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# Lua Syntax¶

## Lexical convention¶

variables are named with sequences of letters, underlines or numbers that begin with letters or underlines. Please do not use token that starts with combination of underline and capitalized letter, for it is how the reserved token of Lua is constructed.

These are the reserved words of Lua, you can't use them as variables.

Caution: Lua is case sensitice. For example, 'and' is a reserved word, but 'And' and 'AND' aren't.

When you enter a reserved word in SciTE, it will automatically turn into a blue bold black body.

## Code specification¶

• Code specification

Lua does not require any delimiters between multiple statements, liek the C language semicolon (;), where line breaks also do not function as statements. Therefore the following writing is legal.

 1 2 3 4 5 6 7 8 a = 1 b = a * 2 a = 1; b = a * 2; a = 1; b = a * 2; a = 1 b = a * 2 

There are 2 kinds of comments in Lua:

The first one is single line comment, like:

 1 2 3 4 5 6 7 8 -- single line comment --[[ mulitiple line comment --]] 

## Variables¶

Lua variables are divided into global variables and local variables.

You don't need to declare a global variable, once you assigned a value to the variable, the global variable is going to work in the whole file. Accessing a variable that is not initialized won't go wrong, but the outcome you;ll get is nil.

 1 2 3 4 5 6 print(b) --> nil b = 10 print(b) -->10 --if you want to delete a global variable, you only need to change the value assigned to it to nil b = nil print(b) --> nil 

Use local to declare a local variable. Different with global variable, local variable is valid only in the code block it is declared. Code block: refers to a control structure, a function body, or a chunk (the file or text string whose variable is declared).

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 local m = 9 if m <= 10 then local m = 5 -- local variable valid within 'then' and 'end' print(m) end --the result is 5 local m = 9 if m <= 10 then print(m) -- this m is the local variable declared in the start end --the result is 9. 

In addition, Lua is a dynamically typed language, and you should not define a type when declaring a variable. The type of the variable is the same as the type of the currently assigned value. We can get the type information of the variable or value through the type function, and the type information will be returned as a string.

  1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 local k = 1 -- k is a local variable print(type(k)) --> number k = "today is Friday" -- assign a string to k print(type(k)) --> string k = {1,2} -- assign a table to k print(type(k)) --> table local d -- d is local variable, it is not assigned during declaration print(type(d)) --> nil d = 11 -- d is assigned after print(type(d)) --> number h = 9 -- h is a global variable print(type(h)) --> number 

## Values and Types¶

There are 8 types of value in Lua:nil, boolean, number, string, userdata, function, thread, and table.

• nil

The special type in Lua with only one possible value: nil. It's major function is to differentiate itself from other values. When a global variable is not assgined with a value, the default value is nil; assign a global variable with nil would delete the variable. Lua use nil to represent a state that is not valid.

• boolean

This type has two optional values: false and true. But be aware that all values ​​in Lua can be used as conditions. In Lua, only when the values ​​are false and nil are considered "false", other values ​​are considered true, such as the number zero and the empty string, which is different from the C language.

• number

Represents a real number. Due to resource limitations, the module Lua development platform provided by our company does not support fractional and floating-point number types. Currently, only integers are supported.

• string

Refers to the sequence of characters. Lua is an 8-bit byte, so a string can contain any numeric character, including the embedded 0. This means you can store arbitrary binary data in a string.

 1 2 3 4 m = "abc" n = "\97\98\99" -- the value of "a" is 97((decimal),"b" is 98,"c" is 99 print(m,n) --> abc abc print(“\09798”) --> a98 

String Like other objects, Lua automatically allocates and allocates memory. A string can contain only one letter or a book. Lua can handle long strings efficiently. 1M strings are very common in Lua. You can enclose a string of characters in single or double quotes to represent a string:

 1 2 a = "hello" b = 'world' 

For uniform style, it is best to use one. For example, double quotes are always used to represent strings. If there is a single quotation mark between the double quotation marks, the system will automatically recognize the single quotation mark; if there are double quotation marks between the double quotation marks, the double quotation marks inside need to be distinguished by the escape character :

