Lua table使用

  days = {"Sunday", "Monday", "Tuesday", "Wednesday",
            "Thursday", "Friday", "Saturday"}

will initialize days[1] with the string "Sunday" (the first element has always index 1, not 0), days[2] with "Monday", and so on:

    print(days[4])  --> Wednesday table从1开始，不是从0开始
还有一点，访问不存在的index不会报错，只是返回nil。

Constructors do not need to use only constant expressions. We can use any kind of expression for the value of each element. For instance, we can build a short sine table as

    tab = {sin(1), sin(2), sin(3), sin(4),
           sin(5), sin(6), sin(7), sin(8)}

To initialize a table to be used as a record, Lua offers the following syntax: table作为record使用

    a = {x=0, y=0}

which is equivalent to

    a = {}; a.x=0; a.y=0

No matter what constructor we use to create a table, we can always add and remove other fields of any type to it:

    w = {x=0, y=0, label="console"}
    x = {sin(0), sin(1), sin(2)}
    w[1] = "another field"
    x.f = w
    print(w["x"])   --> 0
    print(w[1])     --> another field
    print(x.f[1])   --> another field
    w.x = nil       -- remove field "x"

That is, all tables are created equal; constructors only affect their initialization.

Every time Lua evaluates a constructor, it creates and initializes a new table. Consequently, we can use tables to implement linked lists:

每次执行一个构造函数，lua创建和初始化一个新的table，因此，你可以使用table来实现linked list.

    list = nil
    for line in io.lines() do
      list = {next=list, value=line}
    end

This code reads lines from the standard input and stores them in a linked list, in reverse order. Each node in the list is a table with two fields: value, with the line contents, and next, with a reference to the next node. The following code prints the list contents:

    l = list
    while l do
      print(l.value)
      l = l.next
    end

(Because we implemented our list as a stack, the lines will be printed in reverse order.) Although instructive, we hardly use the above implementation in real Lua programs; lists are better implemented as arrays, as we will see in Chapter 11.

We can mix record-style and list-style initializations in the same constructor:

我们可以混合记录性风格和列表形风格来初始化。

    polyline = {color="blue", thickness=2, npoints=4,
                 {x=0,   y=0},
                 {x=-10, y=0},
                 {x=-10, y=1},
                 {x=0,   y=1}
               }

The above example also illustrates how we can nest constructors to represent more complex data structures. Each of the elements polyline[1], ..., polyline[4] is a table representing a record:
这里例子也展示了可以嵌套构造函数来表示复杂的结构。

    print(polyline[2].x)    --> -10 

Those two constructor forms have their limitations. For instance, you cannot initialize fields with negative indices, or with string indices that are not proper identifiers. For such needs, there is another, more general, format. In this format, we explicitly write the index to be initialized as an expression, between square brackets: 用[exp]表示索引

    opnames = {["+"] = "add", ["-"] = "sub",
               ["*"] = "mul", ["/"] = "div"}

    i = 20; s = "-"
    a = {[i+0] = s, [i+1] = s..s, [i+2] = s..s..s}

    print(opnames[s])    --> sub
    print(a[22])         --> ---

That syntax is more cumbersome, but more flexible too: Both the list-style and the record-style forms are special cases of this more general one. The constructor 这个语法更笨重，但是比较灵活。list-style和record-style是这种一般化形式的特例。

    {x=0, y=0}

is equivalent to

    {["x"]=0, ["y"]=0}

and the constructor

    {"red", "green", "blue"}

is equivalent to

    {[1]="red", [2]="green", [3]="blue"}

For those that really want their arrays starting at 0, it is not difficult to write the following:

想要index从0开始，把第一个设置为0，这并不影响其他的，第二个默认为1，因为它是构造函数中的第一个list类型，其他的值跟随他。

 days = {[0]="Sunday", "Monday", "Tuesday", "Wednesday",
            "Thursday", "Friday", "Saturday"}

Now, the first value, "Sunday", is at index 0. That zero does not affect the other fields, but "Monday" naturally goes to index 1, because it is the first list value in the constructor; the other values follow it. Despite this facility, I do not recommend the use of arrays starting at 0 in Lua. Remember that most functions assume that arrays start at index 1, and therefore will not handle such arrays correctly. 

