How to merge two tables overwriting the elements which are in both?

Like Doug Currie said, you can use his function, but there is a problem with his method. If first_table has things in it's k index, the function will over write it.

I'm assuming you're trying to merge these tables, not overwrite index's and value's. So this would be my method, it's very similar but is used for merging tables.

for _, v in pairs(second_table) do table.insert(first_table, v) end

The only problem with this solution is that the index is set as numbers, not as strings. This will work with tables with numbers as the index, and for tables with strings as their index, use Doug Currie's method.

Doug Currie's method:

for k,v in pairs(second_table) do first_table[k] = v end

Extending this great answer, https://stackoverflow.com/a/1283399/1570165, I would like to go with a (pure) functional approach like this one below:

-- example values
local t1 = { a = 0, b = 2 }
local t2 = { a = 1, c = 3 }

-- merge function that takes functional approach

local merge = function(a, b)
    local c = {}
    for k,v in pairs(a) do c[k] = v end
    for k,v in pairs(b) do c[k] = v end
    return c
end

-- t1 and t2 value still same after merge
print(merge(t1, t2)) -- { a = 1, b = 2, c = 3 }
print(t2) -- { a = 1, c = 3 }
print(t1) -- { a = 0, b = 2 }