it could be
accessed from another function than the one in which it was declared.

This is sloppy coding, and Minetest will in warn about this:

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

To correct this, use "local":

```lua
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 (meaning set to nil).

The same applies to functions. Functions are variables of a special type, and
should be made local, because other mods could have functions with the same names.

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

```lua
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*.

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

"local" variables declared outside 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:

```lua
-- 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 the code for a mod in a lot more detail;
however, the most simple approach is to have different files for different
types of things, for example nodes.lua, crafts.lua, craftitems.lua, and so on.