Question

I’m writing a very simple program in Lua to learn more about genetic programming. Below is a mutate function to go to a numbered node (nodeNum) in the tree (pop) and either : swap add with sub (or vice-versa) or replace the node with a random number 1-100.

local count = 0
function mutate(pop, nodeNum)
    for k,v in ipairs(pop) do
        if type(v) == "table" then
            mutate(v, nodeNum)
        else
            count = count + 1
        end

        if count == nodeNum and k == 1 then -- correct node
            if type(v) == "function" then
                if v == add then
                    pop[k] = sub
                else
                    pop[k] = add
                end
            else
                pop[k] = math.random(100)
            end
        end
    end
end

My problem is with count. Calling this function is awkward, since count has to be reset each time :

-- mutate the first 3 elements in program tree t 
mutate(t,1)
count = 0
mutate(t, 2)
count = 0
mutate(t, 3)

I’ve tried variations using do ... end like :

do
    local count
    function mutate(pop, nodeNum)
    if not count then
        count = 0
    ...
end

I’ve also tried giving mutate an extra argument mutate(pop, nodeNum, count) and calling it with mutate(t, 1, 0) but I can’t get either method to work correctly.

I may be missing something really obvious, can anyone see a more elegant solution?

Answer 1

function mutate(pop, nodeNum, count)
    count = count or 0

    for k,v in ipairs(pop) do
        if type(v) == "table" then
            mutate(v, nodeNum, count)
        else
            count = count + 1
        end

        if count == nodeNum and k == 1 then -- correct node
            if type(v) == "function" then
                if v == add then
                    pop[k] = sub
                else
                    pop[k] = add
                end
            else
                pop[k] = math.random(100)
            end
        end
    end
end