我编写了一个Python脚本来解决"Fox, goose and bag of beans puzzle"。我用ABM(基于代理的模型)编写代码。每一件需要运送过河的东西都是一个乘客的物品。河边还有两块陆地是太空物体。在
代码运行良好,可以解决原始问题。但是,一旦我尝试初始化对象(例如peasant2、fox2),就会发生无限循环。我是说我只是初始化了。我从来没有把它们放在实际的模拟中。因此,如果取消对行num 170(#fox2=Passenger('fox','rooster')的注释,就会发生无限循环。有趣的是,你可以初始化额外的谷物或公鸡,但不能初始化农民或狐狸。我想这可能是由于随机模块,所以我试着用
random.seed(some_int)
但这并没有解决任何问题。在
有趣的是,代码在Windows10Python2.7.4上运行良好。我试过用另一个Mac,但它也有无限循环。 是Mac问题还是Python问题?我的密码怎么了?在
无误编码
^{pr2}$具有无限循环的代码
from sets import Set
import random
from itertools import *
class Passenger(object):
""" Anything that gets on board on the boat.
Assumed that there could be multiple captains """
def __init__(self, species, food=None, is_captain=False):
self.species = species
self.food = food
self.is_captain = is_captain
def eat(self, something):
return self.food == something.species
def __str__(self):
return "I am %s" % self.species
class Space(object):
"""docstring for """
def __init__(self, name, residents=[]):
self.name = name
self.residents = residents
self.captains = self.update_captains()
def num_residents(self):
return len(self.residents)
## e.g. send_off([traveller1, traveller2])
def send_off(self, passengers):
''' Remove the passengers who left for the other land.
It means that the number of captains in the land is changed. '''
self.residents = list(Set(self.residents) - Set(passengers))
self.captains = self.update_captains()
## e.g. welcome([sailing_captain, traveller])
def welcome(self, passengers):
''' Append newcomers '''
self.residents += passengers
self.captains = self.update_captains()
def update_captains(self):
return [r for r in self.residents if r.is_captain]
def pick_a_captain(self):
''' Pick a captain randomly '''
return random.choice(self.captains)
def print_resident_species(self):
''' Simply print out every species in the land.
For debug purpose '''
for r in self.residents:
print r.species
def get_resident_species(self):
''' e.g. Returns "fox, grain,"
"fox, grain, peasant" '''
species = [r.species for r in self.residents]
return ', '.join(species)
def __str__(self):
return self.name + ": " + self.get_resident_species()
''' Stand-alone functions '''
def get_captains(residents):
return [r for r in residents if r.is_captain]
def is_peaceful_pair(pair):
''' e.g. is_peaceful_pair([fox, rooster]) => False '''
p1 = pair[0]
p2 = pair[1]
return not p1.eat(p2) and not p2.eat(p1)
def is_peaceful(residents):
''' e.g. is_peaceful([fox, rooster, grain]) => False '''
for pair in list(permutations(residents, r=2)):
if not is_peaceful_pair(pair):
return False
return True
def select_traveller(from_):
for t in from_.residents:
## Figure out if the rest of the residents will get along
if is_peaceful(list(Set(from_.residents) - Set([t]))):
from_.send_off([t])
return t
return None
def get_sailing_captain(from_):
sailing_captain = from_.pick_a_captain()
from_.send_off([sailing_captain])
return sailing_captain
## e.g. travel_to_destination(korea, japan)
## If succeeds, return passengers. If not, return None(stop the simulation)
def travel_to_destination(from_, to):
'''
Randomly pick one traveller and figures out whether the rest will be safe.
Loop until find one and if not, this simulation should end.
'''
if len(from_.captains) == 0:
## No captain, no simulation
print "There is no captain who can sail a boat :("
return None
sailing_captain = get_sailing_captain(from_)
## Shuffle the residents list so that you always get a random traveller
random.shuffle(from_.residents)
traveller = select_traveller(from_)
if traveller != None:
passengers = [sailing_captain, traveller]
to.welcome(passengers)
return passengers
else:
return None
## e.g. travel_back(japan, korea):
##
def travel_back(from_, to):
sailing_captain = get_sailing_captain(from_)
## Shuffle the residents list so that you always get a random traveller
if is_peaceful(from_.residents):
to.welcome([sailing_captain])
return [sailing_captain]
else:
traveller = select_traveller(from_)
passengers = [sailing_captain, traveller]
to.welcome(passengers)
return passengers
def get_passenger_name(passengers):
return tuple(p.species for p in passengers)
def print_land_info(lands):
for l in lands:
print l
peasant = Passenger('human', is_captain=True)
peasant2 = Passenger('human', is_captain=True)
''' IF I UNCOMMENT THE NEXT LINE OUT, THE INFINITE LOOP HAPPENS!!! '''
fox2 = Passenger('fox', 'rooster')
fox = Passenger('fox', 'rooster')
rooster = Passenger('rooster', 'grain')
#rooster2 = Passenger('rooster', 'grain')
grain = Passenger('grain')
#grain2 = Passenger('grain')
korea = Space('Korea', [peasant, fox, rooster, grain])
japan = Space('Japan')
POPULATION = korea.num_residents()
CAPTAIN = get_captains(korea.residents)
i = 1
while True:
print "Loop", i
passengers = travel_to_destination(korea, japan)
if passengers == None:
print "The journey can't be continued"
break
if japan.num_residents() == POPULATION:
print "Everyone has crossed the river safely!"
print_land_info([japan, korea])
break
else:
print "Korea ---> Japan", get_passenger_name(passengers)
print_land_info([japan, korea])
passengers = travel_back(japan, korea)
print "Japan ---> Korea", get_passenger_name(passengers)
print_land_info([japan, korea])
print "========================"
i += 1
编辑: 我根据@hammergene的建议。我把窃听器修好了
travel_back(...)
并补充道
__eq__ and __hash__
致乘客()。不过,我不确定这个问题是否完全解决了。在
无限循环的原因是算法中的一个错误:
travel_back
不是随机洗牌,而是选择第一个不安全的乘客。如果它恰好是刚刚到达的那个,它就变成了no op,它会被无限期地重复。如果在此处添加随机随机洗牌,程序将始终终止:“神秘”依赖于创建额外对象的原因是集合和字典依赖于}操作,它们的默认实现(在自定义类中)只是使用对象的内存地址。在
__hash__
和{在您的例子中,分配一个额外的对象会改变后续分配的内存地址,这反过来又会改变对象在
send_off
操作后的排序方式,并影响到travel_back
将选择哪一个。如果不进行洗牌,它将始终是同一个对象:要么是好的选择(没有循环),要么是坏的,这取决于它们的内存地址。在添加散列/相等运算符将消除是否有一个额外对象的神秘依赖性,并使程序的行为更具确定性:它要么总是陷入无限循环(如果尚未修复
^{pr2}$travel_back
),要么永远不会:相关问题 更多 >
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