python - 计算图中节点距离时的关键错误

Question

我一直收到这个关键错误，但我不明白这是怎么回事。我使用的是 for-in 语句，因此键肯定存在:

def floydWarshall(inFile):
    graph = readGraph(inFile)
    print(graph) # = {'0': {'1': 28, '3': 33}, '2': {'3': 50}, '1': {'4': 44, '2': 10}, '3': {'4': 30}, '4': 999999999}
    nodes = graph.keys()
    print(nodes) # = dict_keys(['0', '2', '1', '3', '4'])

    distance = {}

    for n in nodes:
        distance[n] = {}

        for k in nodes:
        distance[n][k] = graph[n][k]

    for k in nodes:
        for i in nodes:
            for j in nodes:
                distance[i][j] = min (distance[i][j], distance[i][k] + distance[k][j])
    printSolution(distance)

错误:

Traceback (most recent call last):
File "C:/Users/.../prob1.py", line 58, in floydWarshall
    distance[n][k] = graph[n][k]
KeyError: '2' 

关键错误只是在节点中最先出现的任何关键上，每次都改变

Answer 1

并非所有图节点都与所有其他图节点有边，因此使用 graph[n][k] 遍历整个图上的所有节点 k 将导致 KeyError。

也许你想要这样的东西:

for n in nodes:
    distance[n] = {}

    for k in graph[n]: 
        distance[n][k] = graph[n][k]

或者，如果你想在边不存在的情况下将 distance[n][k] 设置为某个默认值:

for n in nodes:
    distance[n] = {}

    for k in nodes: 
        distance[n][k] = graph[n].get(k, default_value)

default_value 对于节点之间的距离通常设置为无穷大