Difference between revisions of "Lua Language"

Revision as of 17:03, 1 June 2024

Comments

In Lua simple comments begin with -- and end at the end of the line. Multi-line comments begin with --[[ and end with ]] .

-- This is a simple comment just to the end of this line
This is not a comment but a future syntax error -- This is a new simple comment
--[[ First line of a multi-line comment
Second line of a multi-line comment
Last line of a multi-line comment]]

Variables, types and operators

In Lua variables are polymorphic (can change the type during theirs lifes). By default a variable is global at Lua Engine level unless the declaration begins with local modifier.

Any variable can host different data types and use specific operators:

nil meaning nothing (a variable not defined, or not assigned, or assigned to nil)
- Comparing operators: comparing any variable to nil ( var == nil or var ~= nil )
a number value (no difference between integer or float values, nor between different precisions)
- Binary arithmetic operators between two numbers: +, -, *, /, // (floor division), % (modulo), ^ (power).
- Unary arithmetic operator: - (example: var1 = -var2)
- Comparing operators between two numbers giving a boolean: >, >=, <, <=, ==, ~=.
- Bitwise operators (only on integer values): & bitwise AND, | bitwise OR, ~ bitwise XOR, ~ bitwise NOT, << left shift, >> right shift.
a boolean value (equal to true or false)
- Binary boolean operators: and, or.
- Unary boolean operator: not.
a string (constants can be surrounded by single 'string'or double quotes "string" or double square brackets [[string]]; the last case accepts new lines inside)
- Concatenation operator: .. between two strings.
- Comparing operators between two strings content giving a boolean: >, >=, <, <=, ==, ~=.
- Length operator: # returns the number of characters
a table
- Indexing operator: [ ] returns the associated value of an index/key in a table
- Indexing operator: . returns the associated value of a string key in a table
- Length operator: # returns the integer index of the first element not equal to nil
a function

The assign operator = is mandatory just for the global variable declaration and can be use later to modify any variable. Lua supports parallel assignment from a list of values to a list of variables: Var1, Var2, Var3 = 7, "GoesInVar2", 12*12. First, all values expressions in the right side list are evaluated, any extra value is ignored and any missing value is replaced by nil. Then these values are assigned to the corresponding variables.

Tips:

Always use the right operators for the right types.
Always use parethesis for complex expressions (the expression not var1==nil or var1~=3 is not equal to not (var1==nil or var1~=3) ).
Use the function type to get a string containing the real type of a variable.
Prefer using local variables versus global variables.

Examples:

local MyVar0           -- local variable having nothing inside so equal to nil
MyVar1 = nil           -- global variable MyVar1 having nothing inside (no type neither)
MyVar2 = UnknownVar    -- another global variable equal to nil (because UnknowVar is not yet defined)
local MyVar3 = 2+0x10  -- local number variable MyVar3 equal to 18 (2+16)
MyVar4 = 'Hello'       -- global string variable MyVar4
local MyVar5 = "Hello" -- local string variable MyVar5 having the same string as MyVar3
MyVar6 = [[First Line
Second Line.]]         -- global multi-line string variable MyVar6 beginning with 'First' and ending with 'Line.'
MyVar2 = (1e3 > 7.55)  -- now MyVar2 becomes a boolean equal to true
MyVar7 = print         -- global function variable MyVar7 which can be used as an alias of print function
MyVar2, MyVar3 = MyVar3, MyVar2 -- parallel assignment: MyVar2 becomes 18 and MyVar3 becomes true

Automatic typecast (coercion):

When some operators or function calls need it, Lua will try to convert automatically numbers to strings or strings to numbers:

The expression '1'+9 is valid and is a number because the string '1' will be automatically converted to the number 1.
But the expression 'one'+9 is a not valid because the string 'one' can't be automatically converted to a number.
The expression 'Waiting '..10 is valid and is a string because the number 10 will be automatically converted to the string '10' by the presence of .. operator.
But the expressions 7..'days' or 'Waiting '..10..' seconds' are invalid because Lua interprets the first dot . after 7 or 10 as a decimal separator and then it is unable to detect the concatenation operator ...
Solution: use spaces and parenthesis as much as you need. The following expressions are valid: ('Waiting '..10)..' seconds' , 'Waiting '.. 10 ..' seconds' , and 7 ..'days' .

Copy operations:

For any copy operation like assignment, function calling arguments and function returns, Lua performs a copy by value (real copy) for nil, numbers, strings, booleans types and performs a copy by reference (not a new copy, just pointing to the original) for tables and functions.

Console output

Lua uses the print function to output numericals and strings to the console (use the menu to open the Lua Engine console). The print function has a variable number of arguments. Each argument (string or number) will be printed to the console with spaces between arguments and a new line will be added at the end. The default format for numbers is the decimal base with automatic use of decimal point and/or decimal power if necessary.

