Algorithm Design Techniques Algorithm Design Techniques v Brute

Algorithm Design Techniques

Algorithm Design Techniques v Brute Force v Greedy Algorithms v Divide and Conquer v Dynamic Programming v Backtracking v Genetic Algorithms 2

Brute Force Ø Based on the problem’s statement and definitions of the concepts involved. Ø Examples: Sequential search Ø Simple sorts: bubble sort etc. Ø 3

Greedy Algorithms "take what you can get now" strategy Work in phases. In each phase the currently best decision is made 4

Greedy Algorithms Examples • Dijkstra's algorithm (shortest path is weighted graphs) • Prim's algorithm, Kruskal's algorithm (minimal spanning tree in weighted graphs) 5

Divide and Conquer • Reduce the problem to smaller problems (by a factor of at least 2) solved recursively and then combine the solutions Examples: Binary Search Mergesort Quick sort Tree traversal In general, problems that can be defined recursively 6

Merge Sort (move to slide show) 7

Dynamic Programming Bottom-Up Technique in which the smallest sub-instances are explicitly solved first and the results of these used to construct solutions to progressively larger sub-instances. Example: Fibonacci numbers computed by iteration and the Tower of Hanoi puzzle. 8

Tower of Hanoi (move to slide show) Conditions for Tower Of Hanoi: • Only one disk may be moved at a time. • Each move consists of taking the upper disk from one of the rods and sliding it onto another rod, on top of the other disks that may already be present on that rod. • No disk may be placed on top of a smaller disk 9

Backtracking Generate-and-Test methods Based on exhaustive search in multiple choice problems Example: Puzzles like eight queens puzzle and traveling salesman problem 10

Solutions to 8 Queens Problem 11

Backtracking – State Space Search • initial state • goal state(s) • a set of intermediate states • a set of operators that transform one state into another. Each operator has preconditions and postconditions. • a cost function – evaluates the cost of the operations (optional) • a utility function – evaluates how close is a given state to the goal state (optional) 12

Genetic Algorithms n Search for good solutions among possible solutions n The best possible solution may be missed n A solution is coded by a string , also called n n chromosome. The words string and chromosome are used interchangeably A strings fitness is a measure of how good a solution it codes. Fitness is calculated by a fitness function Selection: The procedure to choose parents Crossover is the procedure by which two chromosomes mate to create a new offspring chromosome Mutation : with a certain probability flip a bit in the offspring 13

Basic Genetic Algorithm 1. Start: Generate random population of n chromosomes (suitable solutions for the problem) 2. Fitness: Evaluate the fitness f(x) of each chromosome x in the population 3. New population: Create a new population by repeating following steps until the new population is complete 4. Test: If the end condition is satisfied, stop, and return the best solution in current population 14

New Population Ø Selection: Select two parent chromosomes from a population according to their fitness Ø Crossover: With a crossover probability cross over the parents to form a new offspring (children). If no crossover was performed, offspring is an exact copy of parents. Ø Mutation: With a mutation probability mutate new offspring at each locus (position in chromosome). 15