Lecture 4 - Heuristic search strategies Flashcards
What is an evaluation function?
a function that estimates how close a state is to a goal state
What is a heuristic?
An evaluation function that may not be perfect
What is a heuristic search strategy?
A search strategy that uses a heuristic function to determine which node should be expanded next
What is greedy search?
Always chooses the node that the heuristic indicates is closest to the goal state
Is greedy search optimal?
Why?
No
It doesn’t expand each node so may not find the most optimal route
Greedy search typically has a _____ search time
a) low
b) high
a) low
What is A* search?
A combination of uniform cost search and greedy search
If
g(n) is the cost to get from the initial node to node n, and
h(n) is the estimated cost to get to the goal state from node n, then
Expanding the node with the smallest g(n) is what type of search?
Uniform Cost
If
g(n) is the cost to get from the initial node to node n, and
h(n) is the estimated cost to get to the goal state from node n, then
Expanding the node with the smallest h(n) is what type of search?
Greedy Search
If
g(n) is the cost to get from the initial node to node n, and
h(n) is the estimated cost to get to the goal state from node n, then
Expanding the node with the smallest g(n)+h(n) is what type of search?
A* search
In A* search, a solution is found when
a) the goal node is generated
b) the goal node is expanded
b) expanded
What is an admissible heuristic?
A heuristic that never overestimates the true cost of reaching the goal.
Providing an admissible heuristic is used, is A* search complete and optimal?
Yes and yes
What is the main limitation on the use of A* search? Why?
Space complexity - it grows exponentially unless the heuristic is very accurate
Greedy search is also known as
best first search
however,
best first search may also refer to A* search
What is hill climbing?
A search strategy that finds local maxima but may not find the absolute maximum
Give an example of hill climbing using a set of towns and moving from one town to another
Pick a road that seems to be heading in the right direction
When you get to the next town, pick the one that is going in the best direction
Hill climbing _________ allow back tracking
a) does
b) doesn’t
Does not
In many real problems, the only practical search strategy is _____?
Hill Climbing
