File to dictionary only prints one

Question

I have a text file that reads:

a;b  
a;c  
a;d  
b;h  
c;e  
e;f  
e;g  
e;j  
f;b  
g;d  
h;b  
h;e  
i;d  
i;e

but when I print it after making it into a dictionary

def read_graph(file_name):                                                                      
  graph = {}                                                                                      
  for line in open(file_name):
    if ";" in line:
        key, val = map(str.strip, line.split(";"))
        graph[key] = val
  return dict(sorted(graph.items())))

It prints:

{'a': 'b', 'b': 'd', 'c': 'e', 'd': 'g', 'e': 'd', 'f': 'd'}

how do I make it where it prints the keys that repeat?

See also [this question](https://stackoverflow.com/q/20585920/1044698) — Energya, Oct 05 '19 at 08:12

David542 · Accepted Answer · 2019-10-04T23:41:00.770

I assume for this you'd want to use a list of strings instead of a single string as the value, otherwise your dictionary will keep replacing the value for the same key.

Instead of:

{'a': 'b'}

You would probably want a structure such as:

{'a': ['b','c','d']}

Using your function:

def read_graph(file_name):                                                                      
  graph = {}                                                                                      
  for line in open(file_name):
    if ";" not in line: continue
    key, val = line.strip().split(';')
    if key not in graph: graph[key] = list()
    if val not in graph[key]: graph[key].append(val)
  return dict(sorted(graph.items()))


read_graph('file.txt')
{'a': ['b', 'c', 'd'], 'c': ['e'], 'b': ['h'], 'e': ['f', 'g', 'j'], 'g': ['d'], 'f': ['b'], 'i': ['d', 'e'], 'h': ['b', 'e']}

score 0 · Answer 2 · answered Oct 04 '19 at 23:35

Dictionaries in python (and every other language I know) have unique values for each key, and will overwrite them when you put a new value in for an existing key.

Consider a different kind of data structure, like a set of tuples, e.g.

{('a','b'), ('a','c'), ...}

Or, as it looks like you are making a graph, a dictionary where the values are lists of vertices instead of individual vertices, e.g.

{'a':['b','c'],...}

To make the set of tuples, replace the line

        graph[key] = val

with

graph.append((key, val))

To make a dictionary-to-lists, use

if key in graph:
    graph[key].append(val)
else:
    graph[key] = [val]

Hope this helps!

score 0 · Answer 3 · answered Oct 04 '19 at 23:36

You cannot because that is a dictionary, and it is not allowed to have two same keys or it would ambiguous. You could group by key.

def read_graph(file_name):                                                                      
  graph = {}                                                                                      
  for line in open(file_name):
    if ";" in line:
        key, val = map(str.strip, line.split(";"))
        if key not in graph:
            graph[key] = [val]
        else:
            graph[key].append(val)
  return dict(sorted(graph.items())))

So now you have for every key, an array with its values.

score 0 · Answer 4 · answered Oct 04 '19 at 23:37

Since you seem to be working with a graph structure, I would recommend you look at the NetworkX package for Python. They have pre-built graph data-structures for you to use and many algorithms that can operate on them.

import networkx as nx

graph = nx.Graph()
with open(file_name) as f:  # This closes the file automatically when you're done
    for line in f:
        if ";" in line:
            source, dest = map(str.strip, line.split(";"))
            graph.add_edge(source, dest)

In case you still want to use vanilla Python only:

Python's dictionaries can only have one value per key. To store multiple values for a single key, you have to store your keys in a list of values.

my_dict = {
    'a': ['b', 'c', 'd'],
    'b': ['h'],
    ...
}

File to dictionary only prints one

4 Answers4