Iterating over dictionary and performing function

Question

Currently attempting the Travelling Salesman Problem with a simulated annealing solution. All points are stored in a dictionary with the point name as the key and co-ordinates as the value. Having trouble writing a for loop(path_tour function) that goes through every key in a a given path(randomly shuffled dictionary of locations), calculates distances and adds the value to a list to retrun a total length. The current function I have below returns a KeyError, I cant figure out why.

#Calculate distances between points
    def point_distance(point_a, point_b):
        return math.sqrt((point_a[0] - point_b[0])**2 + (point_a[1] - point_b[1])**2)
    
    def get_distance_matrix(points):
        distance_matrix = {}
        for location_a in points:
            distance_matrix[location_a] = {}
            for location_b in points:
                distance_matrix[location_a][location_b] = point_distance(
                        points[location_a], points[location_b])
        return distance_matrix
    
    
    #Calculate length of path
    
    def path_tour(tour):
        path_length = 0
        distances = get_distance_matrix(tour)
        for key, value in tour.items():
            path_length += distances[key][key[1:]]
        return path_length

how the get_distance_matrix is called

example of a path

error message

score 0 · Accepted Answer · answered Jan 25 '22 at 12:57

As you can see from the error, it was trying to look up the key "tudent Information Desk". I assume the location name was "Student Information Desk", so key[1:] removed the first letter. That's obviously not the correct location to look up.

I guess you want the distance from the current to the next location in the tour. That would be something like

locations = list(tour.keys())
for first, second in zip(locations[:-1], locations[1:]):
    path_length += distances[first][second]

I don't get why your tour is a dictionary, though. Shouldn't it be a list to begin with? I know that dictionaries in Python-3 keep their insertion order but this usage seems counter-intuitive.

thank you. I initially started with location points being in a dictionary so I tried to keep that dictionary but you're right, turned out to be counter-intuitive — X13Theo, Feb 04 '22 at 18:17

Iterating over dictionary and performing function

1 Answers1