Creating Weighted Directed Graph in Python based on User Input

Question

I need to create something like this to represent a directed weighted graph based on user input -

graph = {
          'a': {'b': 1, 'c':  4},
          'b': {'c':  3, 'd':  2, 'e':  2},
          'c': {},
          'd': {'b':  1, 'c':  5},
          'e': {'d': -2}
        }

So far,

import pprint

graph = {}
values = {}
v = int(input("Enter number of vertices: "))

print("Enter vertices(keys) : ")
for i in range(v):
    graph.setdefault(input())

edges = {}
for x in graph:
    edges.setdefault(x)

for i in graph:
    graph[i] = edges

print("Enter weights: ")
for i in graph:
    print(i)
    for j in graph[i]:
        var = input()
        graph[i][j] = var

pprint.pprint(graph)

I tried but for some reason, it is replacing the previously read weights with last read weights. Any solutions?

You've neither shown your input format nor an attempt, but see if this helps: http://stackoverflow.com/questions/38165292/how-to-convert-an-adjacency-matrix-to-an-adjacency-list-with-python/38165410#38165410 — Moses Koledoye, Oct 14 '16 at 10:05
Instead of re-inventing the wheel - you may want to look at https://networkx.github.io/ — Jon Clements, Oct 14 '16 at 11:34
Thanks for the library suggestion but I must do it using a dictionary in python. @JonClements — Abhishek, Oct 14 '16 at 12:24

score 1 · Accepted Answer · answered Oct 14 '16 at 12:23

1

for i in graph:
    graph[i] = edges

You're assigning the same dict (edges) to each key of graph. Therefore, when you assign a value to any of them, you're assigning that value to all of them. It looks like what you actually want is copies of edges. In this case, since you haven't assigned any mutable values to edges, a shallow copy is sufficient:

for i in graph:
    graph[i] = edges.copy()

answered Oct 14 '16 at 12:23

glibdud

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It worked! Thanks. However while taking input i.e. taking weights for edges, the order of the keys is random. It will sometimes ask for any vertex & not the one which was inserted first & then second & so on. Any suggestions? – Abhishek Oct 14 '16 at 12:50
1

@Abhishek dicts have no concept of order. If you need to display items in a particular order, you'll need to sort them yourself. Or you can explore [collections.OrderedDict](https://docs.python.org/3.5/library/collections.html#collections.OrderedDict). – glibdud Oct 14 '16 at 12:56

score 0 · Answer 2 · answered Oct 14 '16 at 11:42

Do you have an indentation error?

Instead of

for i in graph:
    print(i)
for j in graph[i]:
    var = input()
    graph[i][j] = var

You maybe intended to write

for i in graph:
    print(i)
    for j in graph[i]:
        var = input()
        graph[i][j] = var

?

Creating Weighted Directed Graph in Python based on User Input

2 Answers2