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非支配排序遗传算法详解

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