Lua -- iterator, module, meta table

next lua Use , iterator 、 Modules and meta tables , Meta table is the key content that needs attention

One 、 iterator

pairs It's an iterator , Its return value is key and value, Let's customize an iterator

Define iterator Syntax ： function Iterator name ( End value parameter , Initial value parameter ) end

Use iterator Syntax ： for The first return value , Second return value in Iterator name , Termination value , Initial value do end

--  The first parameter ： Termination value   The second parameter ： Initial value 
function double(max,init)
        if(init < max) then
                init = init + 1
                return init,init*2
        end
end

-- in Back , Follow the iterator name , Termination value , Initial value   
for i,d in double,10,0  do
        print(i,d)
end

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Two 、 modular

lua5.1 Start supporting modules , Functions can be encapsulated in modules , Reference modules in other scripts

1. New module

--  Define modules 
module = {}

--  Module constants 
module.name = ' My module '
module.time = '2022'

--  Methods of public disclosure   Use module. Method name definition 
function module.func1()
        print("func1")
end

--  Internal methods , Cannot add moudle.
local function func2()
        print("func2")
end

function module.func3()
        func2()
end

--  Return module 
return module

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2. Other scripts introduce modules

--  Introduce modules 
require("module")

print(module.name..module.time)

print(module.func1())
print(module.func3())

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3、 ... and 、 Metatable

Meta table allows you to change table act , Yes table Extend in disguise , It can even achieve the function of simulating classes

1. Meta table definition and use

Meta table and table equally , Use {} Definition , The use of meta tables needs to be combined with ordinary tables , call setmetatable Method

a = {1,2,3} --  Common watch 
b = {} --  Metatable 
c = setmetatable(a,b)

print("table:",a)
print("metatable:",b)
print("ret:",c)
print("getmetatable:",getmetatable(c))

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You can find setmetatable The return value of is a normal table ,getmetattable The return value of is meta table

2. index Metamethod

index The meta method is defined as {__index = value }, There are two uses

2.1 Combine two common tables

--  Two ordinary watches 
e = {'a','b'}
f = {c = 'c'}

--  Meta table will index The meta method is assigned to f
g = {__index = f}

--  by e Table set meta table 
setmetatable(e,g)

print(e['c'])

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2.2 The extension does not correspond to key Method of value taking

--  Common watch 
e = {'a','b'}

--  Meta table will index The meta method is assigned to a function 
g = {
        __index = function(tab,key)
                if(key[tab] == nil) then
                        return 'null'
                end
        end     
}

--  by e Table set meta table 
setmetatable(e,g)

print(e['c'])
print(e[1])

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3. newindex Metamethod

newindex The meta method is defined as {__newindex = value }, There are also two uses

3.1 Store the new element in another normal table

--  Two ordinary watches 
e = {'a','b'}
f = {}

--  Meta table will newindex The meta method is assigned to f
g = {__newindex = f}

--  by e Table set meta table 
setmetatable(e,g)

--  Set new element 
e[5] = 'f'
print(e[5])
print(f[5])

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3.2 Extension insert new element method

--  Common watch 
e = {'a','b'}

--  Meta table will newindex The meta method is assigned an anonymous function 
g = {
        __newindex = function(tab,key,val)
                print(" new key:"..key," new value:"..val)
                rawset(tab,key,val)
        end
}

--  by e Table set meta table 
setmetatable(e,g)
e['a'] = 'hello'

print(e['a'])

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4. Operator overloading

The operator overloading of the meta table is defined as follows ：

The way	Corresponding operator
__add	+
__sub	-
__mul	*
__div	/
__mod	%
__concat	..
__eq	==
__lt	<
__le	<=
__gt	>
__ge	>=

test , Merge two table：

--  Operator overloading 
a = {'a','b'}
b = {'c','d'}

c = {
        __add = function(tab1,tab2)
                --  Get the first one table The length of 
                local t1_len = #tab1
                --  Traversal of the second table
                for k,v in pairs(tab2) do
                        t1_len = t1_len + 1
                        --  Put the second tab The value of is assigned to the first tab
                        tab1[t1_len] = v
                end
                return tab1
        end
}

setmetatable(a,c)
a = a + b

for k,v in pairs(a) do
        print(v)
end

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5. call Metamethod

When a table is called as a function , Will trigger

-- call Metamethod 
a = {1,2}

c = {
        __call = function(tab,...)
                for k,v in pairs({...}) do
                        print(k,v)
                end
        end
}

setmetatable(a,c)

a(1,2,3,5,6,7)

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6. tostring Metamethod

Rewrite the method of printing , and Java It's similar to

a = {'a','c','e'}


c = {
        __tostring = function(tab) 
                local ret = ''
                for k,v in pairs(tab) do
                        ret = ret..v..','
                end
                return ret
        end
}

setmetatable(a,c)

print(a)

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