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Lab 2: Modeling classical planning domains, part 2

Written by Jonas Kvarnström.


Any updates or changes we make in the lab instructions during the course will be announced here.

Lab 2.1: Emergency Services Logistics, Carriers

Since injured people are not necessarily close to the depot location, flying to the depot for every single crate that needs to be delivered is quite inefficient. In this part we will consider an alternative way of moving crates: A helicopter can load up to four crates onto a carrier, which must be at the same location. It can then fly the carrier to a location where there are people needing supplies and unload one or more crates from the carrier. It may continue by flying the carrier to another location, unloading additional crates, and so on, and does not have to return to the depot until after all crates on the carrier have been delivered.

Though you currently only need a single carrier for your single helicopter, there should still be a separate type for carriers. You need to count and keep track of not only which crates are on each carrier but also how many crates there are in total on each carrier, so that carriers cannot be overloaded.

Note that you should not introduce specific slots on carriers. Keeping track of which crate is in which slot leads to redundancy: The model now makes a difference between having crate A in slot 1 and crate B in slot 2, or crate B in slot 1 and crate A in slot 2. This introduces unnecessary redundancy in the seach space and can make the planner spend much more time trying to find a specific crate placement that works. Since we don't need slots, we should simply count crates.


Direct support for numeric state variables (such as the number of crates on a carrier) is comparatively rare. Most planners are non-numeric in this respect, but we can still model a form of pseudo-numbers using standard PDDL objects. For example, we can create a type of pseudo-numbers:

(:types ... num ...)

We can then define a set of value objects in the problem instance:

(:objects ... n0 n1 n2 n3 n4 n5 n6 n7 - num)

To the planner, these are simply objects and have no numeric values. Therefore we have to manually define all operations we may want to apply, such as finding the next number:

(:predicates ... (next ?numA ?numB - num)
(:init ... (next n0 n1) (next n1 n2) (next n2 n3) ... (next n6 n7))

We can then use the value objects in actions and states. For example, in the Schindler Miconic 10 elevator domain, the action of moving up one floor has to increase the floor "number":

(:action move-up
	:parameters (?e - elevator    ?from ?to - num)
	:precondition	(and (at ?elevator ?from)  ;; Where is the elevator?
			(next ?from ?to) ...)	;; "to" must be the next number
	:effects	(and (not (at ?elevator ?from)) (at ?elevator ?to)))	

Note that we did not define "(next n7 [something])". Therefore (next ?from ?to) cannot be true for any destination when you are at floor n7, so we cannot move up from the topmost floor, as expected. Note also that you don't need to define a "before", since you can simply use "next" backwards (if you know ?x and want to know the ?y before it, you require (next ?y ?x)).


You also need at least three new operators that we might call load-crate-on-carrier, fly-carrier, and take-crate-from-carrier. A load-crate-on-carrier action should place a crate that the helicopter is carrying on the carrier, and must therefore be preceded by a suitable pickup action (or whatever you might have called it in domain). Similarly, take-crate-from-carrier must be followed by actually delivering the crate to a person.

You should do the following:

  1. Modify the domain according to the discussion above. Make sure you do this in a new directory and that you keep the older versions of the domain!

  2. Create a new set of problem instances, either manually or by modifying the problem generator.

  3. Run the problems you have created through the same (modern) planners as before, and possibly a few additional planners. What plans are generated? Do they use the carriers? Does this differ among different planners? Discuss your findings briefly in your report for lab 2.

If the plans that are generated do not use the carriers, don't worry: There's an explanation for this.

Lab 2.2: Emergency Services Logistics, Action Costs

As stated above, the intention behind the use of carriers is that a helicopter should not need to go back to the depot for every crate. However, it is not necessarily apparent to a planner that going back and forth across long distances is inefficient. Let us consider the following two informal plan segments for delivering four crates to two distinct locations:

    pick up crate1                 pick up crate1			 
    fly to loc1		           load crate1	  			 
    deliver to person1	           pick up crate2			 
    fly to depot	           load crate2	  			 	 
    pick up crate2	           pick up crate3			 	 
    fly to loc2		           load crate3	  			 
    deliver to person2	           pick up crate4			 
    fly to depot	           load crate4	  			 	 
    pick up crate3	           fly carrier between depot, loc1	 	 	
    fly to loc3		           unload crate1	  		 
    deliver to person3	           deliver to person1			 
    fly to loc3, depot	           fly carrier between loc1, loc2	 	 
    pick up crate4	           unload crate2	  		 	 
    fly to loc4		           deliver to person2			 
    deliver to person4	           fly carrier between loc2, depot	 	
			           unload crate3	  		 		 
			           deliver to person3			 
	  		           fly carrier between loc3, depot	 
			           unload crate4	  		 
	  		           deliver to person4

The solution that uses carriers is longer – and since we have not told the planner how to calculate the cost of a plan, it has to assume that the cost is proportional to the number of actions. Distances are not part of the domain as given to the planner!

