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

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 2

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".