python - Finding the most frequent occurrences of pairs in a list of lists

Question

I've a dataset that denotes the list of authors of many technical reports. Each report can be authored by one or multiple people:

a = [
['John', 'Mark', 'Jennifer'],
['John'],
['Joe', 'Mark'],
['John', 'Anna', 'Jennifer'],
['Jennifer', 'John', 'Mark']
]

I've to find the most frequent pairs, that is, people that had most collaborations in the past:

['John', 'Jennifer'] - 3 times
['John', 'Mark'] - 2 times
['Mark', 'Jennifer'] - 2 times
etc...

How to do this in Python?

Answer 1

Use a collections.Counter dict with itertools.combinations:

from collections import Counter
from itertools import combinations

d  = Counter()
for sub in a:
    if len(a) < 2:
        continue
    sub.sort()
    for comb in combinations(sub,2):
        d[comb] += 1

print(d.most_common())
[(('Jennifer', 'John'), 3), (('John', 'Mark'), 2), (('Jennifer', 'Mark'), 2), (('Anna', 'John'), 1), (('Joe', 'Mark'), 1), (('Anna', 'Jennifer'), 1)]

most_common() will return the pairings in order of most common to least, of you want the first n most common just pass n d.most_common(n)