Differences between two tables in Lua

Question

I have two tables in lua (In production, a has 18 elements and b has 8):

local a = {1,2,3,4,5,6}
local b = {3,5,7,8,9}

I need to return 'a' omitting any common elements from 'b' -- {1,2,4,6} similar to the ruby command a-b (if a and b were arrays).

The best lua logic I have been able to come up with is:

local function find(a, tbl)
    for _,a_ in ipairs(tbl) do if a_==a then return true end end
end

function difference(a, b)
   local ret = {}
   for _,a_ in ipairs(a) do
   if not find(a_,b) then table.insert(ret, a_) end
end

return ret
end

local a = {1,2,3,4,5,6}
local b = {3,5,7,8,9}

local temp = {}
temp = difference(a,b)

print(temp[1],temp[2],temp[3],temp[4])

I need to loop through these table comparison pretty quick (min 10K times a second in production). Is there a cleaner way to do this?

======

This is part of a redis server side script and I have to protect my Redis CPU. Outside of a clean Lua process I have two other options:

1.Create two redis temp keys then run sinter for a big(O) of 42

• 18 sadd(a)
• 8 sadd(b)
• 16 sinter(a,b)

2.Return both a and b to ruby do the array comparison and send back the result.

• Network cost of the back and fourth of a few thousand connections a second will be a drain on resources.

Answer 1

Try this:

function difference(a, b)
    local aa = {}
    for k,v in pairs(a) do aa[v]=true end
    for k,v in pairs(b) do aa[v]=nil end
    local ret = {}
    local n = 0
    for k,v in pairs(a) do
        if aa[v] then n=n+1 ret[n]=v end
    end
    return ret
end