23

basically I have been racking my brains for a good while now as to why my code is not working, I have tested parts separately and have look throughout the web to see if it can help, to no avail. I am getting an error that the traceback is:

Traceback (most recent call last):
File "yes2.py", line 62, in <module>
g.add_edge(row_index,col_index, b)
File "yes2.py", line 27, in add_edge
self.adj[u].append(edge) 
KeyError: 0

The two parts with errors are 

    def add_edge(self, u, v, w=0): 
    if u == v: 
        raise ValueError("u == v") 
    edge = Edge(u,v,w) 
    redge = Edge(v,u,0) 
    edge.redge = redge 
    redge.redge = edge 
    self.adj[u].append(edge) #### LINE 27 ####
    self.adj[v].append(redge) 
    self.flow[edge] = 0 
    self.flow[redge] = 0

and 

g = FlowNetwork()
map(g.add_vertex, ['0','1','2','3','4','5','6'])
with open('network.txt', "r") as file:
for row_index, row in enumerate(file):
    for col_index, value in enumerate(row.split(",")):
        b = int(value)
        if b != 0:
            g.add_edge(row_index,col_index, b) ### LINE 62 ####

And here is the completed code, as without this it may be difficult to see what is happening

class Edge(object):
def __init__(self, u, v, w):
    self.source = u
    self.sink = v 
    self.capacity = w 
def __repr__(self): 
    return "%s->%s:%s" % (self.source, self.sink, self.capacity) 

class FlowNetwork(object): 
def __init__(self): 
    self.adj = {} 
    self.flow = {} 

def add_vertex(self, vertex): 
    self.adj[vertex] = [] 

def get_edges(self, v): 
    return self.adj[v] 

def add_edge(self, u, v, w=0): 
    if u == v: 
        raise ValueError("u == v") 
    edge = Edge(u,v,w) 
    redge = Edge(v,u,0) 
    edge.redge = redge 
    redge.redge = edge 
    self.adj[u].append(edge) 
    self.adj[v].append(redge) 
    self.flow[edge] = 0 
    self.flow[redge] = 0 

def find_path(self, source, sink, path): 
    if source == sink: 
        return path 
    for edge in self.get_edges(source): 
        residual = edge.capacity - self.flow[edge] 
        if residual > 0 and not (edge,residual) in path: 
            result = self.find_path( edge.sink, sink, path + [(edge,residual)] ) 
            if result != None: 
                return result 

def max_flow(self, source, sink): 
    path = self.find_path(source, sink, []) 
    while path != None: 
        flow = min(res for edge,res in path) 
        for edge,res in path: 
            self.flow[edge] += flow 
            self.flow[edge.redge] -= flow 
        path = self.find_path(source, sink, []) 
    return sum(self.flow[edge] for edge in self.get_edges(source)) 

g = FlowNetwork()
map(g.add_vertex, ['0','1','2','3','4','5','6'])
with open('network.txt', "r") as file:
# enumerate allows you to iterate through the list with an index and an object
for row_index, row in enumerate(file):
    # split allows you to break a string apart with a string key
    for col_index, value in enumerate(row.split(",")):
        #convert value from string to int
        b = int(value)
        if b != 0:
            g.add_edge(row_index,col_index, b)

print g.max_flow('1','6')

Many thanks for your time, much appreciated.

4

4 回答 4

22

你得到的错误是它还self.adj没有 key 0。您正在尝试追加到一个尚不存在的列表。

考虑使用 adefaultdict代替，替换这一行（in __init__）：

self.adj = {}

有了这个：

self.adj = defaultdict(list)

您需要在顶部导入：

from collections import defaultdict

现在，而不是提高 a KeyError，self.adj[0].append(edge)将自动创建一个列表以附加到。

于 2014-04-25T15:48:19.197 回答
7

defaultdict 解决方案更好。但是为了完整起见，您还可以在追加之前检查并创建空列表。添加 + 行：

+ if not u in self.adj.keys():
+     self.adj[u] = []
  self.adj[u].append(edge)
.
.
于 2017-07-27T11:09:48.023 回答
3

仅当您的列表或字典在本地功能中不可用时才会出现。

于 2019-06-03T20:04:28.643 回答
0

尝试这个：

class Flonetwork(Object):
    def __init__(self,adj = {},flow={}):
        self.adj = adj
        self.flow = flow
于 2019-07-27T00:14:33.133 回答