In Lua, how do you find out the key an object is stored in?

Question

How would you print() out or find out the index of an object?

For example, if I spawned 20 random rock objects on screen into an array RockTable = {};<

Answer 1

The simplest way is to add an "index" property to each rock:

RockTable = {}

for i=1,20 do

    local rock
    -- do your thing that generates a new 'rock' object

    rock.index = #RockTable + 1
    RockTable[rock.index] = rock

end

If you use a touch listener method, you can retrieve the rock this way:

function touchListener( event )
    local rock = event.target
    local rockIndex = rock.index
    -- ...
end

It is true that you can maintain a second table with indices, but I find my method cleaner - when it is time to remove things, you only have to worry about one table, the main one.

I have a question though: why do you need to retrieve that index? In most cases, well designed event listener functions are enough, you don't need to "find" your objects. Of course I lack information on what you are trying to do, but it is possible that you are over-complicating things.