Lua 入门

变量

var = 50 -- a global variable
print(var) --> 50
do
  local var = 100 -- a local variable
  print(var) --> 100
end
print(var) --> 50
-- The global var (50) still exists 
-- The local var (100) has gone out of scope and can't be accessed any longer.

类型

num = 20 -- a number
num = 20.001 -- still a number
str = "zaldrizes buzdari iksos daor" -- a string
tab = {1, 2, 3} -- a table (these have their own category)
bool = true -- a boolean value
bool = false -- the only other boolean value
print(type(num)) --> 'number'
print(type(str)) --> 'string'
print(type(bool)) --> 'boolean'
type(type(num)) --> 'string'

-- Functions are a type too, and first-class values in Lua.
print(type(print)) --> prints 'function'
old_print = print
print = function (x) old_print "I'm ignoring the param you passed me!" end
old_print(type(print)) --> Still prints 'function' since it's still a function.
-- But we've (unhelpfully) redefined the behavior of print.
print("Hello, world!") --> prints "I'm ignoring the param you passed me!"

特殊类型 nil

Lua 的另一种类型是 nil。nil 类型中唯一的值是 nil。nil 与 Lua 中的所有其他值不同。这是一种非价值的价值。

print(foo) -- This prints nil since there's nothing stored in the variable 'foo'.
foo = 20
print(foo) -- Now this prints 20 since we've assigned 'foo' a value of 20.

-- We can also use `nil` to undefine a variable
foo = nil -- Here we set 'foo' to nil so that it can be garbage-collected.

if nil then print "nil" end --> (prints nothing)
-- Only false and nil are considered false; every other value is true.
if 0 then print "0" end --> 0
if "" then print "Empty string!" --> Empty string!

表达式

a = 3
b = a + 20 a = 2 print(b, a) -- hard to read, can also be written as
b = a + 20; a = 2; print(a, b) -- easier to read, ; are optional though
true and true --> returns true
true and 20 --> 20
false and 20 --> false
false or 20 --> 20
true or 20 --> true
tab or {}
  --> returns tab if it is defined
  --> returns {} if tab is undefined
  -- This is useful when we don't know if a variable exists
tab = tab or {} -- tab stays unchanged if it exists; tab becomes {} if it was previously nil.

a, b = 20, 30 -- this also works
a, b = b, a -- switches values

定义功能

function name(parameter)
    return parameter
end
print(name(20)) --> 20
-- see function category for more information
name = function(parameter) return parameter end -- Same as above

布尔

只有 false 和 nil 评估为 false，其他一切，包括 0 和空字符串评估为 true。

垃圾收集

tab = {"lots", "of", "data"}
tab = nil; collectgarbage()
-- tab does no longer exist, and doesn't take up memory anymore.

表

tab1 = {"a", "b", "c"}
tab2 = tab1
tab2[1] = "d"
print(tab1[1]) --> 'd' -- table values only store references.
--> assigning tables does not copy its content, only the reference.

tab2 = nil; collectgarbage()
print(tab1) --> (prints table address) -- tab1 still exists; it didn't get garbage-collected.

tab1 = nil; collectgarbage()
-- No more references. Now it should actually be gone from memory.

这些是基础知识，但是有一个关于具有更多信息的表的部分。

条件

if (condition) then
    -- do something
elseif (other_condition) then
    -- do something else
else
    -- do something
end

for 循环

Lua 中有两种类型的 for 循环：数字 for 循环和通用 for 循环。

• 数字 for 循环具有以下形式：

  for a=1, 10, 2 do -- for a starting at 1, ending at 10, in steps of 2
    print(a) --> 1, 3, 5, 7, 9
  end
  

  数字 for 循环中的第三个表达式是循环递增的步骤。这使得反向循环变得容易：

   for a=10, 1, -1 do
     print(a) --> 10, 9, 8, 7, 6, etc.
   end
  

  如果省略步骤表达式，Lua 假定默认步长为 1。

   for a=1, 10 do
     print(a) --> 1, 2, 3, 4, 5, etc.
   end
  

  另请注意，循环变量是 for 循环的局部变量。循环结束后它将不存在。

• 通用 for 循环遍历迭代器函数返回的所有值：

  for key, value in pairs({"some", "table"}) do
    print(key, value)
    --> 1 some
    --> 2 table
  end
  

  Lua 提供了几个内置的迭代器（例如，pairs，ipairs），用户可以定义自己的自定义迭代器以及与通用的 for 循环一起使用。

做块

local a = 10
do
    print(a) --> 10
    local a = 20
    print(a) --> 20
end
print(a) --> 10