Examples:

print(MyVar1)                            -- prints an empty line because MyVal1 is nil
print('MyVar3=',MyVar3,MyVar3,MyVar3)    -- prints the line 'MyVar3=2 2 2'
print("MyVar2 = ",MyVar2)                -- prints just the line 'MyVar2 = ' because MyVar2 is not a numerical nor a string
print(MyVar4..' and '..MyVar5..' again') -- prints 'Hello and Hello again'
print(#MyVar5)                           -- prints 5 (the lenght of MyVar5)
MyVar7("9.9+0.1=",9.9+0.1)               -- prints the line '9.9+0.1=10.0' (because MyVar7 is now equal to print)
MyVar7('1/0=',1/0)                       -- prints the line '1/0=inf'

Flow control

Lua provides a conditional test with multiple, optional branches:

if BooleanExpression then
  -- if statement(s) body
elseif BooleanExpression then  -- optional
  -- elseif statement(s) body
else                           -- optional
  -- else/default statement(s) body
end

and several conditional loops:

while BooleanExpression do    -- do statements while BooleanExpression is true
  --statements
end

repeat                        -- repeat statements
  --statements
until BooleanExpression       -- until BooleanExpression becomes true

for Idx = Start,Stop,Step do  -- any numerical value for Start, Stop and Step (Step is optional and defaults to 1)
  --statements
end                           -- loop while Idx <= Stop (for positive Step) or Idx >= Stop (for negative Step)

for KeyIdx,Val in ListExpr do -- it loops over all the ListExpr, giving for each element the KeyIdx and the Val
  --statements                -- KeyIdx and Val available inside the for loop
end

Lua has a special unconditional block mainly used to isolate some local variables:

do
  --statements
end

The break instruction breaks any type of loop and jump to the next instruction after the loop. Any flow control instruction can be embedded in any other flow control instruction.

Examples:

i=5
while i<80 do                 -- it loops for i equal to 5, 10, 20, and 40
  i=i*2
end

i=5
repeat                        -- it loops for i equal to 10, 20, 40, 80
  i=i*2
until i==80

i=5
repeat
 i=i*2
until i==82                   -- warning: this loop will never ending and will block the Lua Engine & Cheat Engine

for i = 10, 14 do             -- it loops for i equal to 10, 11, 12, 13, and 14
end

for i = 10, 14, 2 do          -- it loops for i equal to 10, 12, and 14
end

for i = 10, 14, 2.1 do        -- it loops for i equal to 10 and 12.1
end

for i = 10, 14, -2.1 do       -- it never loops because the start value 10<14 and step is negative
end

for i = 14, 10, -2.1 do       -- it loops for i equal to 14 and 11.9
end

do
  local MyVar=314;
  print(MyVar)                -- prints '314' and new line
end
print(MyVar)                  -- prints nothing but a new line

Functions

Functions are just some special variables, and can be declared at the global level or inside a block or another function.

-- first syntax version
function MyFunction(arg1, arg2, ...)   -- takes 0, 1 to several arguments or undefined number of arguments
  -- statements
  return result1, result2              -- returns 0, 1 to several variables
end

-- second syntax version
MyFunction = function(arg1, arg2, ...) -- takes 0, 1 to several arguments or undefined number of arguments
  -- statements
  return result1, result2              -- returns 0, 1 to several variables
end

Any of preceding 2 declaration versions creates a function variable named MyFunction. Then the user can call it, use it or assign it a new value.

Examples:

MyFunction(MyVar4, MyVar2, MyVar7)   -- calling MyFunction with 3 arguments
MyVar1 = MyFunction(MyVar2, MyVar7)  -- calling MyFunction with 2 arguments and keep the first result in MyVar1
print = MyFunction                   -- now the official print function will be MyFunction
MyFunction = nil                     -- MyFunction will be unavailable now (but print still keeps original MyFunction)
MyFunction = MyVar7                  -- now MyFunction is an alias of the original print function

Any type of arguments or return values are copied by value except for table arguments copied by reference. Any absent argument at function call will have a nil value during calling execution.

Examples:

Func1=function(Tab, Idx)
  if(Idx==nil) then Idx=1 end        -- giving a default value 1
  Tab[Idx]=-100                      -- changing Idx-th element in the real calling argument table (reference not copy)
  Idx=33                             -- changing Idx value, no effect to the real argument, this is a local copy value 
end

function Show(Tab)                   -- prints the first 3 elements of Tab, on the same line
  print(Tab[1],Tab[2],Tab[3])
end

function Show2(Tab)                  -- prints the first 3 elements of Tab, one by line
  for Idx=1,3 do print(Tab[Idx]) end
end

function Show3(Tab)                  -- prints all elements of Tab, one by line
  for Idx in ipairs(Tab) do          -- using the build-in iterator ipairs on input table
    print(Tab[Idx])                  -- prints value using the [] operator
  end
end

function Show4(Tab)                  -- prints all elements of Tab, one by line
  for Idx,Val in ipairs(Tab) do      -- using the build-in iterator ipairs on input table
    print(Val)                       -- prints directly the value
  end
end

local MyTab1 = {10, 20, 30}          -- local table with 3 numbers
local MyIdx = 2
Show(MyTab1)                         -- prints '10 20 30'
Func1(MyTab1,MyIdx)                  -- change value of 2nd element of MyTab1 but no the value of MyIdx
Show(MyTab1)                         -- prints '20 -100 30'
Func1(MyTab1)                        -- change value of 1st element of MyTab1 but no the value of MyIdx
Show(MyTab1)                         -- prints '-100 -100 30'
Show2(MyTab1)                        -- prints on 3 lines '-100' '-100' '30'
Show3(MyTab1)                        -- prints on 3 lines '-100' '-100' '30'
Show4(MyTab1)                        -- prints on 3 lines '-100' '-100' '30'

