一、NSGA算法简介
非支配排序遗传算法(Non-dominated Sorting Genetic Algorithm,NSGA)是将遗传算法的种群个体按照它们的优先级进行分类排序,使得较优的个体能够被更好地保存下来,同时保留种群中的多样性。它首先通过非支配排序将种群分为多个层级,然后在每一层级中根据拥挤度算法对个体进行排序,以维护每一个层级中的多样性。
下面是一段使用NSGA算法进行多目标优化的Python示例代码:
def nsga(population, generations, crossover_rate, mutation_rate): for gen in range(generations): offspring = [] for i in range(len(population)): # 选择两个父代 parent1 = selection(population) parent2 = selection(population) # 对两个父代进行杂交 child = crossover(parent1, parent2, crossover_rate) # 对后代进行变异操作 child = mutation(child, mutation_rate) # 将后代添加到下一代种群中 offspring.append(child) # 合并父代和后代种群 population = population + offspring # 对种群进行非支配排序和拥挤度排序 fronts = non_dominated_sorting(population) sorted_population = sort_by_crowding_distance(population, fronts) # 选择下一代种群 population = sorted_population[:len(population)] return population
二、NSGA算法的非支配排序
NSGA算法的关键在于如何对种群进行非支配排序。给定一组个体,非支配排序将它们分为多个层级,每一层级中的个体都是相对于同一层级中的其他个体来说非支配的。具体步骤如下:
1、计算每一个个体的支配关系,即如果一个个体在某一个目标函数上比另一个个体更好,则该个体支配另一个个体。
2、根据支配关系得到第一层级中的非支配解集合,同时将支配第一层级中的个体进行标记。
3、重复地对每一层级进行处理,直到没有非支配解为止。在处理每一层级时,需要将上一层级中所有被标记的个体排除,并将下一层级中所有新加入的非支配解通过拥挤度算法进行排序,以避免个体聚拢。
下面是非支配排序的代码实现:
def non_dominated_sorting(population): fronts = [] n = [0]*len(population) rank = [0]*len(population) S = [[] for i in range(len(population))] fronts.append([]) for p in range(len(population)): S[p] = [] n[p] = 0 for q in range(len(population)): if population[p].dominates(population[q]): S[p].append(q) elif population[q].dominates(population[p]): n[p] = n[p] + 1 if n[p] == 0: rank[p] = 0 fronts[0].append(p) i = 0 while len(fronts[i]) != 0: Q = [] for p in fronts[i]: for q in S[p]: n[q] = n[q] - 1 if n[q] == 0: rank[q] = i + 1 Q.append(q) i += 1 fronts.append(Q) return fronts[:-1]
三、NSGA算法的拥挤度排序
在计算完每个个体的非支配级别之后,我们需要对每层内的个体进行排序,以便于选择个体生成下一代种群。通常使用拥挤度距离来实现排序,拥挤度表示一个个体周围有多少个个体与其距离相似。在选择下一代种群时,我们希望不仅保留靠前的非支配解,还要尽可能多地保留多样性。因此,当选择靠前的非支配解时,我们倾向于选择拥挤度高的个体,这些个体周围有很多其他个体,说明它们距离其他个体比较远,因此保留它们能够维护多样性。
下面是拥挤度排序的代码实现:
def crowding_distance_assignment(population): n = len(population) for individual in population: individual.crowding_distance = 0 for m in range(len(population[0].fitness)): sorted_population = sorted(population, key=lambda x: x.fitness[m]) sorted_population[0].crowding_distance = sorted_population[n-1].crowding_distance = inf for i in range(1, n-1): sorted_population[i].crowding_distance += sorted_population[i+1].fitness[m] - sorted_population[i-1].fitness[m] for individual in population: individual.crowding_distance /= len(population)
四、NSGA算法的应用举例
下面是一段使用NSGA算法进行多目标优化的Python示例代码,通过调整链表的长度和宽度来优化链表的布局效果:
class LayoutIndividual: def __init__(self, length, width): self.length = length self.width = width self.fitness = [] def evaluate_fitness(self): # 计算适应度 self.fitness = [self.length, self.width] def dominates(self, other): # 判断支配关系 return self.length <= other.length and self.width <= other.width and (self.length < other.length or self.width < other.width) def layout_optimization(objective_functions, max_generations): # 初始化种群 population = [LayoutIndividual(randint(1, 10), randint(1, 10)) for i in range(30)] for gen in range(max_generations): # 计算适应度 for individual in population: individual.evaluate_fitness() # 对种群进行非支配排序和拥挤度排序 fronts = non_dominated_sorting(population) for i, front in enumerate(fronts): crowding_distance_assignment(front) for j, individual in enumerate(front): individual.rank = i individual.distance = individual.crowding_distance # 选择下一代种群 next_population = [] for i in range(30): p1 = random.choice(population) p2 = random.choice(population) child = LayoutIndividual(p1.length, p2.width) next_population.append(child) population = next_population return population