 1 2 3 4 5 6 7 print("one line\nnext line\n\"in quotes\", 'in quotes'") --[[ the result is： one line next line "in quotes", 'in quotes' ]] 

Also remember: The strings in Lua are constant. The String.sub function and other string manipulation functions in Lua do not change the value of the string, but instead return a new string. a normal mistake is :string.sub(s, 2, -2)

You may think the above function will change the value of the string s. Actually it won't. If you want to modify the value of a string variable, you must assign the variable to a new string：s = string.sub(s, 2, -2)

• table

table is the only data structure in Lua, Lua uses table to implement some other data structures like array, matrix, linked list, and queue which are provided by other programming languages.

In Lua, the key of table could be any type except nil. When an element in a table is accessed by an integer subscript, it is an array. In addition, the table has no fixed size, you can dynamically add any number of elements to a table.

 1 2 3 4 5 6 local t = {32,"jeep", name = "cherry"} local p = {["name"] = "peach"} print(t[1]) --> 32 print(t[2]) --> jeep print(t["name"],t.name) --> cherry cherry print(p["name"],p.name) --> peach peach 
• function

function is the first type of value, meaning that a function can be stored in a variable, either as a function argument or as a function's return value. This feature gives the language a lot of flexibility.

• userdata

Userdata can store C data in Lua variables. Userdata has no predefined user data in Lua except for assignment and equality comparison. It is used to describe the application or a new type created by C.

## Expressions¶

Expressions in Lua include numbers, strings, unary and binary operators, and function calls. It can also be a non-traditional table construction.

• Arithmetic Expressions

An arithmetic expression is an expression consisting of an arithmetic operator and its operands. The operand of the arithmetic operator in Lua is a real number.

Arithmetic operator in Lua includes:

binary operators: +-*/^(exponents)%(module)

unary operators: -(negative)

• Relational Expressions

An expression consisting of a relational operator and its action object is a relational expression. The result of all relational expressions is either true or false.

Relational operators supported by Lua are：>、<、>=、<=、==、~=。

Few tips：

== and ~= could be used with any two types of value

if the types of two are not the same, Lua treat them as different. nil is the same as itself.

 1 2 3 4 5 6 7 8 9 print("0" == 0) -->false print(nil == false) -->false --In particular, tables, userdata, and functions are compared by reference. In other words, they are considered equal only when they refer to the same object. For example: a = {x=1,y=2} b = {x=1,y=2} c = a print(a==c) -->true print(b==c) -->false 

Lua compares numbers by the traditional number size, comparing strings in alphabetical order, but the alphabetical order depends on the local environment.

 1 2 print( 3 < 25) -->true print( “3” <"25") -->false (alphabetical order!) 

When a string or number is compared with a different type of value with >, <, >=, <=, an error is reported.

 1 2 print( 3 <“25”) --> attempt to compare number with string print(5>=false) -->attempt to compare boolean with number 
• Logical Expressions

A meaningful expression that joins relational expressions or logical quantities with logical operators is called a logical expression.

There are 3 logical operators: and or not

Logical operators consider false and nil to be false (false), others are true, and 0 is true.

The operations of and and or are not necessarily true and false, but are related to its two operands.

  1 2 3 4 5 6 7 8 9 10 11 12 --[[ a and b -- if a is false, return a, otherwise, return b a or b -- if a is true, return a, otherwise, return b ]]-- print(4 and 5) --> 5 print(nil and 13) --> nil print(false and 13) --> false print(4 or 5) --> 4 print(false or 5) --> 5 print(true and false) --> false print(true or false) --> true 
• String Connection

String operator is ..

In string connection, if the operator is number, Lua will transfer the number into string.

 1 2 3 4 print (“hello” .. “everyone”) -->hello everyone print ( 2 .. “apples”) --> 2 apples s = 2 .. 3 print(s , type(s)) --> 23 string 
• Construction of Table

The table constructor is an expression used to create and initialize a table. The simplest constructor is the empty constructor {}, which is used to create empty tables.

