在Google OR tools的VRP中,可以通过添加约束来强制某些节点不能被最先和最后访问。具体步骤如下:
from ortools.constraint_solver import routing_enums_pb2
from ortools.constraint_solver import pywrapcp
def create_data_model():
# 创建数据模型,包括节点数量、距离矩阵等信息
data = {}
data['num_nodes'] = 5
data['distance_matrix'] = [
[0, 1, 2, 3, 4],
[1, 0, 2, 3, 4],
[2, 2, 0, 3, 4],
[3, 3, 3, 0, 4],
[4, 4, 4, 4, 0]
]
return data
def main():
# 创建数据模型
data = create_data_model()
# 创建路线规划求解器
manager = pywrapcp.RoutingIndexManager(data['num_nodes'], 1, 0)
routing = pywrapcp.RoutingModel(manager)
# 创建距离矩阵回调函数
def distance_callback(from_index, to_index):
return data['distance_matrix'][manager.IndexToNode(from_index)][manager.IndexToNode(to_index)]
transit_callback_index = routing.RegisterTransitCallback(distance_callback)
# 设置节点访问约束
forbidden_nodes = [1, 3] # 假设节点1和节点3不能被最先和最后访问
for node in forbidden_nodes:
routing.AddDisjunction([manager.NodeToIndex(node)], 0)
# 设置目标函数
routing.SetArcCostEvaluatorOfAllVehicles(transit_callback_index)
# 设置搜索参数
search_parameters = pywrapcp.DefaultRoutingSearchParameters()
search_parameters.first_solution_strategy = (
routing_enums_pb2.FirstSolutionStrategy.PATH_CHEAPEST_ARC)
# 求解路线规划问题
solution = routing.SolveWithParameters(search_parameters)
# 输出结果
if solution:
print_solution(manager, routing, solution)
def print_solution(manager, routing, solution):
# 输出解决方案
print('Objective: {} miles'.format(solution.ObjectiveValue()))
index = routing.Start(0)
plan_output = 'Route for vehicle 0:\n'
route_distance = 0
while not routing.IsEnd(index):
plan_output += ' {} ->'.format(manager.IndexToNode(index))
previous_index = index
index = solution.Value(routing.NextVar(index))
route_distance += routing.GetArcCostForVehicle(previous_index, index, 0)
plan_output += ' {}\n'.format(manager.IndexToNode(index))
route_distance += routing.GetArcCostForVehicle(previous_index, index, 0)
plan_output += 'Distance of the route: {} miles\n'.format(route_distance)
print(plan_output)
if __name__ == '__main__':
main()
以上代码中,禁止节点1和节点3被最先和最后访问的约束条件通过routing.AddDisjunction([manager.NodeToIndex(node)], 0)
来设置。你可以根据实际需求修改禁止访问的节点列表。请注意,这只是一个示例代码,实际使用时需要根据具体情况进行调整。
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