How do you copy a Lua table by value?

Question

Recently I wrote a bit of Lua code something like:

local a = {}
for i = 1, n do
   local copy = a
   -- alter the values in the copy
end

Obviously, that wasn't what I wanted to do since variables hold references to an anonymous table not the values of the table themselves in Lua. This is clearly laid out in Programming in Lua, but I'd forgotten about it.

So the question is what should I write instead of copy = a to get a copy of the values in a?

Answer 1

To play a little readable-code-golf, here's a short version that handles the standard tricky cases:

• tables as keys,
• preserving metatables, and
• recursive tables.

We can do this in 7 lines:

function copy(obj, seen)
  if type(obj) ~= 'table' then return obj end
  if seen and seen[obj] then return seen[obj] end
  local s = seen or {}
  local res = setmetatable({}, getmetatable(obj))
  s[obj] = res
  for k, v in pairs(obj) do res[copy(k, s)] = copy(v, s) end
  return res
end

There is a short write-up of Lua deep-copy operations in this gist.

Another useful reference is this Lua-users wiki page, which includes an example on how to avoid the __pairs metamethod.

Answer 2

Table copy has many potential definitions. It depends on whether you want simple or deep copy, whether you want to copy, share or ignore metatables, etc. There is no single implementation that could satisfy everybody.

One approach is to simply create a new table and duplicate all key/value pairs:

function table.shallow_copy(t)
  local t2 = {}
  for k,v in pairs(t) do
    t2[k] = v
  end
  return t2
end

copy = table.shallow_copy(a)

Note that you should use pairs instead of ipairs, since ipairs only iterate over a subset of the table keys (ie. consecutive positive integer keys starting at one in increasing order).

Answer 3

Just to illustrate the point, my personal table.copy also pays attention to metatables:

function table.copy(t)
  local u = { }
  for k, v in pairs(t) do u[k] = v end
  return setmetatable(u, getmetatable(t))
end

There is no copy function sufficiently widely agreed upon to be called "standard".

Answer 4

The full version of deep copy, handling all the 3 situations:

1. Table circular reference
2. Keys which are also tables
3. Metatable

The general version:

local function deepcopy(o, seen)
  seen = seen or {}
  if o == nil then return nil end
  if seen[o] then return seen[o] end

  local no
  if type(o) == 'table' then
    no = {}
    seen[o] = no

    for k, v in next, o, nil do
      no[deepcopy(k, seen)] = deepcopy(v, seen)
    end
    setmetatable(no, deepcopy(getmetatable(o), seen))
  else -- number, string, boolean, etc
    no = o
  end
  return no
end

Or the table version:

function table.deepcopy(o, seen)
  seen = seen or {}
  if o == nil then return nil end
  if seen[o] then return seen[o] end


  local no = {}
  seen[o] = no
  setmetatable(no, deepcopy(getmetatable(o), seen))

  for k, v in next, o, nil do
    k = (type(k) == 'table') and k:deepcopy(seen) or k
    v = (type(v) == 'table') and v:deepcopy(seen) or v
    no[k] = v
  end
  return no
end

Based on the lua-users.org/wiki/CopyTable's and Alan Yates' functions.

Answer 5

An optionally deep, graph-general, recursive version:

function table.copy(t, deep, seen)
    seen = seen or {}
    if t == nil then return nil end
    if seen[t] then return seen[t] end

    local nt = {}
    for k, v in pairs(t) do
        if deep and type(v) == 'table' then
            nt[k] = table.copy(v, deep, seen)
        else
            nt[k] = v
        end
    end
    setmetatable(nt, table.copy(getmetatable(t), deep, seen))
    seen[t] = nt
    return nt
end

Perhaps metatable copy should be optional also?

Answer 6

Here's what I actually did:

for j,x in ipairs(a) do copy[j] = x end

As Doub mentions, if your table keys are not strictly monotonically increasing, it should be pairs not ipairs.

I also found a deepcopy function that is more robust:

function deepcopy(orig)
    local orig_type = type(orig)
    local copy
    if orig_type == 'table' then
        copy = {}
        for orig_key, orig_value in next, orig, nil do
            copy[deepcopy(orig_key)] = deepcopy(orig_value)
        end
        setmetatable(copy, deepcopy(getmetatable(orig)))
    else -- number, string, boolean, etc
        copy = orig
    end
    return copy
end

It handles tables and metatables by calling itself recursively (which is its own reward). One of the clever bits is that you can pass it any value (whether a table or not) and it will be copied correctly. However, the cost is that it could potentially overflow the stack. So and even more robust (non-recursive) function might be needed.