 1 2 b = {x = 0, y = 1,"Monday", 109} print(b[1],b[2],b.x,b.y) -->Monday 109 0 1 

## Function¶

The function has two uses:

1. Complete the specified task, in which case the function is used as a call statement. For example,
 1 2 print(8*9,“star” ) --> 72 star print(os.date()) -->07/30/14 15:17:36调用函数的时候，如果参数列表为空，必须使用()表明是函数调用 
1. calculate and return the value, under this situation, function serves as expression in assignment statement. For example,
 1 2 l = string.len(“1234567”) print(l) --> 7 
• The matching of Lua function arguments and formal parameters is similar to the assignment statement. The excess is ignored and the missing part is complemented by nil.
 1 2 3 4 5 function f(a, b) return a and b end c = f(3)d = f(3,4)e= f(3,4,5) print(c, d, e) --> nil 4 4 
• Lua function supports returning multiple result values
 1 2 3 function foo0 () end -- returns no results function foo1 () return 'a' end -- returns 1 result function foo2 () return 'a','b' end -- returns 2 results 
1. When calling the last parameter of the expression or only one parameter, return as many values ​​as possible according to the variable number function, which is insufficient to fill the nil.
 1 2 3 4 5 6 x,y = foo2() -- x='a', y='b' x = foo2() -- x='a', 'b' is discarded x,y,z = 10,foo2() -- x=10, y='a', z='b' x,y = foo0() -- x=nil, y=nil x,y = foo1() -- x='a', y=nil x,y,z = foo2() -- x='a', y='b', z=nil 
1. Under other situation, the function call only return the first value(if no return value is nil)
 1 2 3 4 5 x,y = foo2(), 20 -- x='a', y=20 x,y = foo0(), 20, 30 -- x='nil', y=20, 30 is discarded print(foo2(), 1) --> a 1 print(foo2() .. "x") --> ax a = {foo0(), foo2(), 4} -- a[1] = nil, a[2] = 'a', a[3] = 4 
• Variable Parameter of Function

The Lua function can accept a variable number of arguments, and the function parameter has three points (...) to indicate that the function has variable parameters. Lua puts the variable arguments of the function in a table called arg. In addition to the arguments, there is a field n in the arg table that represents the number of arguments.

In function parameters, fixed and variable parameters can be declared together, but fixed parameters must be declared before the variable length parameter.

 1 2 3 function test(arg1,arg2,...) --... end 

## Basic Syntax¶

• Assignment Statement

The assignment statements in Lua are basically the same as other programming languages, the only difference being that Lua supports "multiple assignments".

 1 local x,y = “test”, 12 -- “test” is assigned to x, 12 is assigned to y 
• Local variables and blocks

You can use local to define local variables, for example: local a = "china". Local is a reserved word indicating that the variable is a local variable. Unlike global variables, the scope of a local variable is limited to the block in which it resides. The program in Lua can be the execution body of the control structure, the function execution body, or a program block.

 1 2 3 4 5 6 local x =12 if x >10 then local x = 0 print(“x=”,x) -- print out result is： x = 0, but not 12 end 
• Control statement

• if statement

The if statement has three structures:

lua if condition then statements end

if condition then statements else statements end

if condition1 then statements elseif condition2 then statements … -- many elseif statements end 

1. while statement

the syntax of while statement is：

 1 2 3 while condition do statements end 
1. for loop

The for loop statement in the Lua programming language can repeat the specified statement, and the number of repetitions can be controlled in the for statement.

There are two main types of for statements in the Lua programming language:

• Numerical for loop
• Generic for loop

Numerical for loop

luafor var=exp1,exp2,exp3 do<Executive body>end

var changes from exp1 to exp2, incrementing var in increments of exp3 for each change, and executing an "execution". Exp3 is optional. If not specified, the default is 1.

Generic for loop

The generic for loop loops through all the values ​​through an iterator function, similar to the foreach statement in java.

Generic for loop syntax format in Lua programming language:

lua--print all values in afor i,v in ipairs(a)do print(v)end

i is the index of array, v is the value corresponding to index. ipairs is a iterater function Lua offers, it is used to iterate array.

1. break and continue

Break breaks out of the inner loop, continue does not jump out of the loop, but will end this judgment.