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

Invert the table:

function table_invert(t)
  local u = { }
  for k, v in pairs(t) do u[v] = k end
  return u
end

You can then use the inverted table to find the index.

I find this function so useful that it goes into my permanent "Lua utilities" libraries.