Lua add two tables

Question

x = {1, 2, 3}
y = {4, 5, 6}
z = x + y

I have two tables x and y and just want to create a third one which is just the elements of them two (not sorted). I use the above code in an effort but this gives error input:3: attempt to perform arithmetic on a table value (global 'x')...

If `x = { 1, 2, 3 }` and `y = { 3, 4, 5 }`, should `z = x + y = { 1, 2, 3, 4, 5}` (no duplicates) or `{ 1, 2, 3, 3, 4, 5 }` (possible duplicates)? — GoojajiGreg, Apr 18 '17 at 01:37
Possible duplicate of [Concatenation of tables in Lua](http://stackoverflow.com/questions/1410862/concatenation-of-tables-in-lua) — Piglet, Apr 18 '17 at 11:53

score 7 · Accepted Answer · answered Apr 17 '17 at 22:01

It seems you want to concatenate the two tables to obtain {1, 2, 3, 4, 5, 6}.

There is no builtin function or operator for that. You can use this code:

z = {}
n = 0
for _,v in ipairs(x) do n=n+1; z[n]=v end
for _,v in ipairs(y) do n=n+1; z[n]=v end

If you want to use the syntax z = x + y, then set an __add metamethod. (But perhaps a __concat metamethod is more adequate for your meaning.)

score 2 · Answer 2 · answered Apr 27 '17 at 12:06

As the other answers have mentioned, there is no built-in way of doing this. The simplest way of doing it is to define your own function, as in GoojajiGreg's answer. However, if you really want to use the + operator, you can use an __add metamethod.

local metatable = {
    __add = function (t1, t2)
        local ret = {}
        for i, v in ipairs(t1) do
            table.insert(ret, v)
        end
        for i, v in ipairs(t2) do
            table.insert(ret, v)
        end
        return ret
    end
}

local x = {1, 2, 3}
local y = {4, 5, 6}

setmetatable(x, metatable)
setmetatable(y, metatable)

local z = x + y

for i, v in ipairs(z) do
    print(v)
end

-- Output:
-- 1
-- 2
-- 3
-- 4
-- 5
-- 6

score 1 · Answer 3 · answered Apr 17 '17 at 21:51

1

You can set function that will sum tables as __add metamethod in the metatable that should be set for all tables that needs that implicit behavior. See Lua manual section "Metatables and Metamethods" for details.

answered Apr 17 '17 at 21:51

Vlad

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score 1 · Answer 4 · answered Apr 17 '17 at 22:03

You could define a method to return the union of the tables:

local function union ( a, b )
    local result = {}
    for k,v in pairs ( a ) do
        table.insert( result, v )
    end
    for k,v in pairs ( b ) do
         table.insert( result, v )
    end
    return result
end

This doesn't prevent multiple instances of the same value in the resulting table.

score 1 · Answer 5 · answered Jul 16 '19 at 15:43

You can also use table.move to do it:

function extend(t1, t2)
    return table.move(t2, 1, #t2, #t1 + 1, t1)
end

Example usage:

a = {"a", "b"}
b = {"c", "d", "e"}
c = extend(a, b)

After this both a and c contains {"a, "b", "c", "d", "e"}, while b is still {"c", "d", "e"}.

Lua add two tables

5 Answers5