Planning Questions and Answers

Q: What are resource constraints in planning?

Fuel is consumed and a crane is occupied and released, and a planner must respect both kinds of limit, which requires numeric reasoning rather than pure logic. Constraint and linear programming techniques are often combined with the planner for the purpose. Computing budgets, permissions and plan length are different constraints. Extending a symbolic formalism with numbers is a step that changes which solving technologies apply.

Q: Why do resource constraints often require a hybrid solving approach?

A planner reasons well about causal structure and poorly about arithmetic, so scheduling and resource allocation are often handed to a constraint or linear solver while the planner handles the actions. Representation is possible, plan length is a separate matter, and undecidability arises only in particular cases. Combining tools whose strengths are complementary, rather than extending one to do everything, is the same argument that produced hybrid constraint and integer solvers.

Q: What is the difference between planning and scheduling?

A planner decides what must be done and in what causal order, while a scheduler decides when each action happens and which resource performs it. Real applications need both and the boundary between them is often blurred deliberately. Timing of the computation, domain and output count are not the distinction. Separating the causal from the temporal and resource questions is what allows each to be attacked by the technology best suited to it. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Q: Consider the following statements about classical planning. 1. States are represented as sets of fluents assumed complete 2. The delete relaxation gives a heuristic computable in polynomial time 3. Partial order planners commit to a total order at every step Which of the statements given above is or are correct?

The first two are correct. A partial order planner deliberately leaves actions unordered until a threat forces a decision, which is the principle of least commitment. Committing at every step would make it an ordinary state space planner. The three statements between them cover representation, heuristics and search strategy, which is most of what distinguishes one planner from another.

Q: Consider the following pairings in planning. Delete relaxation - ignores negative effects Mutex relation - records pairwise incompatibility Causal link - records which action supplies a precondition Conformant plan - branches on the results of sensing How many of the pairs given above are correctly matched?

The first three are correct. A conformant plan is a single sequence that works whatever the initial state is, with no sensing at all; branching on observations is a contingent plan. The distinction determines what the executive must be capable of, and confusing the two leads to planning for a capability the agent does not have.

Q: What is a plan?

A plan is the action sequence, since the agent needs to know what to do rather than merely what will result. Partial order planners produce a set of actions with orderings, any linearisation of which serves. Final states, goal sets and search records are different objects. Being clear about what the planner is asked to produce determines what bookkeeping the search must carry, which is the same consideration as recovering a path in ordinary search.

Q: What is the goal of a planning problem?

A goal is a partial description, so any state satisfying it will do and the planner need not know which one it will reach. Requiring an exact state would be far more restrictive and rarely what is wanted. Final actions and rewards are different notions, the last belonging to decision theoretic planning. Specifying an outcome by the conditions it must meet, rather than exhaustively, is what makes goals expressible at all in a large state space. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Q: What is the initial state of a planning problem?

Classical planning assumes the initial state is completely known, which together with the closed world assumption makes every fluent's truth determinate. Relaxing that assumption gives conformant and contingent planning. First actions and goals are different components, and partial knowledge is exactly what the classical formulation excludes. Knowing which assumptions a formalism makes is what identifies which extension is needed when they fail.

41.

What are resource constraints in planning?

A.Limits on consumable or reusable quantities the actions require
B.Limits on the computing resources the planner may consume
C.Restrictions on which actions each agent is permitted to perform
D.Bounds on the number of actions a plan may contain

Answer: A

Fuel is consumed and a crane is occupied and released, and a planner must respect both kinds of limit, which requires numeric reasoning rather than pure logic. Constraint and linear programming techniques are often combined with the planner for the purpose. Computing budgets, permissions and plan length are different constraints. Extending a symbolic formalism with numbers is a step that changes which solving technologies apply.

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42.

Why do resource constraints often require a hybrid solving approach?

A.Resource constraints make the problem undecidable in general
B.Numeric reasoning is handled better by specialised solvers
C.Resources cannot be represented in a planning language
D.The planner would otherwise produce plans that are too long

Answer: B

A planner reasons well about causal structure and poorly about arithmetic, so scheduling and resource allocation are often handed to a constraint or linear solver while the planner handles the actions. Representation is possible, plan length is a separate matter, and undecidability arises only in particular cases. Combining tools whose strengths are complementary, rather than extending one to do everything, is the same argument that produced hybrid constraint and integer solvers.

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43.

What is the difference between planning and scheduling?

A.Planning applies to the physical tasks and scheduling to the computational ones
B.Planning produces one plan whereas scheduling produces several
C.Planning chooses the actions; scheduling assigns them times and resources
D.Planning is done offline whereas scheduling is done during execution

Answer: C

A planner decides what must be done and in what causal order, while a scheduler decides when each action happens and which resource performs it. Real applications need both and the boundary between them is often blurred deliberately. Timing of the computation, domain and output count are not the distinction. Separating the causal from the temporal and resource questions is what allows each to be attacked by the technology best suited to it. Framing the question as which quantity the change acts upon makes the answer follow without calculation.

