Integer Programming: Algorithms - 3

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1 Week 9 Integer Programming: Algorithms - 3 OPR 992 Applied Mathematical Programming OPR Applied Mathematical Programming - p. 1/12

2 Dantzig-Wolfe Reformulation Example Strength of the Linear Programming Master OPR Applied Mathematical Programming - p. 2/12

3 Dantzig-Wolfe Reformulation Dantzig-Wolfe Reformulation Example Strength of the Linear Programming Master Consider an IP of the form max K k=1 (ck ) T x k s.t. K k=1 (A k) T x k b k x k X k, k = 1,..., K Assuming that X k = {x k,t } T k t=1, k = 1,..., K, we can introduce λ k,t {0, 1} such that X k = T k t=1 λ k,t x k,t, and T k t=1 λ k,t = 1. OPR Applied Mathematical Programming - p. 3/12

.., K Assuming that X k = {x k,t } T k t=1, k = 1,.

4 Example Dantzig-Wolfe Reformulation Example Strength of the Linear Programming Master Solve an instance of STSP with the following distance matrix: OPR Applied Mathematical Programming - p. 4/12

STSP with the following distance matrix: 7 2 1 5 3 6 8

5 Strength of the Linear Programming Master Dantzig-Wolfe Reformulation Example Strength of the Linear Programming Master K z LP M = max{ c k x k : k=1 K A k x k = b, x k conv(x k ), k = 1,..., K}. k=1 z LP M = w LD = z CUT. Choose the right algorithm based on speed. OPR Applied Mathematical Programming - p. 5/12

k = b, x k conv(x k ), k = 1,..., K}. k=1 z LP M = w LD = z CUT.

6 OPR Applied Mathematical Programming - p. 6/12

7 Introduction When should we use a heuristic? A solution is required rapidly. The instance is too large to formulate as whole problem of reasonable size. Once formulated, known algorithms cannot solve it in real time. It is much easier to find solutions by inspection than by solving using a general-purpose algorithm. OPR Applied Mathematical Programming - p. 7/12

Once formulated, known algorithms cannot solve it in real time.

8 Tabu Search Local search for a constrained problem of the form can involve a goal function min{c(x) : g(x) = 0} c(x) + αg(x). For different values of α, the local search can cycle among solutions. Solution: Make certain solution forbidden to avoid cycling. OPR Applied Mathematical Programming - p. 8/12

For different values of α, the local search can cycle among solutions.

9 Simulated Annealing Choose a neighbor randomly. The probability of choosing a neighbor depends on the effect on the goal function. Parameters of a simulated annealing algorithm: 1. The initial temperature T 2. The cooling ratio r 3. The loop length L 4. The definition of frozen, or the stopping criterion. OPR Applied Mathematical Programming - p. 9/12

Parameters of a simulated annealing algorithm: 1. The initial temperature T 2.

10 Simulated Annealing - The Algorithm 1. Get an initial solution S. 2. Get an initial temperature T and a cooling ratio r with 0 < r < While not yet frozen, do the following: (a) Perform the following loop L times: i. Pick a random neighbor S of S. ii. Let = f(s ) f(s). iii. If 0, set S = S. iv. If > 0, set S = S with probability e /T. (b) Reduce the temperature by setting T rt. 4. Return the best solution found. OPR Applied Mathematical Programming - p. 10/12

While not yet frozen, do the following: (a) Perform the following loop L times: i. Pick a random neighbor S of S. ii.

11 Genetic Algorithms Start with a finite population of solutions and evolve it from one generation to the next. Each iteration performs the following steps: 1. Evaluation: The fitness of the individuals is evaluated. 2. Parent Selection: Certain pairs of solutions (parents) are selected based on their fitness. 3. Crossover: Each pair of parents combines to produce one or two new solutions (offspring). 4. Mutation: Some of the offspring are randomly modified. 5. Population Selection: Based on their fitness, a new population is selected replacing some or all of the original population by an identical number of offspring. OPR Applied Mathematical Programming - p. 11/12

Crossover: Each pair of parents combines to produce one or two new solutions (offspring). 4. Mutation: Some of the offspring are randomly modified. 5.

12 Local Search Wrap-Up In any local search algorithm, it is important to balance: 1. Communication 2. Diversification 3. Intensification OPR Applied Mathematical Programming - p. 12/12

Genetic Algorithm. Based on Darwinian Paradigm. Intrinsically a robust search and optimization mechanism. Conceptual Algorithm

Genetic Algorithm. Based on Darwinian Paradigm. Intrinsically a robust search and optimization mechanism. Conceptual Algorithm 24 Genetic Algorithm Based on Darwinian Paradigm Reproduction Competition Survive Selection Intrinsically a robust search and optimization mechanism Slide -47 - Conceptual Algorithm Slide -48 - 25 Genetic