不推荐在Lua最开始位置为0，因为大多数函数式是从1开始的。

You can always put a comma after the last entry. These trailing commas are optional, but are always valid:

    a = {[1]="red", [2]="green", [3]="blue",} 同php一样，table后面放分号不影响正确性。

Such flexibility makes it easier to write programs that generate Lua tables, because they do not need to handle the last element as a special case。

Finally, you can always use a semicolon instead of a comma in a constructor. We usually reserve semicolons to delimit different sections in a constructor, for instance to separate its list part from its record part:

    {x=10, y=45; "one", "two", "three"}
最后，你可以使用分号代替逗号，我们通常保留分号来区分不同的段。

删除table
赋值为nil即可。
lua table作为array：

Tables as arrays

First, remember that tables are still just key/value containers, Lua doesn't actually have an array type. But tables can be treated like arrays, which is explained here:

Table constructors can contain a comma separated list of objects to create an "array":

> t = {"a", "b", "c"}
> = t[1]
a
> = t[3]
c

This is a syntax shortcut for:

> t = {[1]="a", [2]="b", [3]="c"}
> = t[1]
a
> = t[3]
c

So it's still just a key/value association.

You can also mix the array syntax with the usual key=value syntax:

> t = {"a", "b", [123]="foo", "c", name="bar", "d", "e"}
> for k,v in pairs(t) do print(k,v) end
1       a
2       b
3       c
4       d
5       e
123     foo
name    bar

求array length,

> t = {"a", "b", "c"}
> = #t
3

The # operator doesn't count all the items in the table (!), instead it finds the last integer (not-fractional number) key. Because of how it's implemented, its results are undefined if all the integer keys in the table aren't consecutive (that is, don't use it for tables used as sparse arrays[2]).

> a={}
> a[1000]=1
> print(#a)

输出0.

请记住对于所有未初始化的元素的索引结果都是nil。Lua将nil作为定界数组结尾的标志。当一个数组有
“空隙hole”是，即中间含有nil时，长度操作符会认为这些nil元素就是结尾标志，因此应该避免对于那些
含有“空隙”的数组使用#。大多数数组不会包含“空隙”，因此，#一般是安全的。
如果真的需要处理那些含有“空隙”的数组，使用使用table.maxn.他将返回一个table的最大正index。

print(table.maxn(a)) ->1000

There are two ways to add an item to the end of an array:

> t = {}
> table.insert(t, 123)
> t[#t+1] = 456
> = t[1]
123
> = t[2]
456

table.insert takes an optional index parameter to insert into the middle of an array. It shifts up any other integer keys above the index:

> t = {"a", "c"}
> table.insert(t, 2, "b")
> = t[1], t[2], t[3]
a b c

table.remove removes an item from an array, shifting down any remaining integer keys:

> t = {"a", "b", "c"}
> table.remove(t, 2)
> = t[1], t[2]
a c

To loop over an array, use ipairs. Unlike pairs, it only gives you the consecutive integer keys from 1, and it guarantees their order. With pairs, the number keys will not necessarily be given in the correct order!

> t = {"a", "b", "c"}
> for i, v in ipairs(t) do print(i, v) end
1       a
2       b
3       c

To join together an array of strings, there's table.concat. It takes optional separator, start, and end parameters. Here we only use the separator:

> t = {"a", "b", "c"}
> = table.concat(t, ";")
a;b;c

For a list of all the table.* functions and their complete documentation, see [[3]].

Table values are references

Unlike basic types such as numbers, when you store a table in a new variable, pass it to a function, etc., you don't create a new copy of the table. Instead you get a new reference (think of it like a handle, or pointer) to the same table: table值是引用.

> t = {}
> u = t
> u.foo = "bar"
> = t.foo
bar
> function f(x) x[1] = 2 end
> f(t)
> = u[1]
2

Tables are freed from memory by the garbage collector sometime after the last reference to them is gone (not necessarily instantly). The garbage collector is designed to work correctly even in the case where a table (directly or indirectly) contains a reference to itself.

A related thing to remember is that table comparison works by reference. Comparing tables using == will return false even if the two tables have the same contents, they must actually be references to the same table.

Finally, if you want to copy a table, you'll have to do it manually. Lua offers no standard function for it, mainly because of all the different ways you can copy a table.

Tables as unordered sets

Often people new to Lua will create an array to store a group of objects, even if the order isn't necessary. The problem with this is that removal is slow (need to shift down other items), and checking if an item is in the array is slow (need to loop over all the items).

This can be solved by storing items in the keys and setting values to a dummy value (like true), so you can use a table like an unordered set with fast insertion, removal, and lookup.

The main differences are that there's no easy way to get the count (you have to use a loop), and you can't store the same item twice in the set.

So if you need to store a group of items, it's best to consider both sets and arrays to see what fits your situation best.

local items = {}

-- add some items to the set
items["foo"] = true
items[123] = true

-- is "foo" in the set?
if items["foo"] then
  -- do stuff
end

-- remove item from the set
items[123] = nil

参考：http://www.lua.org/pil/3.6.html

 http://lua-users.org/wiki/TablesTutorial