Returning the index of a value in a Lua table

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).

Your code is efficient but wrong. (Consider {[false]=0}.) The correct code is

if next(myTable) == nil then
   -- myTable is empty
end

For maximum efficiency you'll want to bind next to a local variable, e.g.,

...
local next = next 
...
... if next(...) ...

(When next is local, the code finds primitive function next by a constant-time indexing operation into an array of "upvalues." When next is left global, finding next involves indexing index the "environment" hash table, which contains the values of the global variables. This indexing operation is still constant-time, but it is significantly slower than the array lookup for a local variable.)