Sorting a list of tuples made up of two numbers by consecutive numbers

Question

The list of tuples is the output from a capacitated vehicle-routing optimization and represents the arcs from one stop to another, where 0 represents the depot of the vehicle (start and end point of vehicle). As the vehicle must drive several laps it may return to the depot before all stops were made. The solver will always return the starting-arcs first, which in the example below means that the first consecutive tuples starting with 0 , namely (0, 3), (0, 7), (0, 8) will determine how many laps (= 3) there are.

How can I sort the arcs in consecutive order so that one vehicle could drive the arcs one after another?

Input:

li = [(0, 3), (0, 7), (0, 8), (3, 0), (4, 0), (7, 3), (8, 4), (3, 0)]

Output:

[(0, 3), (3, 0), (0, 7), (7, 3), (3, 0), (0, 8), (8, 4), (4, 0)]

What I tried so far:

    laps = 0
    for arc in li:
        if arc[0] == 0:
            laps = laps + 1
    new_list = []
    for i in range(laps):
        value = li.pop(0)
        new_list.append([value])
    for i in range(laps):
        while new_list[i][-1][1] != 0:
            arc_end = new_list[i][-1][1]
            for j in range(len(li)):
                if li[j][0] == arc_end:
                    value = li.pop(j)
                    new_list[i].append(value)
                    break
    flat_list = [item for sublist in new_list for item in sublist]
    return flat_list

Answer 1

You are trying to solve the problem of finding cycles in a directed graph. The problem itself is not a difficult one to solve, and Python has a very good package for solving such problems - networkx . It would be a good idea to learn a bit about fundamental graph algorithms. There is also another stack overflow question about this algorithm, which you can consult, at Finding all cycles in a directed graph