Lua: How to search in a table where keys are tables (or objects)

Question

I want to save a lua table, where the keys are other lua tables. I know that this is possible, but I want to be able to look up the table using copies of these tables. In particular, I want to be able to:

t = {}
key = { a = "a" }
t[key] = 4
key2 = { a = "a" }

and then I want to see:

t[key2]

and get 4.

I know that I can turn it keyinto a row and put it in a table t. I also thought about writing a custom hash function, or about that, inserting tables. Is there a better way to get this functionality? What other options do I have?

Answer 1

Lua , , "". , , , , Lua , . :

t = {}
key = { a = "a" }
t[key] = 4
key2 = key
...
t[key2] -- returns 4

, , , . key -, 4 , . .

... , . .

- , . true, "", false, . . :

equivalent({a=1},{a=1})          -- true
equivalent({a=1,b=2}, {a=1})     -- false
equivalent({a={b=1}}, {a={b=2}}) -- false

, , . , "" , . ; , .

local function equivalent(a,b)
  if type(a) ~= 'table' then return a == b end

  local counta, countb = 0, 0

  for k,va in pairs(a) do
    if not equivalent(va, b[k]) then return false end
    counta = counta + 1
  end

  for _,_ in pairs(b) do countb = countb + 1 end

  return counta == countb
end

. , , .

, t equivalent . .

t . , ; ( store). t , store, equivalent, .

:

local function equivalent(a,b)
... -- same code as before
end

local store = {} -- this is the table that stores the values

t = setmetatable({}, {
  __newindex = store,
  __index = function(tbl, key)
    for k,v in pairs(store) do
      if equivalent(k,key) then return v end
    end
  end
})

:

t[{a = 1}] = 4

print(t[{a = 1}]) -- 4
print(t[{a = 1, b = 2}]) -- nil

Answer 2

Lua. , - "" . , , .

- , -, , (, , ) . , . pairs() , table.sort(), pairs() , , "equal".

​​, :

function hash(t) ... end
t = {}
key1 = { a = "a", b = "b" }
t[hash(key1)] = 4
key2 = { a = "a", b = "b" }
print(t[hash(key2)]) -- should print "4" if the hash function works correctly

-, , . ?

, , , -. , , . , :

function pkey(...)
    local n, args = select('#', ...), { ... }
    for i=1,n do args[i] = args[i].value end -- extract your info here
    return table.concat(args, ' ') -- space or other separator, such as ':'          
end
tab[pkey(phrase1, phrase2, phrase3)] = "value"

Answer 3

, :

• t[{a=1}] = true , store ( -)
• , , , . , , .
• O (n) (n - , lua - O (1)); , -

( , kikito , - .)

/ - , kikito , . , __newindex , :

t = setmetatable({}, {
    __newindex = function(tbl, key, value)
        for k,v in pairs(store) do
            if equivalent(k,key) then
                tbl[k] = value
                return
            end
        end
        store[key] = value
    end,
    __index = function(tbl, key)
        for k,v in pairs(store) do
            if equivalent(k, key) then return v end
        end
    end
})

, -. HashTable, :

local function HashTable(Hash, Equals)
    --Hash is an optional function that takes in any key and returns a key that lua can use (string or number). If you return false/nil, it will be assumed that you don't know how to hash that value.
    --    If Hash is not provided, table-keys should have a GetHash function or a .Hash field
    --Equals is an optional function that takes two keys and specify whether they are equal or not. This will be used when the same hash is returned from two keys.
    --    If Equals is not provided, items should have a Equals function; items are in this case assumed to not be equal if they are different types.
    local items = {} --Dict<hash, Dict<key, value>>
    local function GetHash(item)
        return Hash and Hash(item) or type(item) == "table" and (item.GetHash and item:GetHash() or item.Hash) or item
    end
    local function GetEquals(item1, item2)
        if Equals then return Equals(item1, item2) end
        local t1, t2 = type(item1), type(item2)
        if t1 ~= t2 then return false end
        if t1 == "table" and item1.Equals then
            return item1:Equals(item2)
        elseif t2 == "table" and item2.Equals then
            return item2:Equals(item1)
        end
        return false
    end
    return setmetatable({}, {
        __newindex = function(_, key, value)
            local hash = GetHash(key)
            local dict = items[hash]
            if not dict then
                if value ~= nil then --Only generate a table if it will be non-empty after assignment
                    items[hash] = {[key] = value}
                end
                return
            end
            for k, v in pairs(dict) do
                if GetEquals(key, k) then --Found the entry; update it
                    dict[k] = value
                    if value == nil then --check to see if dict is empty
                        if next(dict) == nil then
                            items[hash] = nil
                        end
                    end
                    return
                end
            end
            --This is a unique entry
            dict[key] = value
        end,
        __index = function(_, key)
            local hash = GetHash(key)
            local dict = items[hash]
            if not dict then return nil end
            for k, v in pairs(dict) do
                if GetEquals(key, k) then
                    return v
                end
            end
        end
    })
end

:

local h = HashTable(
    function(t) return t.a or 0 end, --Hash
    function(t1, t2) return t1.a == t2.a end) --Equals
h[{a=1}] = 1
print(h[{a=1}]) -- 1
h[{a=1}] = 2
print(h[{a=1}]) -- 2
print(h[{a=1,b=2}]) -- 2 because Hash/Equals only look at 'a'

, Hash/Equals.

, O (1).

(: , .)

Answer 4

, . , -, , , . , , pairs() , O(1) O(n).

Answer 5

, , : , , token, , ... .., , , .

, , :

• .
• .

:

index["I"] = {
    ["like"] = {
        ["banana"] = 1,
        ["hot"] = {
            ["chick"] = 1
        }
    }    
}

This way you can count the number of hits with a single crawl step and count the occurrences at the same time you index, but as I said earlier, it depends on your goal, and that will mean re-splitting the phrase to find the occurrences through your index.