Random/Exhaustive Search

1
Random/Exhaustive Search

• Generate and Test
• Generate a candidate solution and test to see if
  it solves the problem
• Repeat
• Information used by this algorithm
• You know when you have found the solution

2
Hill Climbing

• Generate a candidate solution by modifying the
  last solution, S
• If the new solution, N, is better than S then
  S N
• Repeat
• Local Search
• Information used by this algorithm
• Compare two candidate solutions and tell which is
  better

3
Population of Hill Climbers

• Randomly generate initial population of hill
  climbers (Randomly generate initial candidate
  solutions)
• Do hill climbing in parallel
• After time t, choose best solution in population
• Information used by this algorithm
• Same as hill climbing

4
Genetic Algorithms

• Population of information exchanging hill
  climbers
• Concentrates resources in promising areas of the
  search space
• Information used
• Same as hillclimbing

5
Hard problems

• Computational complexity, problem size n
• Binary Search O(log(n))
• Linear Search O(n)
• Bubble Sort O(n2)
• Scheduling NP-complete
  (at least exponential
  O(an)

6
Hard problems

• Poorly defined

Robotics How do we catch a ball, navigate, play basketball
User Interfaces Predict next command, adapt to individual user
Medicine Protein structure prediction, Is this tumor benign, design drugs
Design Design bridge, jet engines, Circuits, wings
Control Nonlinear controllers
7
Search as a solution to hard problems

• Strategy generate a potential solution and see
  if it solves the problem
• Make use of information available to guide the
  generation of potential solutions
• How much information is available?
• Very little We know the solution when we find it
• Lots linear, continuous,
• Modicum Compare two solutions and tell which is
  better

8
Search tradeoff

• Very little information for search implies we
  have no algorithm other than RES. We have to
  explore the space thoroughly since there is no
  other information to exploit
• Lots of information (linear, continuous, ) means
  that we can exploit this information to arrive
  directly at a solution, without any exploration
• Modicum of information (compare two solutions)
  implies that we need to use this information to
  tradeoff exploration of the search space versus
  exploiting the information to concentrate search
  in promising areas

9
Exploration vs Exploitation

• More exploration means
• Better chance of finding solution (more robust)
• Takes longer
• More exploitation means
• Less chance of finding solution, better chance of
  getting stuck in a local optimum
• Takes less time

10
Choosing a search algorithm

• The amount of information available about a
  problem influences our choice of search algorithm
  and how we tune this algorithm
• How does a search algorithm balance exploration
  of a search space against exploitation of
  (possibly misleading) information about the
  search space?
• What assumptions is the algorithm making?

11
Genetic Algorithm

• Generate pop(0)
• Evaluate pop(0)
• T0
• While (not converged) do
• Select pop(T1) from pop(T)
• Recombine pop(T1)
• Evaluate pop(T1)
• T T 1
• Done