How do you copy a Lua table by value?

In Lua, a table is a highly flexible data structure that can represent arrays, dictionaries, and sets. By default, table assignment in Lua is by reference, meaning that if you directly assign a table to another variable, both variables reference the same table object. If you need to deep copy a table, you must manually traverse and copy its contents.

lua
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  -- Non-table types can be copied directly
        copy = orig
    end
    return copy
end

-- Usage example:
local tbl = {1, 2, {3, 4}}
local tbl_copy = deepcopy(tbl)

print(tbl[3][1])       -- Output 3
tbl_copy[3][1] = 300
print(tbl[3][1])       -- Still outputs 3, indicating true copying

The deepcopy function recursively copies a table and its nested tables. It first checks the type of the original object; if it is a table, it creates a new table and recursively copies each key and value from the original table to the new one. For non-table types, such as numbers or strings, it copies them directly. Additionally, the function handles any existing metatables.

Recursive copying ensures the independence of all levels within the table, so modifying the copied table does not affect the original table. This is particularly useful when dealing with multi-level nested data structures.

Although effective, this method may consume more resources when dealing with very large or deeply nested tables. Therefore, in practical applications, you should consider whether to perform a deep copy based on specific circumstances, such as the size and complexity of the table.