lua - table maintenance (particle system related)

Question

The update() function below gets called on every frame of a game. If the Drop particle has y value greater than 160 I want to remove it from the table. The problem is that I get "attempt to compare number with nil" errors, on the line notated below:

local particles = {};

function update()
    local num = math.random(1,10);
    if(num < 4) then
        local drop = Drop.new()
        table.insert ( particles, drop );
    end

    for i,val in ipairs(particles) do
        if(val.y > 160) then --ERROR attempt to compare number with nil
            val:removeSelf(); --removeSelf() is Corona function that removes the display object from the screen
            val = nil;
        end
    end
end

What am I doing wrong? Obviously val is nil, but I don't understand why the table iteration would find val in the first place since I set it to nil when it's y value gets larger than 160.

Answer 1

Thanks for the answers, they were all helpful. Here is what ended up working for me. The table.remove call is necessary to keep the loop running properly.

for i = #particles, 1, -1 do
    if particles[i].y > 160 then
        local child = table.remove(particles, i)
        if child ~= nil then
            display.remove(child)
            child = nil
        end
    end
end