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pythondefaultdict

Keep identical values in default dict

I have the following two lists:

prefix = ['AGG', 'CAG', 'CAG', 'GAG', 'GGA', 'GGG', 'GGG']
suffix = ['GGG', 'AGG', 'AGG', 'AGG', 'GAG', 'GGA', 'GGG']

I am trying to use defaultdict to get this result:

AGG -> GGG
CAG -> AGG,AGG
GAG -> AGG
GGA -> GAG
GGG -> GGA,GGG

(Yes this is a Rosalind problem for those of you who know. I REALLY want to learn how to use dictionaries effectively though, so I am NOT looking for homework answers. Just dictionary help.)

This is my code: 

from collections import defaultdict
nodes = defaultdict(set)
for pre, suf in zip(prefix, suffix):
    nodes[pre].add(suf)
for k,v in nodes.items():
    print(k,v)

This is my result: 

CAG {'AGG'}
GAG {'AGG'}
GGG {'GGG', 'GGA'}
AGG {'GGG'}
GGA {'GAG'}

So I have two issues:

1) I need AGG, the value for CAG, to be preserved as two identical instances.

2) I cannot figure out how to print from my dictionary nicely, without brackets and quotes and to add in the arrow.

Thank you in advance!

Solution

  • Use a defaultdict of list, instead of set. Sets removes duplicates.

    Your code is already fine, you'll just have to change the 

    nodes[pre].add(suf)

    to

    nodes[pre].append(suf)

    For the printing, it will be like

    print('{} -> {}'.format(k, ','.join(v)))