python - 寻找图中的负循环

Question

我尝试了以下代码，以找到负循环。运行代码时遇到错误:“NameError:名称'NetworkXUnbounded'未定义”

我正在尝试在这里运行贝尔曼福特算法。 Algo 引发“NetworkXUnbounded”类型的异常，我尝试使用如下所示的异常处理来处理该异常。

import pandas as pd 
import networkx as nx
import numpy as np
df = pd.read_csv("fx_rate_3.csv") 
G = nx.from_pandas_edgelist(df,source='A', target='B', edge_attr ['weight'], create_using=nx.DiGraph())

def find_path(digraph, start):
    try:
        path = nx.bellman_ford_predecessor_and_distance(digraph, start, 'weight') 
        return path
    except NetworkXUnbounded:
        cycles = nx.simple_cycles(digraph)
        for cycle in cycles:
            print (cycle) 

Answer 1

在使用之前，您需要先从networkX模块导入NetworkXUnbounded。像这样的事情:

import pandas as pd 
import networkx as nx
import numpy as np
from networkx import NetworkXUnbounded  # <---- Import the exception before using it

df = pd.read_csv("fx_rate_3.csv") 
G = nx.from_pandas_edgelist(df,source='A', target='B', edge_attr ['weight'], create_using=nx.DiGraph())

def find_path(digraph, start):
    try:
        path = nx.bellman_ford_predecessor_and_distance(digraph, start, 'weight') 
        return path
    except NetworkXUnbounded:
        cycles = nx.simple_cycles(digraph)
        for cycle in cycles:
            print (cycle)

还要注意，即使无向图中存在负权重边，也可能会发生此异常:

If the (di)graph contains a negative cost (di)cycle, the algorithm raises an exception to indicate the presence of the negative cost (di)cycle. Note: any negative weight edge in an undirected graph is a negative cost cycle.

参见the documentation了解更多信息。