Question

我试图用python实现Karger算法来计算图的最小切割，并用一个小输入图测试我的代码。基本上我为每次迭代制作原始输入图的副本，但是在运行一次迭代的代码之后，原始输入图以某种方式被修改。我的代码在哪里出错了？

import random

def random_contraction(graph):
    u = random.choice(graph.keys())
    v = random.choice(graph[u])
        for node in graph[v]:
        for vertex in graph[node]:
            if vertex == v:
                graph[node].remove(vertex)
                graph[node].append(u)
    graph[u] = graph[u] + graph[v]
    graph[u] = [node for node in graph[u] if (node != u and node != v)]
    graph.pop(v, None)

def min_cut(graph):
    while len(graph) > 2:
        random_contraction(graph)
    node = random.choice(graph.keys())
    return len(graph[node])

graph_input = {0:[1,3,4,5], 1:[0,2,4],2:[1,3,5],3:[0,2],4:[0,1],5:[0,2]}
list_min_cut = []
for dummy_idx in range(100):
    print "original", graph_input 
    #check the original input graph at the beginning of iteration
    graph = dict(graph_input)
    #make a copy of original input graph
    list_min_cut.append(min_cut(graph))
print min(list_min_cut)

Answer 1

使用dict()进行复制时，只会创建浅副本。这意味着复制不是递归完成的，因此新的dict将包含对原始列表的引用，而不是它们的副本。

要执行深副本，您可以使用deepcopy：

from copy import deepcopy
...

for dummy_idx in range(100):
    print "original", graph_input 
    graph = deepcopy(graph_input)

每次迭代后都会修改python原始字典

1 个答案: