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pythonlistdictionary
Kacper Reicher
Kacper Reicher

Reputation: 25

How to combine list of lists into a dictionary where the first element of a nested list is the key

I'm trying to combine a list, similar to the following

l = [['A', '1'], ['A', '2'], ['B', '1'], ['C', '1'], ['C', '2']]

into a dictionary, where the first element of a nested list is the key

d = {'A': ['1', '2'], 'B': ['1'], 'C': ['1', '2']}

Is it possible to do this fairly easily?

Upvotes: 1

Views: 1102

Answers (5)

IoaTzimas
IoaTzimas

Reputation: 10624

You can use itertools.groupby:

l = [['A', '1'], ['A', '2'], ['B', '1'], ['C', '1'], ['C', '2']]
d={}
for i,k in itertools.groupby(sorted(l, key=lambda x: x[0]), key=lambda x: x[0]):
    d[i]=list(itertools.chain(*(p[1:] for p in k)))

>>> print(d)
{'A': ['1', '2'], 'B': ['1'], 'C': ['1', '2']}

Upvotes: 1

user2390182
user2390182

Reputation: 73470

Simplest is the following, using dict.setdefault:

d = {}
for k, v in l:
    d.setdefault(k, []).append(v)

With a collections.defaultdict, the code gets even cleaner:

from collections import defaultdict

d = defaultdict(list)
for k, v in l:
    d[k].append(v)

Upvotes: 6

sam
sam

Reputation: 2311

Iterate over l and append in the keys of the dictionbary like this:

d = {}
for item in l:
    

if item[0]  in d.keys():
    d[item[0]].append(item[1])
if item[0] not in d.keys():
    d[item[0]] = [item[1]]

Upvotes: 0

balderman
balderman

Reputation: 23815

Using defaultdict can help here

from collections import defaultdict

l = [['A', '1'], ['A', '2'], ['B', '1'], ['C', '1'], ['C', '2']]
d = defaultdict(list)
for x in l:
  d[x[0]].append(x[1])
print(d)

output

defaultdict(<class 'list'>, {'A': ['1', '2'], 'B': ['1'], 'C': ['1', '2']})

Upvotes: 0

Sushil
Sushil

Reputation: 5531

Here is a traditional way of doing it. Just iterate over the list and append the keys and values to the dictionary. Here is the full code:

l = [['A', '1'], ['A', '2'], ['B', '1'], ['C', '1'], ['C', '2']]
dictionary = {}
for key,value in l:
    if key in dictionary.keys():
        dictionary[key].append(value)
    else:
        dictionary[key] = [value]
print(dictionary)

Output:

{'A': ['1', '2'], 'B': ['1'], 'C': ['1', '2']}

Upvotes: 0

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