Concatenate table sequences in lua

Question

Is there an easy way to concatenate two tables which are sequences? For example

a = {1, 2, 3}
b = {5, 6, 7}
c = cat(a,b)

where c would be the table {1,2,3,5,6,7}?

Try having a look at [this](http://stackoverflow.com/questions/1283388/lua-merge-tables) and [this](http://stackoverflow.com/questions/1410862/concatenation-of-tables-in-lua) — Robin Gertenbach, Jan 21 '16 at 10:33
Excellently and precisely worded question. Unfortunately, it is a duplicate of poorer quality questions. — Tom Blodget, Jan 22 '16 at 00:05

warspyking · Answer 1 · 2016-01-22T00:06:51.390

3

function cat(t, ...)  
    local new = {unpack(t)}  
    for i,v in ipairs({...}) do  
        for ii,vv in ipairs(v) do  
            new[#new+1] = vv  
        end  
    end  
    return new  
end

It uses iteration to add the elements of each array to a new one.

It's also worth noting that {unpack(t)} will only work if you have less than a specific number of elements, due to how tuples work in Lua. This varies across versions and depending on what you're doing, but if it's small you probably have nothing to worry about.

edited Jan 22 '16 at 00:06

answered Jan 21 '16 at 10:43

warspyking

3,045
4
20
37

You were missing a single space in the first line, that's why the code block didn't work ;) Remember: four spaces make a code :) – Henrik Ilgen Jan 21 '16 at 14:39
Please note that `{unpack(t)}` only works for up to about one million table elements (or 15000 if you have 16 bit `int`s). – siffiejoe Jan 21 '16 at 15:46
@siffie Last I checked it was 8K... – warspyking Jan 21 '16 at 19:01
@Henrik Can't believe I missed that. Thanks :D – warspyking Jan 21 '16 at 19:02
@warspyking It's about 8K for Lua 5.1. Both Lua 5.2 and 5.3 can handle 999970 elements in my test cases (depending on how much stack space you already use). – siffiejoe Jan 21 '16 at 22:57
@siffiejoe Oh, I primarily use Lua 5.1, that's why I thought it was 8K, – warspyking Jan 21 '16 at 23:38

Concatenate table sequences in lua

1 Answers1