Point table to another table in Lua - lua

Question

is there any way to point table to another table? for example:
local a = {}
local b = {}
a.name = "Josh"
print(a.name) -- Prints 'Josh'
print(b.name) -- Prints 'Josh' aswell
a.name = "I don't have a name"
print(a.name) -- Print 'I don't have a name'
print(b.name) -- Prints 'I don't have a name' aswell
I hope you get my point.. thanks
EDIT:
Okay, so here is the idea:
I am making a dinamic function that is like this
local table = { 1, 2, "hey" }
function drawimage(name, posx, posy referencetable)
_tabledata[name] = { posx = posx, posy = posy, reference = {}}
setmetatable(_tabledata[name].reference, { __index = referencetable })
end
drawimage("Header", 0, 50, table)
All good and fine, values work and we are all happy.. the problem occurs when the reference table changes it's value in this way
local data = { 123123, 545454, "heyou" } -- Data is sent from another script via a trigger
table = data
Since I am not updating it by an index (ie: table[1] = 9999) reference variable is 'unsynced' with the real one, I hope you understand :)
EDIT2:
Okay here is a self working example of my main problem
local maintable = { "Stack", "Overflow" }
local maintablecopy = {}
maintablecopy = maintable
print("maintable[1] = " ..maintable[1]) -- Prints Stack
print("maintable[2] = " ..maintable[2]) -- Prints Overflow
print("")
print("maintablecopy[1] = " ..maintablecopy[1]) -- Prints Stack
print("maintablecopy[2] = " ..maintablecopy[2]) -- Prints Overflow
print("")
print("Changing values..")
local newdata = { "Hello", "World" }
maintable = newdata
print("")
print("maintable[1] = " ..maintable[1]) -- Prints Hello
print("maintable[2] = " ..maintable[2]) -- Prints World
print("")
print("maintablecopy[1] = " ..maintablecopy[1]) -- Prints Stack -- PROBLEM
print("maintablecopy[2] = " ..maintablecopy[2]) -- Prints Overflow -- PROBLEM
print("Using setmetatable..")
maintable = { "Stack", "Overflow" }
maintablecopy = {}
setmetatable(maintablecopy, { __index = maintable })
print("maintable[1] = " ..maintable[1]) -- Prints Stack
print("maintable[2] = " ..maintable[2]) -- Prints Overflow
print("")
print("maintablecopy[1] = " ..maintablecopy[1]) -- Prints Stack
print("maintablecopy[2] = " ..maintablecopy[2]) -- Prints Overflow
print("")
print("Changing values..")
local newdata = { "Hello", "World" }
maintable = newdata
print("")
print("maintable[1] = " ..maintable[1]) -- Prints Hello
print("maintable[2] = " ..maintable[2]) -- Prints World
print("")
print("maintablecopy[1] = " ..maintablecopy[1]) -- Prints Stack -- PROBLEM
print("maintablecopy[2] = " ..maintablecopy[2]) -- Prints Overflow -- PROBLEM
Why I cannot directly point it to the table when the variable updates? becouse I have 20 tables to update, it would be easier to do this
local _dynamics = {}
local tbl1 = { "Hey", 8787 }
local tbl2 = { 123, "There" }
local tbl3 = { "You", 1111 }
function dynamicFunction(name, posx, posy, textsize, reference)
_dynamics[name] = { posx = posx, posy = posy, textsize = textsize, reference = reference }
end
dynamicFunction("first", 0, 0, 5, tbl1)
dynamicFunction("second", 0, 0, 5, tbl2)
dynamicFunction("third", 0, 0, 5, tbl3)
for key in pairs(_dynamics) do
local inf = _dynamics[key]
for i = 1, #inf.reference do
print(inf.reference[i])
if i == #inf.reference then
print("")
end
end
end
print("")
print("")
tbl1 = { "aaaaa", "bbbbbbbbbb" }
tbl2 = { "ccccccccccc", "ttttttttttt" }
tbl3 = { "rrrrrrr", "yyyyyyyyyyy" }
for key in pairs(_dynamics) do
local inf = _dynamics[key]
for i = 1, #inf.reference do
print(inf.reference[i])
if i == #inf.reference then
print("")
end
end
end
print("Values should get updated on the reference variable, but it doesn't.. this would save me to do a check for every single variable")
You can run it on http://www.compileonline.com/execute_lua_online.php to see yourself what I mean.
Sorry if it's a mess but my english is not the best :D

Answer 1

You want the __index metamethod:
local a = { name="Josh" }
local b = {}
print(a.name) --> Josh
print(b.name) --> nil
setmetatable(b,{__index=a})
print(b.name) --> Josh
a.name = "Gilligan"
print(a.name) --> Gilligan
print(b.name) --> Gilligan
-- but note! the shadow
b.name = "overridden"
print(a.name) --> Gilligan
print(b.name) --> overridden
b.name = nil
print(a.name) --> Gilligan
print(b.name) --> Gilligan
For more details, I offer up this article of mine:
http://phrogz.net/lua/LearningLua_ValuesAndMetatables.html
Response to Edit2:
Let me summarize the problems with some of your code:
local maintablecopy = {}
maintablecopy = maintable
With the above code you create one table, set maintablecopy to reference that table, and then you completely abandon it when you set maintablecopy to instead reference a different table. This demonstrates a lack of understanding of how variables work.
local newdata = { "Hello", "World" }
maintable = newdata
Again, you are not "copying" newdata into maintable, you are changing the variable to reference the same table here.
maintable = { "Stack", "Overflow" }
maintablecopy = {}
setmetatable(maintablecopy, { __index = maintable })
-- …
local newdata = { "Hello", "World" }
maintable = newdata
Again, same problem. Here are some ways to change your code:
Replace a Table's Contents
Instead of maintable = newdata you could do this:
function copytable(from,to_table)
-- erase all old keys
for k,_ in pairs(to_table) do to_table[k] = nil end
-- copy the new ones over
for k,v in pairs(from) do to_table[k] = v end
end
local a = { name="Foo" }
local b = {}
copytable(a,b)
print(a.name == b.name) --> true
local c = { name="NEW" }
copytable(c,b)
print(c.name == b.name) --> true
However, doing this will not cause b to update if c changes.
c.name = "EVEN NEWER"
print(c.name == b.name) --> false
Update the __index
local a = { name="Foo" }
local b = setmetatable({},{__index=a})
print(a.name == b.name) --> true
-- cause b to follow c now instead of a
local c = { name="NEW" }
getmetatable(b).__index = c
print(c.name == b.name) --> true
c.name = "EVEN NEWER"
print(c.name == b.name) --> true
In general, you need to step back and describe the original problem you are trying to solve, instead of this XY problem.