Merge function in Lua

Question

Hey, I'm trying to implement the merge function from merge sort in Lua. I know the algorithm pretty well, but I'm new to Lua. I keep getting a "bad argument #1 to 'insert' (table expected, got nil)" I believe the error is pointing at my recursive call. I can't figure it out and I have a feeling it's something pretty trivial. I just need a Lua guru to give me some guidance. Thanks. Here's my function:

function merge(l1, l2)
if # l1 == 0 then
    return l2
elseif # l2 == 0    then
    return l1
else
    if l1[1] <= l2[1] then
        tmp = l1[1]
        table.remove(l1,1)
        return table.insert(merge(l1,l2),tmp)

    else 
        tmp = l2[1]
        table.remove(l2,1)
        return table.insert(merge(l1,l2),tmp)
    end
end
end

Answer 1

I am not completely sure what you want to achieve with the merge function, but a clear problem is that table.insert doesn't return a table, it returns nil.

Storing the result of the merge in table t, using table.insert on t and finally returning t doesn't seem to break as before.

function merge(l1, l2) 
    if #l1 == 0 then
        return l2
    elseif # l2 == 0 then
        return l1
    else
        if l1[1] <= l2[1] then
            local tmp = table.remove(l1, 1)
            local t = merge(l1, l2) 
            table.insert(t, tmp)
            return t
        else 
            local tmp = table.remove(l2, 1)
            local t = merge(l1, l2) 
            table.insert(t, tmp)
            return t
        end 
    end 
end

A note on the few other modifications I did. table.remove returns the removed element, so you don't have to first access the element and then remove it. I recommend using the keyword local for variables, because in Lua all variables are global by default.