lua中的坑

　　在工作中使用lua也有一年了，代码也写了不少，踩过不少坑，这里记录一下。

• table.sort

　　table.sort是lua自带的排序函数，数据量小时，也还是不错的。不过要注意你传入的compare函数。例如:

local tb = { 9,8,3,777,0,36548,556,0 }

table.sort( tb,function(a,b) return a>=b end )

上面的代码够简单了，但是你运行起来，却是报错了。

Program starting as '"D:programseroBraneStudioinlua53.exe" -e "io.stdout:setvbuf('no')" "D:work_codeluaTestTest.lua"'.
Program 'lua53.exe' started in 'D:work_codeserver' (pid: 6572).
D:programseroBraneStudioinlua53.exe: D:work_codeluaTestTest.lua:77: invalid order function for sorting
stack traceback:
[C]: in function 'sort'
D:work_codeluaTestTest.lua:77: in main chunk
[C]: in ?
Program completed in 0.04 seconds (pid: 6572).

原因在于compare函数中不能出现等于，即a>b或a<b都OK，但不能是a>=b或a<=b。如果使用了=，当数组中出现两个权重一样的元素时，就会报错。具体原因你可以看table.sort的官方文档，有句话：

If comp is given, then it must be a function that receives two list elements and returns true when the first element must come before the second in the final order (so that, after the sort, i < j implies not comp(list[j],list[i]))

如果是报invalid order function for sorting也就罢了，网上查一下就能找到答案。然而我在项目中的报错却比较奇怪：

function Society_city:do_player_hurt_sort()
    local player_hurt = {}
    for ty,hurt_info in pairs( self.player_hurt ) do
        for pid,info in pairs( hurt_info ) do
            local _info = {}
            _info.ty = ty
            _info.pid = pid
            _info.hurt = info.hurt
            _info.times = info.times

            table.insert( player_hurt,_info )
        end
    end

    table.sort( player_hurt,function(a,b) return a.hurt >= b.hurt end )

    return player_hurt
end   

报错信息为：

society_city.lua:679: attempt to index local 'a' (a nil value)

我一直以为是数组中加入了一个nil，但查了半天，原来是>=的原因。

• table编译

在我们的项目中，策划将配置填到excel表，然后用工具将excel表批量转为lua表作为配置。这样在lua中直接require就可以使用配置了。转出来的配置表通常是这样的：

local t = 
{
    [1] = 
    {
        ['id'] = 1,
        ['icon'] = 1001,
        ['name'] = '默认头像框',
        ['description'] = '初始赠送',
        ['type_id'] = 1,
        ['sort'] = 1,
    },

    [2] = 
    {
        ['id'] = 2,
        ['icon'] = 1002,
        ['name'] = '10级头像框',
        ['description'] = '达到$1级即可获得',
        ['type_id'] = 2,
        ['num'] = 10,
        ['sort'] = 2,
    },

    [3] = 
    {
        ['id'] = 3,
        ['icon'] = 1003,
        ['name'] = '20级头像框',
        ['description'] = '达到$1级即可获得',
        ['type_id'] = 2,
        ['num'] = 20,
        ['sort'] = 3,
    },

    [4] = 
    {
        ['id'] = 4,
        ['icon'] = 1004,
        ['name'] = '30级头像框',
        ['description'] = '达到$1级即可获得',
        ['type_id'] = 2,
        ['num'] = 30,
        ['sort'] = 4,
    },

    [5] = 
    {
        ['id'] = 5,
        ['icon'] = 1005,
        ['name'] = '40级头像框',
        ['description'] = '达到$1级即可获得',
        ['type_id'] = 2,
        ['num'] = 40,
        ['sort'] = 5,
    },

    [6] = 
    {
        ['id'] = 6,
        ['icon'] = 1006,
        ['name'] = '50级头像框',
        ['description'] = '达到$1级即可获得',
        ['type_id'] = 2,
        ['num'] = 50,
        ['sort'] = 6,
    },

    [7] = 
    {
        ['id'] = 7,
        ['icon'] = 1007,
        ['name'] = '60级头像框',
        ['description'] = '达到$1级即可获得',
        ['type_id'] = 2,
        ['num'] = 60,
        ['sort'] = 7,
    },

}

在某些功能中，是需要将id从小到大遍历的。由于策划配置时总是严格按小到大的，转换出来的配置表key值也是从小到大的。然而，当我们require这个table时，顺序却是乱的。即

for k,v in pairs( t ) do
print( k,v )
end

输出：

Program 'lua.exe' started in 'D:work_codeserver' (pid: 8044).
2    table: 0x003d8ff8
5    table: 0x003d92c8
3    table: 0x003d90e8
7    table: 0x003d8a58
1    table: 0x003da560
4    table: 0x003d91d8
6    table: 0x003d92f0
Program completed in 0.03 seconds (pid: 8044).

这样的代码，第一个取得的id并不一定是1。也就是说，lua虚拟机在编译这个文件时，里面的元素并不是按顺序的，即使key值是int型并且按从小到大并严格自增。