Question

我正在阅读有关解决TSP问题的Google OR软件包或工具。但是，我对代码的一部分有些困惑。

from ortools.constraint_solver import pywrapcp
from ortools.constraint_solver import routing_enums_pb2

# Distance callback
def create_distance_callback(dist_matrix):
  # Create a callback to calculate distances between cities.

  def distance_callback(from_node, to_node):
    return int(dist_matrix[from_node][to_node])

  return distance_callback

def main():
  # Cities
  city_names = ["New York", "Los Angeles", "Chicago", "Minneapolis", 
"Denver", "Dallas", "Seattle",
                "Boston", "San Francisco", "St. Louis", "Houston", 
"Phoenix", "Salt Lake City"]
  # Distance matrix
  dist_matrix = [
    [   0, 2451,  713, 1018, 1631, 1374, 2408,  213, 2571,  875, 1420, 
2145, 1972], # New York
    [2451,    0, 1745, 1524,  831, 1240,  959, 2596,  403, 1589, 1374,  
357,  579], # Los Angeles
    [ 713, 1745,    0,  355,  920,  803, 1737,  851, 1858,  262,  940, 
1453, 1260], # Chicago
    [1018, 1524,  355,    0,  700,  862, 1395, 1123, 1584,  466, 1056, 
1280,  987], # Minneapolis
    [1631,  831,  920,  700,    0,  663, 1021, 1769,  949,  796,  879,  
586,  371], # Denver
    [1374, 1240,  803,  862,  663,    0, 1681, 1551, 1765,  547,  225,  
887,  999], # Dallas
    [2408,  959, 1737, 1395, 1021, 1681,    0, 2493,  678, 1724, 1891, 
1114,  701], # Seattle
    [ 213, 2596,  851, 1123, 1769, 1551, 2493,    0, 2699, 1038, 1605, 
2300, 2099], # Boston
    [2571,  403, 1858, 1584,  949, 1765,  678, 2699,    0, 1744, 1645,  
653,  600], # San Francisco
    [ 875, 1589,  262,  466,  796,  547, 1724, 1038, 1744,    0,  679, 
1272, 1162], # St. Louis
    [1420, 1374,  940, 1056,  879,  225, 1891, 1605, 1645,  679,    0, 
1017, 1200], # Houston
    [2145,  357, 1453, 1280,  586,  887, 1114, 2300,  653, 1272, 1017,    
0,  504], # Phoenix
    [1972,  579, 1260,  987,  371,  999,  701, 2099,  600, 1162,  
1200,  504,   0]] # Salt Lake City

  tsp_size = len(city_names)
  num_routes = 1
  depot = 0

  # Create routing model
  if tsp_size > 0:
    routing = pywrapcp.RoutingModel(tsp_size, num_routes, depot)
    search_parameters = 
              pywrapcp.RoutingModel.DefaultSearchParameters()
    # Create the distance callback.
    dist_callback = create_distance_callback(dist_matrix)
    routing.SetArcCostEvaluatorOfAllVehicles(dist_callback)
    # Solve the problem.
    assignment = routing.SolveWithParameters(search_parameters)
    if assignment:
      # Solution distance.
      print "Total distance: " + str(assignment.ObjectiveValue()) + " 
               miles\n"
      # Display the solution.
      # Only one route here; otherwise iterate from 0 to 
      # routing.vehicles() - 1
      route_number = 0
      index = routing.Start(route_number) # Index of the variable for 
                                          # the starting node.
      route = ''
      while not routing.IsEnd(index):
        # Convert variable indices to node indices in the displayed 
        #  route.
        route += str(city_names[routing.IndexToNode(index)]) + ' -> '
        index = assignment.Value(routing.NextVar(index))
      route += str(city_names[routing.IndexToNode(index)])
      print "Route:\n\n" + route
    else:
      print 'No solution found.'
  else:
    print 'Specify an instance greater than 0.'

if __name__ == '__main__':
  main()

此代码来自https://developers.google.com/optimization/routing/tsp#euclid_distance。

我的问题是关于距离回调部分，其中定义了create_distance_callback。我的第一个问题是from_node和to_node参数来自哪里？对于它们是什么，它们来自何处以及它们服务于什么目的，我确实感到困惑，因为这段代码将距离矩阵转换成我不确定的东西。如果有人能为我解释那部分代码，以及最终对距离矩阵发生什么影响，将不胜感激，因为在执行该代码后，矩阵似乎变成了不是矩阵的东西。

谢谢。

Python：Google ortools软件包示例代码

0 个答案: