我是否缺少对给定 state 中的操作的检查？

Question

The problem: Three traditional, but jealous, couples need to cross a river. Each couple consists of a husband and a wife. They find a small boat that can contain no more than two persons. Find the simplest schedule of crossings that will permit all six people to cross the river so that none of the women shall be left in company with any of the men, unless her husband is present. It is assumed that all passengers on the boat onboard before the next trip and at least one person has to be in the boat for each crossing.

Had to edit out code, was requested by professor.

I've been working on this problem for 6 hours and I am stumped. My professor is busy and cannot help.

Answer 1

I toke a careful look on your code. It is indeed a very interesting problem and quite complex. After some while I realized that what maybe causing your problem is that your are checking the conditions before the crossing is made and not afterwords. A saw the template you provided and I guess we can try to stick with to logic proposed by 1- make the action method return all possible crosses (without checking the states yet) 2- given each action, get the corresponding new state and check if that state is valid. 3- make the value() method to check if we are making progress on the optimization.

class Problem:
    def __init__(self, initial_state, goal):
        self.goal = goal
        self.record = [[0, initial_state, "LEFT", []]]
            # list of results [score][state][boat_side][listActions]

    def actions(self, state, boat_side):
        side = 0 if boat_side == 'LEFT' else 1
        boat_dir = 'RIGHT' if boat_side == 'LEFT' else 'LEFT'

        group = [i for i, v in enumerate(state) if v == side]
        onboard_2 = [[boat_dir, a, b] for a in group for b in group if 
            a < b and (     # not the same person and unique group 
            (a%2==0 and b - a == 1) or (        # wife and husband
            a%2==0 and b%2==0) or (             # two wife's
            a%2==1 and b%2==1)                  # two husbands
        )]
        onboard_1 = [[boat_dir, a] for a in group]
        return onboard_1 + onboard_2

    def result(self, state, action):
        new_boat_side = action[0]
        new_state = []
        for i, v in enumerate(state):
            if i in action[1:]:
                new_state.append(1 if v == 0 else 0)
            else:
                new_state.append(v)

        # check if invalid
        for p, side, in enumerate(new_state):
            if p%2 == 0: # is woman
                if side != new_state[p+1]: # not with husband
                    if any(men == side for men in new_state[1::2]):
                        new_state = False
                        break

        return new_state, new_boat_side

    def goal_test(self, state):
        return state == self.goal

    def value(self, state):
        # how many people already crossed
        return state.count(1)


# optimization process
initial_state = [0]*6
goal = [1]*6
task = Problem(initial_state, goal)

while True:
    batch_result = []
    for score, state, side, l_a in task.record:
        possible_actions = task.actions(state, side)
        for a in possible_actions:
            new_state, new_boat_side = task.result(state, a)
            if new_state: # is a valid state
                batch_result.append([
                    task.value(new_state),
                    new_state,
                    new_boat_side,
                    l_a + a,
                ])

    batch_result.sort(key= lambda x: x[0], reverse= True)
        # sort the results with the most people crossed
    task.record = batch_result[:5]
        # I am only sticking with the best 5 results but
        #   any number should be fine on this problem
    if task.goal_test(task.record[0][1]):
        break

#   for i in task.record[:5]: # uncomment these lines to see full progress
#       print(i)
#   x = input() # press any key to continue

print(task.record[0][3])

I hope it helped, please fill free to say if anything is still not so clear.