  lua-l archive

• Subject: Re: Lua Basics: table duplication?
• From: Philippe Lhoste <PhiLho@...>
• Date: Mon, 29 Aug 2005 11:34:40 +0200

William Trenker wrote:
Sometimes I want to make a duplicate copy of a table, not just assign
another reference to it.  Is this the standard way:

a={1,2,3, m="m", "n" , o={4}, p = function() end}

b={}
table.foreach(a, function(k,v) b[k]=v end)

close, but you'll still end up with references to child tables, rather than copies. this may be ok for you, otherwise you have to recurse child tables

function clone(node)
if type(node) ~= "table" then return node end
local b = {}
table.foreach(node, function(k,v) b[k]=clone(v) end)
return b
end

a={1,2,3, m="m", "n" , o={4}, p = function() end}

b=clone(a)

(and that still will only copy references to userdata and functions)

Well, if you kill the original table, these data will continue to live, so I suppose that's the main purpose.
Note that this function is too simplistic, it will gag on self-references:

a.aa = a

clone() with throw a stack overflow...

A number of copy routines has been given on this mailing list, you should check the archives or perhaps the Wiki, and perhaps PiL (Programming in Lua, Roberto's book. Still have to read it...).

Here is the result of my tentative to improve the above code (reformatted to my style...):

function CloneTable(node)
local visitRef = {}

local CT
CT = function (node)
if type(node) ~= "table" then
--~       print(node)
return node
end

if visitRef[node] then
--~       print"Visited!"
return nil
end
visitRef[node] = true

return CT(node)
end

local subTable = {}
table.foreach(node,
function(k, v)
subTable[k] = CT(v)
end)
return subTable
end

But we loose the self-references. Returning 'node' instead of 'nil' seems to work, but with strange results.

--
Philippe Lhoste
--  (near) Paris -- France
--  http://Phi.Lho.free.fr
--  --  --  --  --  --  --  --  --  --  --  --  --  --