Creating a graph using input from text file in python

Question

I have the following txt file:

31  1262    -1
531 13930   1
531 16139   -1
531 17567   1
531 20653   1

The first column show the starting nodes and the second column shows the ending nodes. I want to create a graph like

graph = {'0': set(['1', '2']),
         '1': set(['0', '3', '4']),
         '2': set(['0']),
         '3': set(['1']),
         '4': set(['2', '3'])}

using the input from text file. I saved the first column in start_vertex = [] and the second column in end_vertex = []. I wrote the code below but i cant save the graph.

for i in range(len(lines)):
    v = start_vertex[i]
    if v == start_vertex[i]:
        graph = graph + {'v' : set(['end_vertex[i]'])}

And the full code is here:

file = open("network.txt","r")
lines = file.readlines()
start_vertex = []
end_vertex = []
sign = []
graph = []

for x in lines:
    start_vertex.append(x.split('\t')[0])
    end_vertex.append(x.split('\t')[1])
    sign.append(x.split('\t')[2])
file.close()


def dfs(graph, start, visited = None):
    if visited is None:
        visited = set()
        visited.add(start)
        print(start)
        for next in graph[start] - visited:
            dfs(graph, next, visited)
        return visited

for i in range(len(lines)):
    v = start_vertex[i]
    if v == start_vertex[i]:
        graph = graph + {'v' : set(['end_vertex[i]'])}

If you are working with graphs, try the awesome NetworkX package. — Sayan Sil
– Sayan Sil, Commented Nov 11, 2017 at 19:49

Wasi Ahmad · Accepted Answer · 2017-11-11 20:23:33Z

1

From your question, it is not clear what is your exact input and what is the expected output. You didn't mention what the third column means in the input text file. Besides, your program has severe problems.

I am guessing you are trying to store the input graph as follows.

lines = [(31, 1262, -1), (531, 13930, 1), (531, 16139, -1), (531, 17567, 1), (531, 20653, 1)]
start_vertex, end_vertex = [], []
for line in lines:
    start_vertex.append(line[0])
    end_vertex.append(line[1])

graph = {}
for i in range(len(lines)):
    v = start_vertex[i]
    if v in graph:
        graph[v] |= set([end_vertex[i]])
    else:
        graph[v] = set([end_vertex[i]])

print(graph)

Output:

{531: {13930, 16139, 20653, 17567}, 31: {1262}}

which is in the form: {start_v1: {end_v4, end_v5, ...}, start_v2: {end_v3}}.

answered Nov 11, 2017 at 20:23

Wasi Ahmad

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