Lua talbe's pairs & ipairs & traverse loop, table.pack and table.unpack

Lua 表有几种形式用得比较多,
一种是sequence, 一种是table, 还有一种是list.

1. sequence指没有气泡的一些元素. 例如{"hello", "nihao","yes"}是一个sequence, 索引为1开始的自增数字.

但是{"hello", "nihao", nil, "yes"} 不算序列. 因为中间有个nil气泡.

> t = {"hello", "nihao", nil, "yes"}
> for i,v in ipairs(t) do
>> print (i,v)
>> end
1       hello
2       nihao
使用ipairs遍历t表只输出前两个值

2. table指key, value形式存储的表.

例如tbl = {a="nihao", b=nil, c="hello", d="yes"}

使用pairs可以遍历这个表, 会忽略value=nil的元素.

> tbl = {a="nihao", b=nil, c="hello", d="yes"}
> for k,v in pairs(tbl) do
>>  print(k,v)
>> end
a       nihao
d       yes
c       hello

其实pairs是调用了next(_s, _var)函数, 所以我们知道generic for语法的话, 可以直接使用next来写for循环.

> for k,v in next, tbl do
>>   print(k,v)
>> end
a       nihao
d       yes
c       hello

直接调用next的效果

> =next(tbl,nil)
a       nihao
> return next(tbl,"a")
d       yes
> return next(tbl,"d")
c       hello
> return next(tbl,"c")
nil

pairs原型

function pairs(t)
  return next, t, nil
end

3. list指链表.

> a = {"hello", next=nil}  -- 链表的头
> a = {"nihao", next=a}  -- next指向头
> a = {"yes", next=a}  -- next指向上层
> a = {"ok", next=a}  -- next指向上层 

打印链表的值.

> print(a[1])
ok
> print(a.next[1])
yes
> print(a.next.next[1])
nihao
> print(a.next.next.next[1])
hello

生成一个链表和输出链表的值的例子

> list = nil
>     for line in io.lines() do
>>       list = {next=list, value=line}
>>     end
a
b
c
d
e
^D
>  l = list
>     while l do
>>       print(l.value)
>>       l = l.next
>>     end
e
d
c
b
a

链表的generic for循环函数用法 .

iterator函数, 函数中不保存state值. 所以也称为stateless iterator

> function getnext(list, node)
>>   if not node then
>>     return list
>>   else
>>     return node.next
>>   end
>> end

factory 函数

> function traverse(list)
>>   return getnext, list, nil
>> end

使用io.lines生成链表

> list = nil
> for line in io.lines() do
>>   list = {val = line, next = list}
>> end
hello
yes
digoal
hehe
^D

使用factory函数循环

> for node in traverse(list) do
>>   print (node.val)
>> end
hehe
digoal
yes
hello

直接使用stateless iterator函数和state值循环

> for node in getnext,list do
  print (node.val)
end
hehe
digoal
yes
hello

4. 

table.pack 和table.unpack

table.pack把多个值打包成一个序列表.
> a = table.pack("a","b","c","nil")
> for i,v in ipairs(a) do
>>   print (i,v)
>> end
1       a
2       b
3       c
4       nil
table.unpack把序列表解散成多值返回.
> x,y,z = table.unpack(a)
> print(x,y,z)
a       b       c
> x,y,z = table.unpack({a=1,b=2,c=3})  -- unpack不处理非序列表元素.
> print(x,y,z)
nil     nil     nil
> x,y,z = table.unpack({a=1,b=2,c=3,"h","j","k"})  -- 只解包序列部分
> print(x,y,z)
h       j       k