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General Computer Science 320201 GenCS I & II Lecture ... - Kwarc

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completeness does it always find a solution if one exists?<br />

time complexity number of nodes generated/expanded<br />

space complexity maximum number of nodes in memory<br />

optimality does it always find a least-cost solution?<br />

Time and space complexity measured in terms of:<br />

b maximum branching factor of the search tree<br />

d depth of a solution with minimal distance to root<br />

m maximum depth of the state space (may be ∞)<br />

c○: Michael Kohlhase 421<br />

Complexity means here always worst-case complexity.<br />

Note that there can be infinite branches, see the search tree for Romania.<br />

4.1.3 Uninformed Search Strategies<br />

Uninformed search strategies<br />

Definition 598 (Uninformed search) Use only the information available in the problem<br />

definition<br />

Frequently used strategies:<br />

Breadth-first search<br />

Uniform-cost search<br />

Depth-first search<br />

Depth-limited search<br />

Iterative deepening search<br />

c○: Michael Kohlhase 422<br />

The opposite of uninformed search is informed or heuristic search. In the example, one could add,<br />

for instance, to prefer cities that lie in the general direction of the goal (here SE).<br />

Uninformed search is important, because many problems do not allow to extract good heuristics.<br />

Breadth-first search<br />

Idea: Expand shallowest unexpanded node<br />

Implementation: fringe is a FIFO queue, i.e. successors go in at the end of the queue<br />

A<br />

B C<br />

D E F G<br />

H I J K L M N O<br />

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