Action costs

The basic idea behind action costs in PDDL is as follows. Previously you have only defined boolean predicates. With action costs, you also define a numeric state variable called total-cost. This state variable should initially be zero and should be explicitly increased by each action that is performed. Planners should then be aware of the function with this specific name and aim to find a plan that minimizes the value of total-cost.

Some actions may have a constant cost. For example, the cost of picking up a crate may not depend on which crate is being picked up or which helicopter is involved. Such actions simply increase the total cost by a constant. The cost of flying, on the other hand, definitely depends on the distance that is flown. Such parameter-dependent costs are specified by defining a numeric function, giving it values in the initial state, and using it in the actions.

You should extend the previous domain and problem instances by introducing action costs, as discussed below. Specifically, flying long distances should be expensive. If you use the problem generator, the flight_cost function should be of some help. If not, you might for example assign a cost of 100 to flying between the depot and another location, and a cost of 10 to flying between different locations.

  1. Add :action-costs to the requirements section of the domain specification.
  2. After predicates but before actions, you define a total-cost function (state variable) and a fly-cost function as follows, where "location" is the name of your location domain.
    	(:functions (total-cost) - number
    	            (fly-cost ?from ?to - location) - number)
  3. For actions with constant cost, you add a new effect increasing total-cost by a constant amount: (increase (total-cost) 10), for example. Only non-negative integers are allowed.
  4. For flight actions, you add a new effect increasing total-cost by a non-constant amount: (increase (total-cost) (fly-cost ?from ?to)), for example.
  5. Every problem file must be amended to initialize total-cost. In the :init section, you add (= (total-cost) 0). This should be added to the problem generator.
  6. Similarly, every problem file must be amended to initialize fly-cost. The problem generator already generates random coordinates for each location and has a function called flight_cost that generates a flight cost depending on the distance between two locations. You can then use this information to specify values for fly-cost for all pairs of locations (including flying from one location to itself!) in the :init section of the problem file. For example, you might add (= (fly-cost loc1 loc1) 0) (= (fly-cost loc1 loc2) 20) and so on. Again, only non-negative integers are allowed.
  7. Finally, every problem file must specify the following at the end, after the initial state and goal (but before the final closing parenthesis):
        (:metric minimize (total-cost))
  8. Run the problems you have created through the same planners as before. What plans are generated? Do they seem better than before? Do helicopters use carriers in those cases where this is better according to the distances (which may not always be the case, if destinations are widely dispersed)? Discuss this briefly in your report.

Please ask us if you have any questions, and see the discussion on modeling action costs in IPC-2008 if you are interested in technical details.

Lab 2.3: Emergency Services Logistics, Optimal Planners

The planners you have used so far may (or may not) take action costs into account, but they don't make any guarantees about the plans that they find. There are also optimizing planners that guarantee that only optimal plans are returned.

You should do the following:

  1. For the modeling part of the lab, try some of these planners on your domain and problems. Do they handle all PDDL constructs that you have used? If not, you need to simplify your domain to work with plain STRIPS functionality. Various simplifications were discussed during one of the lectures. Ask your lab assistant if you encounter problems!
  2. Test the performance of these planners on your domain and problems, in terms of speed and quality. Are the plans better, and if so, how much better? Does it take more time? Do you think it is worth it for this domain? Discuss this briefly in your report.

Finishing and handing in the results

When you have finished the lab, you should once again ask a lab assistant to take a look at your final domain to briefly check whether the model you developed appears reasonable.

When you hand in your results:

  • Create a single ZIP file, named fffll111-fffll222-lab2.zip where fffll111 and fffll222 are your login names (assuming you are working in pairs).
  • The ZIP file should contain in all domain files, problem files and problem generators you have created or modified, clearly named. It should also contain the report as a simple text file or as a PDF document (no Word or OpenOffice documents, please).
  • Send the ZIP file by e-mail to Mikael Nilsson.

Page responsible: Jonas Kvarnström
Last updated: 2017-03-28