在_lua_中，数组下标从1开始计数。
官方：Lua lists have a base index of 1 because it was thought to be most friendly for non-programmers, as it makes indices correspond to ordinal element positions.
在初始化一个数组的时候，若不显式地用键值对方式赋值，则会默认用数字作为下标，从1开始。由于在_lua_内部实际采用哈希表和数组分别保存键值对、普通值，所以不推荐混合使用这两种赋值方式。
local color={first="red", "blue", third="green", "yellow"}
print(color["first"]) --> output: red
print(color[1]) --> output: blue
print(color["third"]) --> output: green
print(color[2]) --> output: yellow
print(color[3]) --> output: nil