Difficulty teaming up neighbours of each cone

Question

I have a nested list of junctions between cones.

a = [0,1]
b = [2,4]
c = [2,0]
d = [4,3]

e=[a,b,c,d]

I want to write a program that lists eve

Answer 1

Using descriptive names will help make your code more readable.

a = [0,1]
b = [2,4]
c = [2,0]
f = [4,6]
junctions=[a,b,c,d,f]
neighbour_list =[]

If we didn't know in advance what the cones were, we would need to find all the cones in the junctions

# find all the cones in the junctions
cones = list()
for junction in junctions:
    for cone in junction:
        cones.append(cone)
# The previous could be replaced with a list comprehension
# cones = [cone for junction in junctions for cone in junction]

If you want to only consider cones that have junctions use

##cones = set(cones)

If you want to consider all cones within the range represented in the junctions use

cones = range(min(cones), max(cones) + 1)

It is a bit easier to build the lists if the cone loop is the outer loop because we are trying to find the neighbours for each cone:

# for each cone look through the junctions to find neighbors
for cone in sorted(cones):    #search for neighbors from the smallest to the largest cone
    neighbors = list()    #make a new list for each cone
    for junction in junctions:
        if junction[0] == cone:
            neighbors.append(junction[1])
        elif junction[1] == cone:
            neighbors.append(junction[0])
    neighbour_list.append(neighbors)

>>> print neighbour_list
[[1, 2], [0], [4, 0], [4], [2, 3, 6], [], [4]]
>>> 

It seems like neighbour_list is missing some information - you can't readily tell which cone the neighbors are for. You could use zip to add the information:

>>> print zip(sorted(cones), neighbour_list)
[(0, [1, 2]), (1, [0]), (2, [4, 0]), (3, [4]), (4, [2, 3, 6]), (5, []), (6, [4])]
>>> 

Or a slight change in the program could add the info neighbour_list

neighbour_list.append((cone,neighbors))

Answer 2

I recommend using a dictionary instead.

a = [0, 1]
b = [2, 4]
c = [2, 0]
d = [4, 3]

e = [a, b, c, d]
neighbour_list = {}

for x, y in e:
    neighbour_list.setdefault(x, [])
    neighbour_list[x].append(y)
    neighbour_list.setdefault(y, [])
    neighbour_list[y].append(x)
neighbour_list = list(neighbour_list.values())
print(neighbour_list)

Running it gives:

[[1, 2], [0], [4, 0], [4], [2, 3]]