Reading/Writting deep inside sparse tables

[Banknorris]

Hi just wanted to hear opinions on something I came up with to deal with elements in a sparse very deep table (in practical terms this happens to me with saved variables). For example, if I want to access a[3][7]["aa"][2][2], I need to first check if a, a[3], a[3][7], a[3][7]["aa"] and a[3][7]["aa"][2] exist, otherwise I would get an "attempt to index 'a' (a nil value)". With the following metamethods I can just:

local a = new_table()
print(a(3,7,"aa",2,2)) --this prints nil as it should (since value was not set) but no errors
a[3][7]["aa"][2][2] = "zzz" --sets everything in the path not set already: a[3], a[3][7], a[3][7]["aa"] etc
print(a(3,7,"aa",2,2)) --this prints "zzz"

The only problem I can see is that you must use the proper forms depending if you are reading or writting, for example:

local b = new_table()
print(b[1][1][1]) --instead of printing nil, this is the same as b[1]=new_table() b[1][1]=new_table() b[1][1][1]=new_table() and will print b[1][1][1] address (an empty table with a metatable attached)
b(1,1,1) = "rrr" --this will not run: syntax error near '='

But I couldn't find a way to do both things (read and write) with the same format. Sorry if this sounds too trivial but it sounded like an Eureka! moment to me.

local mt = {}
 
mt["__index"] = function(t,k)
    local v = setmetatable({},mt)
    rawset(t,k,v)
    return v
end
 
mt["__call"] = function(t,...)
    local i,n,v = 1,select("#",...),{...}
    while t and i<=n do
        t = rawget(t,v[i])
        i = i+1
    end
    return t
end
 
local new_table = function()
    return setmetatable({},mt)
end

How good do you think this is?

[SDPhantom]

Actually, you should be able to write a value to it using the following syntax with no change to the code.

b[1][1][1] = "rrr"

Note with few exceptions, you can't make any function calls or evaluations happen on the left side of the assignment operator.

[Banknorris]

Originally Posted by SDPhantom Actually, you should be able to write a value to it using the following syntax with no change to the code. b[1][1][1] = "rrr"

Well that was my first example (line 3). In case people are missing what is the point, the point is that by using those meta-methods, you can avoid doing this:

b[1]={} b[1][1]={} b[1][1][1]="rrr"

and instead just write the last line since the meta-methods will do this job for you.

b[1][1][1]="rrr"

also, instead of this:

if b[1] and b[1][1] and b[1][1][1]=="rrr" then

you can just

if b(1,1,1)=="rrr" then

[Phanx]

Originally Posted by Banknorris also, instead of this: if b[1] and b[1][1] and b[1][1][1]=="rrr" then you can just if b(1,1,1)=="rrr" then

But function calls are so slow.

[AlleyKat]

I made better infinite table example for u

local setmetatable = setmetatable;
local rawget = rawget;
local type = type;
local secondLayer, getVal;
     
local newTable = function()
    return setmetatable({}, secondLayer);
end;
 
getVal = function(s, v, ...)
    local t = rawget(s, v);
    if t == nil or type(t) ~= "table" then
        return t;
    else
        return getVal(t, ...);
    end
end
 
secondLayer = { __index = function(self, key)
    local new = newTable();
    self[key] = new;
    return new;
end, __call = getVal};
 
-----
local a = newTable();
a[1][2][3] = 1;

... or not

[SDPhantom]

Originally Posted by Banknorris also, instead of this: if b[1] and b[1][1] and b[1][1][1]=="rrr" then you can just if b(1,1,1)=="rrr" then

With the same __index metamethod, you don't need to call the table's __call metamethod anyway.

Let's look at the following code:

if b[1][1][1]=="rrr" then

If any of the indexing operations result in a nil return, the __index metamethod will jump in anyway and return a new table along with setting the returned table to the associated index. The end result would be comparing an empty table with "rrr" in such case.