Convert two lists to dictionary with values as list

Question

I can convert two lists to dictionary

>>> keys = ['a', 'b', 'c']
>>> values = [1, 2, 3]
>>> dictionary = dict(zip(keys, values))
>>> print dictionary

How to convert it to dictionary with keys but values as list.

keys = ['a', 'b', 'c' ,'a']
values=[1, 2, 3, 4]

Output:

{'a': [1,4], 'c': [3], 'b': [2]}

I am using this in dependency parser to get corresponding adjectives for nouns in text. Note I have to do this for huge text so efficency matters.

Please state the computational time of approach as well.

You can't use `zip` here as you have multiple instances of the same key — EdChum, Sep 05 '15 at 22:43
No keys and values are the same length. Until something weird happens will check once. — Abhishek Bhatia, Sep 05 '15 at 23:00
http://stackoverflow.com/questions/10664856/make-dictionary-with-duplicate-keys-in-python http://stackoverflow.com/questions/11141383/is-there-a-way-to-preserve-duplicate-keys-in-python-dictionary — Padraic Cunningham, Sep 05 '15 at 23:01
So don't do this: `dict(zip(set(keys), [[values[i] for i, x in enumerate(keys) if x == s] for s in set(keys)]))` I mean do that, because it's hilarious and it works, but definitely don't do that. — Andrew LaPrise, Sep 05 '15 at 23:11

score 5 · Accepted Answer · answered Sep 05 '15 at 22:46

5

I'd simply loop over the key/value pairs and use setdefault to add them to the dictionary:

>>> keys = ['a', 'b', 'c' ,'a']
>>> values=[1, 2, 3, 4]
>>> d = {}
>>> for k,v in zip(keys, values):
...     d.setdefault(k, []).append(v)
...     
>>> d
{'c': [3], 'b': [2], 'a': [1, 4]}

answered Sep 05 '15 at 22:46

DSM

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I might add a link to the function https://docs.python.org/2/library/stdtypes.html#dict.setdefault - so that future browsers can have a look what it does... – J Richard Snape Sep 08 '15 at 08:49

score 2 · Answer 2 · answered Sep 05 '15 at 22:46

2

Presuming both lists have the same length:

>>> import collections
>>> keys = ['a', 'b', 'c' ,'a']
>>> values = [1, 2, 3, 4]
>>> r = collections.defaultdict(list)

>>> for i, key in enumerate(keys):
...     r[key].append(values[i])

answered Sep 05 '15 at 22:46

Ozgur Vatansever

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Mathias Rav · Answer 3 · 2015-09-05T23:04:13.070

1

The function itertools.groupby takes a list and groups neighboring same elements together. We need to sort the zipped list to ensure that equal keys end up next to each other.

import itertools
keys = ['a', 'b', 'c', 'a']
values = [1, 2, 3, 4]
data = sorted(zip(keys, values))
dictionary = {}
for k, g in itertools.groupby(data, key=lambda x: x[0]):
    dictionary[k] = [x[1] for x in g]
print(dictionary)
# {'c': [3], 'b': [2], 'a': [1, 4]}

edited Sep 05 '15 at 23:04

answered Sep 05 '15 at 22:47

Mathias Rav

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why is `sorted` needed? – Ozgur Vatansever Sep 05 '15 at 22:50
because that's the contract of groupby, it groups contiguous items with the same key. – dsvensson Dec 01 '16 at 13:29

score 0 · Answer 4 · answered Sep 05 '15 at 22:46

0

There's probably a better way to do this. But here's one solution:

keys = ['a', 'b', 'c' ,'a']
values=[1, 2, 3, 4]

combined = {}

for k, v in zip(keys, values):
    l = combined.get(k, [])
    l.append(v)
    combined[k] = l

answered Sep 05 '15 at 22:46

Eugene Yurtsev

134
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Convert two lists to dictionary with values as list

4 Answers4