Very good example. I thought it was clever how you used the qualifyers. You also coverd most bases in terms of the inventory system. I think the only thing not coverd is what happens if there are more items than you have slots for.
If you only have 4 inventory slots, then whenever you pick up an item you make it check if the 4th line in the list is equal to "", aka nothing. If the 4th line is empty then it means you have room.
I didn't want to have too much confusing code but I guess it would have probably been good to add it.