Lua unpack function usage

unpack, accepts a table as a parameter, and then returns all elements of the array according to the subscript

unpack

lua version <= 5.1

local t = {nil , 3}
retunrn unpack(t)   // nil , 3

table.unpack

lua version > 5.1

local t = {nil, 3}
return table.unpack(t) // nil, 3

About the processing of indefinite number of parameters

Version 5.1 and earlier can be processed directly

local function fun1(...)
    print(unpack(arg))  
end

fun1(1,nil,3) //1,nil,3

After version 5.2, manual conversion is required

local function fun2(...)
    local arg = { ... }
    print(table.unpack(arg))
end
fun2(1,nil,3) //1,nil,3

table.length problem

There is a very interesting problem.
If the parameter is {nil, 2, nil}, it cannot be printed normally.
Why? After reading the documentation, I found that it is related to #table

table calculation method

That is to say, when there is a nil value, the calculation result of #table is not certain

local function fun(...)
   local arg = {...}
   print(table.unpack(arg))
   for k, v in pairs(arg) do
      print(k, v)
   end
   print(arg [1],arg[2],arg[3], #arg)
   print('***************')
end

fun(nil, 2, nil)
fun(1,2,nil)
fun(1,nil,nil)
fun(nil,nil,3)

2   2
nil 2   nil 0
***************
1   2
1   1
2   2
1   2   nil 2
***************
1
1   1
1   nil nil 1
***************
nil nil 3
3   3
nil nil 3   3
***************

In conclusion, the result of #table is undefined when there are nil values ​​in the table.
So when using table in the future, try not to set nil, but to table.remove