11-twoup - Optimization Problems • Each instance has a...

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Unformatted text preview: Optimization Problems • Each instance has a number of candidate solutions . • An objective function (usually) maps each candidate solution to a numeric value. • The goal is to find a candidate solution that either minimizes or maximizes the objective function, depending on the problem. • Example: Maximum subsequence sum. 1 Greedy Algorithms • Builds an optimal solution incrementally. • At each step, the partial solution is extended according to some criterion that is reasonably easy to evaluate. – We must prove that whenever we have a partial solution that can be extended to an optimal solution, this extension will result in a partial solution that can be extended to an optimal solution. • Not all optimization problems admit greedy algorithms; however, when a greedy algorithm is possible, it can usually be implemented efficiently. 2 Job Scheduling Input: A set of jobs { x 1 , . . . , x m } , each having a natural number deadline and value . Output: An array sched [1 ..n ] such that • for 1 ≤ i ≤ n , ≤ sched [ i ] ≤ m ; • if sched [ i ] negationslash = 0 , then i is no greater than the deadline of x sched [ i ] ; • each j , 1 ≤ j ≤ m , appears at most once in sched [1 ..n ] ; and • the sum of the values of the scheduled jobs is maximized. 3 Generalization • The first step in constructing any greedy algorithm is to generalize the problem so that the input also includes a partial solution. • A straightforward generalization includes as input a set of (unscheduled) jobs and a schedule of other jobs. • In making the next scheduling decision, we don’t need to know which jobs occupy the occupied time slots — only that these slots are occupied. • The generalization we use takes as input a set of unscheduled jobs and a boolean array indicating which time slots are available. 4 Scheduling Criterion • We can transform this problem to a smaller instance by assigning a job to an unoccupied time slot so that this schedule can be extended to an optimal schedule....
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This note was uploaded on 09/06/2009 for the course CIS 11274 taught by Professor Howell during the Spring '09 term at Kansas State University.

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11-twoup - Optimization Problems • Each instance has a...

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