[英]Recursion: how to avoid Python set changed set during iteration RuntimeError
我有一些代碼可以解決圖形着色問題(廣泛定義為將“顏色”分配給無向圖的問題,確保沒有兩個由邊連接的頂點具有相同的顏色)。 我正在嘗試使用約束傳播來實現解決方案,以提高標准遞歸回溯算法的效率,但遇到以下錯誤:
File "C:\Users\danisg\Desktop\coloring\Solver.py",
line 99, in solve
for color in self.domains[var]:
RuntimeError: Set changed size during iteration
這里,對於每個頂點,我為該特定頂點保留一set
可能的特定值:
self.domains = { var: set(self.colors) for var in self.vars }
在我進行賦值后,我將此約束傳播到相鄰域,以限制搜索空間:
for key in node.neighbors: # list of keys corresponding to adjacent vertices
if color in self.domains[key]: # remove now to prune possible choices
self.domains[key].remove(color)
這不是拋出實際錯誤的地方(在我的代碼中,我指出問題在try-except
塊中的位置),但可能是問題的根源。
我是否有正確的想法,如果不是正確的實施? 更重要的是,我該如何解決這個問題? 另外,是否有必要保留單獨的domains
字典? 或者我們可以使domain
成為圖中每個節點的屬性嗎?
這是調用此代碼的solve
函數:
def solve(self):
uncolored = [var for var in self.vars if self.map[var].color == None]
if len(uncolored) == 0:
return True
var = min(uncolored, key = lambda x: len(self.domains[var]))
node = self.map[var]
old = { var: set(self.domains[var]) for var in self.vars }
for color in self.domains[var]:
if not self._valid(var, color):
continue
self.map[var].color = color
for key in node.neighbors:
if color in self.domains[key]:
self.domains[key].remove(color)
try:
if self.solve():
return True
except:
print('happening now')
self.map[var].color = None
self.domains = old
return False
我的實現使用Node
對象:
class Solver:
class Node:
def __init__(self, var, neighbors, color = None, domain = set()):
self.var = var
self.neighbors = neighbors
self.color = color
self.domain = domain
def __str__(self):
return str((self.var, self.color))
def __init__(self, graph, K):
self.vars = sorted( graph.keys(), key = lambda x: len(graph[x]), reverse = True ) # sort by number of links; start with most constrained
self.colors = range(K)
self.map = { var: self.Node(var, graph[var]) for var in self.vars }
self.domains = { var: set(self.colors) for var in self.vars }
以下是其他兩個使用/有用的功能:
def validate(self):
for var in self.vars:
node = self.map[var]
for key in node.neighbors:
if node.color == self.map[key].color:
return False
return True
def _valid(self, var, color):
node = self.map[var]
for key in node.neighbors:
if self.map[key].color == None:
continue
if self.map[key].color == color:
return False
return True
我正在使用的示例圖可以在這里找到。
讀取數據的功能:
def read_and_make_graph(input_data):
lines = input_data.split('\n')
first_line = lines[0].split()
node_count = int(first_line[0])
edge_count = int(first_line[1])
graph = {}
for i in range(1, edge_count + 1):
line = lines[i]
parts = line.split()
node, edge = int(parts[0]), int(parts[1])
if node in graph:
graph[node].add(edge)
if edge in graph:
graph[edge].add(node)
if node not in graph:
graph[node] = {edge}
if edge not in graph:
graph[edge] = {node}
return graph
應該如下調用:
file_location = 'C:\\Users\\danisg\\Desktop\\coloring\\data\\gc_50_3'
input_data_file = open(file_location, 'r')
input_data = ''.join(input_data_file.readlines())
input_data_file.close()
graph = read_and_make_graph(input_data)
solver = Solver(graph, 6) # a 6 coloring IS possible
print(solver.solve()) # True if we solved; False if we didn't
我認為問題在於:
for color in self.domains[var]:
if not self._valid(var, color):
continue
self.map[var].color = color
for key in node.neighbors:
if color in self.domains[key]:
self.domains[key].remove(color) # This is potentially bad.
如果key == var
當self.domains[key].remove(color)
則更改當前正在迭代的集合的大小。 你可以通過使用避免這種情況
for color in self.domains[var].copy():
使用copy()將允許您迭代集合的副本,同時從原始項目中刪除項目。
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