Discuss in Forum

44.

Consider the following statements about classical planning.

1. States are represented as sets of fluents assumed complete

2. The delete relaxation gives a heuristic computable in polynomial time

3. Partial order planners commit to a total order at every step

Which of the statements given above is or are correct?

A.1 and 3 only
B.2 and 3 only
C.1, 2 and 3
D.1 and 2 only

Answer: D

The first two are correct. A partial order planner deliberately leaves actions unordered until a threat forces a decision, which is the principle of least commitment. Committing at every step would make it an ordinary state space planner. The three statements between them cover representation, heuristics and search strategy, which is most of what distinguishes one planner from another.

Discuss in Forum

45.

Match each planning technique with its characteristic feature.

List I

A. Progression

B. Regression

C. Partial order planning

D. Hierarchical planning

List II

P. forward from the initial state

Q. backward from the goal

R. least commitment on ordering

S. decomposition of abstract tasks

Select the correct match.

A.A-P, B-Q, C-R, D-S
B.A-Q, B-P, C-S, D-R
C.A-R, B-S, C-P, D-Q
D.A-P, B-R, C-Q, D-S

Answer: A

The four differ in what they search over: states forward, states backward, plans, and task decompositions. Naming the search space is the quickest way to predict a planner's branching factor and the heuristics that will apply to it, and it organises the whole topic more usefully than a chronological account of the algorithms would.

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46.

Consider the following pairings in planning.

Delete relaxation - ignores negative effects

Mutex relation - records pairwise incompatibility

Causal link - records which action supplies a precondition

Conformant plan - branches on the results of sensing

How many of the pairs given above are correctly matched?

A.All four
B.Only three
C.Only one
D.Only two

Answer: B

The first three are correct. A conformant plan is a single sequence that works whatever the initial state is, with no sensing at all; branching on observations is a contingent plan. The distinction determines what the executive must be capable of, and confusing the two leads to planning for a capability the agent does not have.

Discuss in Forum

47.

Three of the following are heuristics derived from a relaxation. Which is the odd one out?

Mutex propagation in a planning graph

The additive heuristic over goal atoms

The maximum heuristic over goal atoms

The relaxed plan length heuristic

A.The maximum heuristic over goal atoms
B.The relaxed plan length heuristic
C.Mutex propagation in a planning graph
D.The additive heuristic over goal atoms

Answer: C

The additive, maximum and relaxed plan heuristics are all computed from the delete relaxation. Mutex propagation records interactions the relaxation deliberately ignores, so it strengthens rather than relaxes the model. Distinguishing techniques that simplify a problem to obtain a bound from those that add information to tighten one is what makes the relationship between them intelligible.

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48.

What is a plan?

A.A description of the state of the world after all actions are complete
B.A set of the goals that the agent is currently attempting to achieve
C.A record of the search the planner performed while finding a solution
D.A sequence of actions achieving the goal from the initial state

Answer: D

A plan is the action sequence, since the agent needs to know what to do rather than merely what will result. Partial order planners produce a set of actions with orderings, any linearisation of which serves. Final states, goal sets and search records are different objects. Being clear about what the planner is asked to produce determines what bookkeeping the search must carry, which is the same consideration as recovering a path in ordinary search.

Discuss in Forum

49.

What is the goal of a planning problem?

A.A condition the final state must satisfy
B.The single state the agent is required to reach exactly
C.The action the agent must perform at the end of the plan
D.The reward the agent receives once the plan is complete

Answer: A

A goal is a partial description, so any state satisfying it will do and the planner need not know which one it will reach. Requiring an exact state would be far more restrictive and rarely what is wanted. Final actions and rewards are different notions, the last belonging to decision theoretic planning. Specifying an outcome by the conditions it must meet, rather than exhaustively, is what makes goals expressible at all in a large state space. Most errors here come from carrying a result across from a neighbouring case where it does not hold.

Discuss in Forum

50.

What is the initial state of a planning problem?

A.A partial description of what the agent currently knows
B.A complete description of the world when planning begins
C.The first action of the plan that the agent will perform
D.The condition the agent hopes to bring about eventually

Answer: B

Classical planning assumes the initial state is completely known, which together with the closed world assumption makes every fluent's truth determinate. Relaxing that assumption gives conformant and contingent planning. First actions and goals are different components, and partial knowledge is exactly what the classical formulation excludes. Knowing which assumptions a formalism makes is what identifies which extension is needed when they fail.

Discuss in Forum

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