converting a dictionary to another one

Question

I have a dictionary, the keys are integer values and the values are a list of strings. I want to make it vice versa, the string become key and integers become values. Like:

first design:

{1:["a"], 2:["a","b"], 3:["a"], 4:["b", "cd"], 6:["a","cd"]}

second design:

{"a": [1,2,3,6], "b":[2,4], "cd":[4,6]}

any clue? Thanks

I think this is what you are looking for [Invert Dictionary-python](https://stackoverflow.com/questions/29633848/python-reverse-inverse-a-mapping-but-with-multiple-values-for-each-key) — Gambit1614, Jul 31 '17 at 04:04
@MohammedKashif that one's different in that the values in the lists are unique. — perigon, Jul 31 '17 at 04:05

mattjegan · Answer 1 · 2017-07-31T04:18:56.323

A simple approach would be to create a defaultdict that provides an empty list for new keys, and then for each list in your current dict, create a key for each element and append the original dict key to the new list:

from collections import defaultdict

orig = {1: ['a'], 2: ['a', 'b'], 3: ['a'], 4: ['b', 'cd'], 6: ['a', 'cd']}
d = defaultdict(list)

for k, v in orig.items():
    for string in v:
        d[string].append(k)

# d = {'a': [1, 2, 3, 6], 'b': [2, 4], 'cd': [4, 6]}

score 2 · Answer 2 · answered Jul 31 '17 at 04:04

2

For the love of one-liners (note: heavily inefficient!):

d2 = {val: [key for key in d if val in d[key]] for lst in d.itervalues() for val in lst}

answered Jul 31 '17 at 04:04

perigon

2,160
11
16

score 2 · Answer 3 · answered Jul 31 '17 at 04:11

Code

first_map = {1: ["a"], 2: ["a", "b"], 3: ["a"], 4: ["b", "cd"], 6: ["a", "cd"]}
second_map = {}
for key, value in first_map.items():
    for i in value:
        if i in second_map:
            second_map[i].append(key)
        else:
            second_map[i] = [key]
print(second_map)

Cory Madden · Answer 4 · 2017-07-31T05:27:07.893

1

z = {1:["a"], 2:["a","b"], 3:["a"], 4:["b", "cd"], 6:["a","cd"]}
d = dict()

for k, v in z.iteritems():
    for i in v: 
        if i in d: 
            d[i].append(k)
        else:   
            d[i] = [k]

Using iteritems() over items() is a little better because the former creates a generator in Python 2.

edited Jul 31 '17 at 05:27

answered Jul 31 '17 at 04:07

Cory Madden

5,026
24
37

1

Maybe better ```iteritems```, since OP indicates Python 2.7? – perigon Jul 31 '17 at 04:10

score 1 · Answer 5 · answered Jul 31 '17 at 04:09

dict1= {1:["a"], 2:["a","b"], 3:["a"], 4:["b", "cd"], 6:["a","cd"]}
dict2= {}
for key, value in dict1.items():
   for val in value:
       if val in dict2:
           dict2[val].append(key)
           dict2[val]= list(set(dict2[val]))
       else:
           dict2[val]= [key]

score 1 · Answer 6 · answered Jul 31 '17 at 04:21

1

from collections import defaultdict

second_dict = defaultdict(list)

for key, value in first_dict.items():

    for item in value:

        second_dict[item].append(key)

answered Jul 31 '17 at 04:21

stamaimer

6,227
5
34
55

converting a dictionary to another one

6 Answers6