But that's overkill for the very simple case of wanting to copy an array into another variable.

Answer 7

The (unfortunately lightly documented) stdlib project has a number of valuable extensions to several of the libraries shipped with the standard Lua distribution. Among them are several variations on the theme of table copying and merging.

This library is also included in the Lua for Windows distribution, and should probably be a part of any serious Lua user's toolbox.

One thing to make sure of when implementing things like this by hand is the proper handling of metatables. For simple table-as-structure applications you probably don't have any metatables, and a simple loop using pairs() is an acceptable answer. But if the table is used as a tree, or contains circular references, or has metatables, then things get more complex.

Answer 8

Don't forget that functions are also references, so if you wanted to completely 'copy' all of the values you'd need to get separate functions, too; however, the only way I know to copy a function is to use loadstring(string.dump(func)), which according to the Lua reference manual, doesn't work for functions with upvalues.

do
    local function table_copy (tbl)
        local new_tbl = {}
        for key,value in pairs(tbl) do
            local value_type = type(value)
            local new_value
            if value_type == "function" then
                new_value = loadstring(string.dump(value))
                -- Problems may occur if the function has upvalues.
            elseif value_type == "table" then
                new_value = table_copy(value)
            else
                new_value = value
            end
            new_tbl[key] = new_value
        end
        return new_tbl
    end
    table.copy = table_copy
end

Answer 9

Warning: the marked solution is INCORRECT!

When the table contains tables, references to those tables will still be used instead. I have been searching two hours for a mistake that I was making, while it was because of using the above code.

So you need to check if the value is a table or not. If it is, you should call table.copy recursively!

This is the correct table.copy function:

function table.copy(t)
  local t2 = {};
  for k,v in pairs(t) do
    if type(v) == "table" then
        t2[k] = table.copy(v);
    else
        t2[k] = v;
    end
  end
  return t2;
end

Note: This might also be incomplete when the table contains functions or other special types, but that is possible something most of us don't need. The above code is easily adaptable for those who need it.

Answer 10

That's as good as you'll get for basic tables. Use something like deepcopy if you need to copy tables with metatables.

Answer 11

I think the reason why Lua doesn't have 'table.copy()' in its standard libraries is because the task is not precise to define. As shown already here, one can either make a copy "one level deep" (which you did), a deepcopy with or without caring of possible duplicate references. And then there's metatables.

Personally, I would still like them to offer a built-in function. Only if people wouldn't be pleased with its semantics, they would need to go do it themselves. Not very often, though, one actually has the copy-by-value need.

Answer 12

In most of the cases when I needed to copy a table, I wanted to have a copy that doesn't share anything with the original, such that any modification of the original table has no impact on the copy (and vice versa).

All the snippets that have been shown so far fail at creating a copy for a table that may have shared keys or keys with tables as those are going to be left pointing to the original table. It's easy to see if you try to copy a table created as: a = {}; a[a] = a. deepcopy function referenced by Jon takes care of that, so if you need to create a real/full copy, deepcopy should be used.

Answer 13

Use penlight library here: https://stevedonovan.github.io/Penlight/api/libraries/pl.tablex.html#deepcopy

local pl = require 'pl.import_into'()
local newTable = pl.tablex.deepcopy(oldTable)

Answer 14

This might be the simplest method:

local data = {DIN1 = "Input(z)", DIN2 = "Input(y)", AINA1 = "Input(x)"}

function table.copy(mytable)  --mytable = the table you need to copy

    newtable = {}

    for k,v in pairs(mytable) do
        newtable[k] = v
    end
    return newtable
end

new_table = table.copy(data)  --copys the table "data"

Answer 15

In my situation, when the information in the table is only data and other tables (excluding functions, ...), is the following line of code the winning solution:

local copyOfTable = json.decode( json.encode( sourceTable ) )

I'm writing Lua code for some home automation on a Fibaro Home Center 2. The implementation of Lua is very limited with no central library of functions you can refer to. Every function needs to be declared in the code so to keep the code serviceable, so one line solutions like this are favorable.