LOCAL SEARCH AND CONTINUOUS SEARCH. Local search algorithms In many optimization problems, the path...
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Transcript of LOCAL SEARCH AND CONTINUOUS SEARCH. Local search algorithms In many optimization problems, the path...
Local search algorithms
In many optimization problems, the path to the goal is irrelevant; the goal state itself is the solution
In such cases, we can use local search algorithms keep a (sometimes) single "current" state, try
to improve it
Example: n-queens
Put n queens on an n × n board with no two queens on the same row, column, or diagonal
Example: n-queens
Put n queens on an n × n board with no two queens on the same row, column, or diagonal
Example: n-queens
Put n queens on an n × n board with no two queens on the same row, column, or diagonal
Local Search
Operates by keeping track of only the current node and moving only to neighbors of that node
Often used for: Optimization problems Scheduling Task assignment …many other problem where the goal is to
find the best state according to some objective function
Hill-climbing search
Consider next possible moves (i.e. neighbors) Pick the one that improves things the most
“Like climbing Everest in thick fog with amnesia”
Hill-climbing search: 8-queens problem
h = number of pairs of queens that are attacking each other, either directly or indirectly
h = 17 for the above state
Hill-climbing search: 8-queens problem
• 5 steps later…• A local minimum with h = 1 (a common problem
with hill climbing)
Try, try again
Run algorithm some number of times and return the best solution Initial start location is usually chosen randomly
If you run it “enough” times, will get answer (in the limit)
Drawback: takes lots of time
Sideways moves
If stuck on a ridge, if we wait awhile and allow flat moves, will become unstuck—maybe
Questions How long is awhile? How likely to become unstuck?
Any other extensions?
First-choice hill climbing Generate successors randomly until a good
one is found Look three moves ahead
Unstuck from certain areas More inefficient Might not be any better Move quality: as good or better
Comparison of approaches for 8-queens problem
Technique Success rate Average number of moves
Hill Climbing 14% 3.9
Hill Climbing + 6 restarts if needed
65% 11.5
Hill Climbing + up to 100 sideways moves if needed
94% 21
• Tradeoff between success rate and number of moves
• As success rate approaches 100% number of moves will increase rapidly
Nice properties of local search Can often get “close”
When is this useful?
Can trade off time and performance
Can be applied to continuous problems E.g. first-choice hill climbing More on this later…
Simulated annealing
Insight: all of the modifications to hill climbing are really about injecting variance Don’t want to get stuck in local maxima or
plateu
Idea: explicitly inject variability into the search process
Properties of simulated annealing
More variability at the beginning of search Since you have little confidence you’re in right
place
Variability decreases over time Don’t want to move away from a good solution
Probability of picking move is related to how good it is Sideways or slight decreases are more likely than
major decreases
How simulated annealing works At each step, have temperature T
Pick next action semi-randomly Higher temperature increase randomness Select action according to goodness and
temperature Decrease temperature slightly at each time
step until it reaches 0 (no randomness)
Local Beam Search
Keep track of k states rather than just one
Start with k randomly generated states
At each iteration, all the successors of all k states are generated
If any one is a goal state, stop; else select the k best successors from the complete list and repeat. Results in states getting closer together over
time
Stochastic Local Beam Search
Designed to prevent all k states clustering together
Instead of choosing k best, choose k successors at random, with higher probability of choosing better states.
Terminology: stochastic means random.
Genetic algorithms
Inspired by nature
New states generated from two parent states. Throw some randomness into the mix as well…
Genetic Algorithms
Initialize population (k random states) Select subset of population for mating Generate children via crossover
Continuous variables: interpolate Discrete variables: replace parts of their
representing variables Mutation (add randomness to the children's
variables) Evaluate fitness of children Replace worst parents with the children
Genetic algorithms
Fitness function: number of non-attacking pairs of queens (min = 0, max = 8 × 7/2 = 28) 24/(24+23+20+11) = 31% 23/(24+23+20+11) = 29% … etc.
Genetic algorithms
Probability of selection is weighted by the normalized fitness function.
Crossover from the top two parents.
Genetic Algorithms
1. Initialize population (k random states)2. Calculate fitness function3. Select pairs for crossover4. Apply mutation5. Evaluate fitness of children6. From the resulting population of 2*k
individuals, probabilistically pick k of the best.
7. Repeat.
Searching Continuous Spaces Continuous: Infinitely many values. Discrete: A limited number of
distinct, clearly defined values.
In continuous space, cannot consider all next possible moves (infinite branching factor) Makes classic hill climbing impossible
Example
Want to put 3 airports in Romania, such that the sum of squared distances from each city on the map to its closest airport is minimized.
State: coordinates of the airports
Objective function:, ¿∑𝑖=1
3
∑𝑐∈𝐶 𝑖
(𝑥𝑖−𝑥𝑐 )2+( 𝑦 𝑖+𝑦𝑐 )2
Searching Continuous Space
Discretize the state space Turn it into a grid and do what we’ve always
done.
Searching Continuous Space Calculate the gradient of the objective
function at the current state.
Take a step of size in the direction of the steepest slope
Problem: Can be hard or impossible to calculate. Solution: approximate the gradient through sampling.
Step size
Very small takes a long time to reach the peak
Very big can overshoot the goal
What can we do…? Start high and decrease with time Make it higher for flatter parts of the space
Summary
Local search often finds an approximate solution (i.e. it end in “good” but not “best” states)
Can inject randomness to avoid getting stuck in local maxima
Can trade off time for higher likelihood of success
Real World Problems
“many real world problems have a landscape that looks more like a widely scattered family of balding porcupines on a flat floor, with miniature porcupines living on the tip of each porcupine needle, ad infinitum.”
-Russell and Norvig
“One of the popular myths of higher education is that professors are sadists who live to inflict psychological trauma on undergraduates. …”
… “I do not “take off” points. You earn them. The difference is not merely rhetorical, nor is it trivial. In other words, you start with zero points and earn your way to a grade.”
… “this means that the burden of proof is on you to demonstrate that you have mastered the material. It is not on me to demonstrate that you have not. ”
Dear Student: I Don't Lie Awake At Night Thinking of Ways to Ruin Your Life
Art Caden, for Forbes.com
Link to the Article