Search¶

Why?¶

• To achieve goals or to maximize our utility we need to predict what the result of our actions in the future will be.
• There are many sequences of actions, each with their own utility.
• We want to find, or search for, the best one.

Problem¶

• initial state
• actions: set of possible actions in current state x.
• transition model: Result(x,a) = state that follows from applying action
• goal test
• A solution is a sequence of steps / path

Trees¶

• Exploration of state space by generating successors of already-explored states (a.k.a.~expanding states).
• Every states is evaluated: is it a goal state?
• Repeated states: Failure to detect repeated states can turn a linear problem into an exponential one!
• must keep track of all possible states (uses a lot of memory)
``````**function** TREE-SEARCH(problem) **returns** a solution, or failure
initialize the frontier using the initial state of problem
**loop do**
**if** the frontier is empty **then return** failure
choose a leaf node and remove it from the frontier
**if** the node contains a goal state **then return** the corresponding solution
expand the chosen node, adding the resulting nodes to the frontier

**function** GRAPH-SEARCH(problem) **returns** a solution, or failure
initialize the frontier using the initial state of problem
**initialize the explored set to be empty**
loop do
**if** the frontier is empty **then return** failure
choose a leaf node and remove it from the frontier
**if** the node contains a goal state **then return** the corresponding solution
**_add the node to the explored set_**
expand the chosen node, adding the resulting nodes to the frontier
**only if not in the frontier or explored set**
``````

Implementation¶

• A state is a (representation of) a physical configuration
• A node n is a data structure constituting part of a search tree contains info such as: state, parent node, action, path cost g(n).
• The Expand function creates new nodes, filling in the various fields and using the SuccessorFn of the problem to create the corresponding states.

Search Strategy¶

picking the order of node expansion

Evaluation Metrics¶
• completeness
• time complexity
• space complexity
• optimality (least-cost)

• b: maximum branching factor

• d: depth of the least-cost solution
• m: maximum depth of the state space (may be ∞)

While searching you have no clue whether one non-goal state is better than any other. Your search is blind. You don’t know if your current exploration is likely to be fruitful.

Various blind strategies:

• Uniform-cost search
• Depth-first search
• Iterative deepening search
• Bi-Directional Search
Criterion Breadth- First Uniform- Cost Depth- First Depth- Limited Iterative Deepening Bidirectional (if applicable)
Time bd b bm b' bd 64n
Space b b bm bl bd ber
Optimal? Yes Yes No No Yes Yes
Complete? Yes Yes No Yes, if I > d Yes Yes
• Direct techniques (blind search) are not always possible
• Weak techniques can be effective if applied correctly on the right kinds of tasks. (Heuristic Search Techniques)

• Heuristic search techniques make use of domain specific information - a heuristic.

Generate and Test¶

``````do while goal not accomplished
generate a possible solution
test solution to see if it is a goal
``````

Hill Climbing¶

Variation on generate-and-test:

• generation of next state depends on feedback from the test procedure.
• Test now includes a heuristic function that provides a guess as to how good each possible state is.
• There are a number of ways to use the information returned by the test procedure.
• Searching for a goal state = Climbing to the top of a hill
Simple Hill Climbing¶
• Use heuristic to move only to states that are better than the current state.
• Always move to better state when possible.
• The process ends when all operators have been applied and none of the resulting states are better than the current state.

Problems:

• Will terminate when at local optimum.
• The order of application of operators can make a big difference.
• Can’t see past a single move in the state space.

Simulated Annealing

• Based on physical process of annealing a metal to get the best (minimal energy) state. • Hill climbing with a twist: – allow some moves downhill (to worse states) – start out allowing large downhill moves (to much worse states) and gradually allow only small downhill moves.

A variation of hill climbing in which, at the beginning of the process, some downhill moves may be made. • To do enough exploration of the whole space early on, so that the final solution is relatively insensitive to the starting state.

• Lowering the chances of getting caught at a local maximum, or plateau, or a ridge.

The search initially jumps around a lot, exploring many regions of the state space.

• The jumping is gradually reduced and the search becomes a simple hill climb (search for local optimum).