Tables

Tables are a very special build-in data type providing the only way to create user-defined data structures. Tables are heterogeneous associative arrays where each element is a pair of a key/index and a data/value. Data and key types can be different between elements. Lua provides automatic numeric key creation and build-in functions for access, iteration, adding etc.

A table declaration uses { } and inside elements are separated by , . Implicit keys/indexes are automatically generated and start with 1. Explicit keys/indexes declaration uses a special syntax for each element:[key]=value . A key can't be nil nor a NaN number.

Tables syntactic sugar:

Keys of type string, without spaces:
- Can be declared without surrounding squared breaks and unquoted (like the key of the first element of Tab3 in examples).
- Can use the point operator . instead of squared brackets operator [ ]. These make tables usage similar to structures/records/classes/namespaces in C/C++/Pascal.
Automatic insertion:
- Assigning an unknown (but valid) key in a existing table will automatically create/add this element.

Examples:

-- Creates an empty table
Tab0 = {}                            

-- Creates an homogenous table of 3 pairs (number key/index, number value): (1,5), (2,10), and (3,20)
Tab1 = {5, 10, 20}                   

-- Creates an heterogeneous table of 3 pairs: (number 1,number 0), (number 2,string "second"), (number 3,function print)
Tab2 = {0, "hours", print}

-- creates an heterogeneous table Tab3 of 3 pairs: (string 'no_space',number 8), (string 'string with spaces',boolean true), (number 12,table Tab2)
-- Warning: the value of the 3th element is not a new copy of Tab2 but really Tab2 (Lua uses copy by reference for tables!)
Tab3 = {no_space = 8, ['string with spaces'] = true, [12] = Tab2}

-- The next quasi-unreadable instruction will print '8 hours'
Tab2[3](Tab3["no_space"],Tab3[12][2])
-- Explanation: Tab2[3] value is print, Tab3["no_space"] is 8, Tab3[12] is Tab2 and Tab2[2] is "hours"

Tab3[12][1]=Tab1[2]    -- write 10 (value of 2nd element of Tab1) in the value of Tab2[1] (first element of Tab2)
Tab3.no_space = -8     -- write -8 in the value of the first element of Tab3
Tab3.FourthElement=3.7 -- create a new element in Tab3 with key 'FourthElement' and value 3.7

function ShowTab(tab)
  for i,val in ipairs(tab)
    print('tab[',i,']=',val);
  end
end

ShowTab(Tab1);         -- prints each element of Tab1, one by line

function ShowTypeTab(tab)
  for key,val in pairs(tab)
    print('type(tab[',key,']) =',type(val))
  end
end

ShowTypeTab(Tab1);     -- type(tab[ 1 ]) = number
                       -- type(tab[ 2 ]) = number
                       -- type(tab[ 3 ]) = number

ShowTypeTab(Tab3);     -- type(tab[ string with spaces ]) = boolean
                       -- type(tab[ no_spaces ]) = number
                       -- type(tab[ ForthElement ]) = number
                       -- type(tab[ 12 ]) = table

Tab2D = {{9,2,3},
         {6,5,4},
         {7,8,1}}      -- embedded tables emulate a 3x3 matrix
print(Tab2D[1][1],Tab2D[2][2],#Tab2D,#Tab2D[3],#Tab2D[4])
-- prints '9 5 3 3' (lenght of Tab2D is 3 and lenght of Tab2D[3] is 3 too)

Table functions: Lua provides several functions in the table (namespace) table:

table.insert(tab, val, pos) : insert val in the table tab as new element, after the optional key pos or at the end.
table.remove(tab, pos) : remove from table tab the element with key pos.
table.sort(tab) : sort all elements of tab in increasing order of theirs values.
table.concat(tab, sep) : concatenates all values of tab using an optional separator.

Examples:

numbs = {'One','Two','Three','Four'}
print(table.concat(numbs,' ')) -- 'One Two Three Four'
table.insert(numbs,'Five')
print(table.concat(numbs,' ')) -- 'One Two Three Four Five'
table.sort(numbs)
print(table.concat(numbs,' ')) -- 'Five Four One Three Two'
table.remove(numbs,3)
print(table.concat(numbs,' ')) -- 'Five Four Three Two'

Difference between revisions of "Lua Language"

Revision as of 17:03, 1 June 2024

Contents

Introduction

Syntax

Comments

Variables, types and operators

Console output

Flow control

Functions

Tables

External links

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