Question

我在lua有一张桌子

test = {fist=1,second=2,third=3}

我想将此表分成两个新表，其值为：

keys = {first, second, third}
values = {1,2,3}

我的尝试：

local keys = {}
local values = {}

for key , value in pairs(test) do
    table.insert(keys, key)
    table.insert(values, value)

end

如果有更好的选择，请告诉我。

Answer 1

假设t中的值是连续的整数，则此代码应该起作用：

out1={}
out3={}
for k,v in pairs(t) do
        out1[v]=k
        out3[v]=v
end

请注意，即使t中的对以非指定顺序访问，新表也会正确填充。

Answer 2

假设任意键和值：

local keys, values = {}, {}

for k, v in pairs(input_table) do
    keys[#keys+1] = k
    values[#keys] = v
end

这是采用上述假设的最快方法这样的表插入是基准测试并且证明更快，至少在香草Lua中。

Answer 3

我的快速解决方案是这样的：

local function split_table(input_table)
    local out_table1 = {}
    local out_table2 = {}

    for key, value in pairs(input_table) do --simply iterating input table
        table.insert(out_table1, key)
        table.insert(out_table2, value)
    end

    return out_table1, out_table2 --returning both output tables
end

Answer 4

可以使用元表和影子表来创建历史记录跟踪表，记录添加和修改密钥的原始顺序。

这是一个经过轻微测试的示例，另存为track.lua：

-- tracking table that keeps a history of keys in the
-- order they were added to the table.


-- safe markers to store to track the use of nil or NaN 
-- as either keys or values. Neither can be a key, and 
-- nil cannot be a value in a sequence. Note that the history
-- iterator will assume that the record of keys is a valid
-- sequence.
local nilmarker, nanmarker = newproxy(), newproxy()


-- Make a value that can server as either key or value in a
-- table, even if it is nil or NaN, neither of which can be
-- a table key or a value in a valid sequence.
local function safemark(v)
    if v == nil then return nilmarker end
    if v ~= v then return nanmarker end
    return v
end


-- Set a key and track it's history, potentially including
-- deletions since we use safe markers in the tracking tables.
local function t_newindex(t,k,v)
    local mt = getmetatable(t)
    if mt.__index ~= mt.shadow or not mt.shadow[k] then
        mt.k[#mt.k+1] = safemark(k)
        mt.v[#mt.v+1] = safemark(v)
    end
    mt.shadow[k] = v
    return mt and mt.shadow and mt.shadow[k]
end

-- Look up a key in the shadow table
local function t_index(t,k)
    return getmetatable(t).shadow[k]
end

-- simple module table
local tracked = {}

-- create a new table with tracked keys and values. If called
-- with no argument or false, only key creation and initial values
-- are tracked. If called with true, then every value change will
-- be tracked.
function tracked.new(fullhistory)
    local mt = {
        __newindex = t_newindex,
        shadow = {},
        k = {},
        v = {},
    }
    mt.__index = fullhistory and t_index or mt.shadow
    return setmetatable({},mt)
end


-- return a human-readable string describing a value,
-- paying attention to our private marks for nil and NaN
local function tracked.safe(v)
    if v == nilmarker then return "~~nil~~" end
    if v == nanmarker then return "~~nan~~" end
    return tostring(v)
end

-- return an iterator in history order of the keys and values
-- as they were created and updated. The history records nil
-- and NaN via private markers. To test for those markers, use
-- tracked.safe() to convert the possibly marked values to strings.
function tracked.history(t)
    local i = 0
    local mt = getmetatable(t)
    local k,v = mt.k, mt.v
    return function()
        i = i + 1
        return k[i], v[i]
    end
end

return tracked

假设它在模块路径中可用为“track.lua”，那么它可以像这样使用：

C:\Users\Ross\Documents\tmp\SOQuestions>lua
Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> track = require "q19953449"
> t = track.new(true)
> t.a = 1
> t.b = 2
> t.a = 0
> t.c = 3
> for k,v in track.history(t) do print(k,v) end
a       1
b       2
a       0
c       3
> t.c = nil
> for k,v in track.history(t) do print(k,v) end
a       1
b       2
a       0
c       3
c       userdata: 007FD638
> t.d = 0/0
> for k,v in track.history(t) do print(k,v) end
a       1
b       2
a       0
c       3
c       userdata: 007FD638
d       userdata: 007FD658
> =t.d
-1.#IND
> =t.c
nil
> ^Z

从lua第一个输出表中的一个输入表输出两个表将具有所有键，第二个将具有输入表的所有值

4 个答案: