Introduction to Lua - 邱璇洛 (ゝ∀･)

Development Environment: MacOS
Lua 5.4.4
External Link: Runoob Tutorial - Lua

Introduction
Lua, named after the moon, is a very simple, fast, and powerful scripting language, known for being underestimated.

Basic Syntax
Lua is very simple, so simple that you can easily master it in an afternoon.
Hello World!

print("Hello World")

Comments

-- Single-line comment
--[[
    Multi-line comment
--]]

Variables

By default, variables are always considered global.
Global variables do not need to be declared. Once a value is assigned to a variable, it creates the global variable. Accessing an uninitialized global variable will not cause an error, but the result will be: nil
—— Runoob Tutorial/Lua Basic Syntax

The local keyword is used to set a local variable.

-- Global variable
variable = 123
-- Set local variable
local local_variable = "string"
-- Long variable
many_line_text = [[
 /    _ 
(__(/(/ 
Lua 5.4.4  Copyright (C) 1994-2022 Lua.org, PUC-Rio        
]]

Basic Types
Except for replacing NULL with nil, there is no difference. (Learn more).

-- Null
nil_variable = nil
print(nil_variable)

Variable Operations

Concatenating two variables#

-- Concatenating variables
local link_str_a = "astr"
local link_str_b = "bstr"
local str_c = link_str_a..link_str_b
print(str_c)

Type conversion#

-- Type conversion
local int_a = tostring(10)
print(type(int_a))
int_a = tonumber(int_a)
print(type(int_a))

Various ways to output variables in the command line#

-- Global variable
variable = 123
-- Set local variable
local local_variable = "string"

print("var = ", variable, "local var = ", local_variable) 
print(string.format("var = %d, local var = %s", variable, local_variable))

Arrays and Indices
Note⚠️: Lua arrays start counting from 1, not 0!!!

-- Arrays
array = {233, "qwq", 996, "乐"}
print(array[1])
-- Inserting
-- Insert at the end
table.insert(array, "Inserted new element")
-- Insert in the middle
table.insert(array, 2, "The second element has changed")
-- Insertion does not change the previous elements, the previous second element becomes the third
print(array[5]) -- Inserted new element
print(array[2]) -- The second element has changed
print(array[3]) -- qwq
-- Removing
-- Remove the second element
table.remove(array, 2)
print(array[2]) -- The second element has changed, the third element becomes the second
-- Assigning the removed element
local remove_number = table.remove(array, 1)
-- Now, remove_number = 233(array[1]), array[1] = "qwq"
print(array[1])
print(remove_number)

-- Indices
str_array = {
    str_a = "a",
    int_a = 123456,
    -- Strange index
    [";;,fun"] = "233"
}
print(str_array["str_a"])
-- or
print(str_array.int_a)
-- and 
print(str_array[";;,fun"])

-- Assignment
str_array["abuc"] = "223"
print(array.abuc)

Functions and Conditions

-- Functions and if
function calculate(symbol, a, b)
    -- In Lua, the not equal sign is represented by `~=`
    --[[
        `or`  == `||` 
        `and` == `&&`
        `not` == true -> false or false -> true
    --]]
    if ((type(symbol) ~= "string" or (type(a) or type(b)) ~= "number")) then
        print("Error: Unknown input type")
        return nil
    end
    if (symbol == "+") then
        return a+b
    elseif (symbol == "-") then
        return a-b
    elseif (symbol == "*") then
        return a*b
    elseif (symbol == "/") then
        return a/b
    else
        print("Error: Unknown symbol type")
        return nil
    end
end

print(calculate("+", 1, 2))

Various Loops

-- Loops
while_var = 0

-- Check before the loop
while (while_var < 3) do
    while_var = while_var + 1
end
print("while_var ->", while_var)

-- Check after the loop
repeat
    while_var = while_var + 1
until (while_var > 5)
print("while_var ->", while_var)

-- i=3, i = i-1 each time until i==1
for i=3, 1, -1 do
    print(i)
end

-- Generic for loop
for i, v in ipairs(array) do
    print(i, v)
end

-- Lua supports nested loops
more_array = {}
for i=1, 3 do
    more_array[i] = {}
    for j=1, 3 do
        more_array[i][j] = i*j
    end
end
-- Accessing arrays
for i=1,3 do
    for j=1,3 do
       print(string.format("%d ", more_array[i][j]))
    end
 end

Multi-File Programming (Packages)
In Lua, we can easily call packages using require(package name). However, note that require by default looks for package files in the project's root directory, not the working directory of the code file. If you put the package elsewhere, like this:

Project Directory
    Code Directory (src)
        main.lua
        mod.lua

You need to tell require where it is when calling it:

-- Look for the mod package in the src directory of the project
require(src.mod)

Example
Create mod.lua in the same directory:

-- mod.lua
-- This is a package
mod = {}

mod.constant = "This is a constant"
mod.constant_two = "This is another constant"

-- This is a public function
function mod.add(a, b)
    return a+b
end

-- This is a private function
local function subtraction(a, b)
    return a-b
end

-- Call
function mod.open_sub(a, b)
    subtraction(a, b)
end

return mod

Call in main.lua:

-- Call the package
require("mod")
-- Use
print(mod.add(1, 2))

Miscellaneous

Getting the length of an array or variable#

-- Get the length
local local_variable = 123
array = {1, 2, 3, 4, 5}
-- Add `#`
print(#array)
print(#local_variable)