Duplicating a table?

[Aqualyt]

a = {};
a[key1] = {};
a[key2] = {};

b = {};

How would I make b a duplate of a?

[Iriel]

Assuming you're dealing with a simple 'shallow copy' (i.e. not deeply nested tables that also have to be copied)

b = {};
for k,v in pairs(a) do b[k] = v end

[Aqualyt]

Thanks!

[Dhargo]

if you need a true copy (no pointer references to the other table's elements)

function tablecopy(tbl)
  if type(tbl) ~= "table" then return tbl end
  local t = {}
  for i,v in pairs(tbl) do
    t[i] = tablecopy(v)
  end
  return t
end

I think that's right, but I wrote it while killing the last few minutes of a very long day

[ckknight]

without metatables:

function tablecopy(tbl)
  if type(tbl) ~= "table" then return tbl end
  local t = {}
  for i,v in pairs(tbl) do
    t[i] = tablecopy(v)
  end
  table.setn(t, table.getn(tbl))
  return t
end

with metatables (shallow copy):

function tablecopy(tbl)
  if type(tbl) ~= "table" then return tbl end
  local t = {}
  for i,v in pairs(tbl) do
    t[i] = tablecopy(v)
  end
  table.setn(t, table.getn(tbl))
  return setmetatable(t, getmetatable(tbl))
end

with metatables (deep copy):

function tablecopy(tbl)
  if type(tbl) ~= "table" then return tbl end
  local t = {}
  for i,v in pairs(tbl) do
    t[i] = tablecopy(v)
  end
  table.setn(t, table.getn(tbl))
  return setmetatable(t, tablecopy(getmetatable(tbl)))
end

I think. the main difference from Dhargo's is the setn and the metatables. You'll probably just want the shallow metatable copy.

[Iriel]

The challenges with deep copies is knowing what to, and what not to copy, as well as properly handling self referential structures.

Quite often you dont want to deep copy metatable entries (though perhaps you want to shallow copy the metatable).

You usually dont want to copy tables-as-keys, but sometimes it's applicable.

Any self respecting deep copy method probably wants to maintain a temporary mapping table to avoid infinite recursion when it encounters a self-referential table. Since we can use tables as table keys, it's remarkably easy in lua to reconstruct such copies.