Python: How to sort a list of lists by the most common first element?

Question

How do you sort a list of lists by the count of the first element? For example, if I had the following list below, I'd want the list to be sorted so that all the 'University of Georgia' entries come first, then the 'University of Michigan' entries, and then the 'University of Florida' entry.

l = [['University of Michigan','James Jones','phd'],
     ['University of Georgia','Anne   Greene','ba'],
     ['University of Michigan','Frank Kimball','ma'],
     ['University of Florida','Nate Franklin','ms'],
     ['University of Georgia','Sara Dean','ms'],
     ['University of Georgia','Beth Johnson','bs']]

thefourtheye · Accepted Answer · 2014-02-27T16:12:30.953

9

from collections import Counter
c = Counter(item[0] for item in l)
print sorted(l, key = lambda x: -c[x[0]])

Output

[['University of Georgia', 'Anne   Greene', 'ba'],
 ['University of Georgia', 'Sara Dean', 'ms'],
 ['University of Georgia', 'Beth Johnson', 'bs'],
 ['University of Michigan', 'James Jones', 'phd'],
 ['University of Michigan', 'Frank Kimball', 'ma'],
 ['University of Florida', 'Nate Franklin', 'ms']]

Vanilla dict version:

c = {}
for item in l:
    c[item[0]] = c.get(item[0], 0) + 1
print sorted(l, key = lambda x: -c[x[0]])

defaultdict version:

from collections import defaultdict
c = defaultdict(int)
for item in l:
    c[item[0]] += 1
print sorted(l, key = lambda x: -c[x[0]])

edited Feb 27 '14 at 16:12

answered Feb 27 '14 at 15:54

thefourtheye

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@zhangxaochen You are welcome :) You are good at numpy yourself :) – thefourtheye Feb 27 '14 at 16:00
Do not forget that Counter does not exist in Python 2.6.6 so that this cannot be used if a later Python is not available. – sabbahillel Feb 27 '14 at 16:03
@sabbahillel Thanks :) Please check the plain dict version :) – thefourtheye Feb 27 '14 at 16:06
another way for the last version is with a `defaultdict` – acushner Feb 27 '14 at 16:10
1

@AdamCushner Please check my answer. Included that as well :) – thefourtheye Feb 27 '14 at 16:12

score -1 · Answer 2 · edited May 23 '17 at 12:03

Getting solution from here: How to sort a list of lists by a specific index of the inner list?

from operator import itemgetter

L=[['University of Michigan','James Jones','phd'],['University of Georgia','Anne   Greene','ba'],['University of Michigan','Frank Kimball','ma'],['University of Florida','Nate Franklin','ms'],['University of Georgia','Sara Dean','ms'],['University of Georgia','Beth Johnson','bs']]

print 'Before:', L
print ' After:', sorted(L, key=itemgetter(0))

Output

Before: [['University of Michigan', 'James Jones', 'phd'], ['University of Georgia', 'Anne   Greene', 'ba'], ['University of Michigan', 'Frank Kimball', 'ma'], ['University of Florida', 'Nate Franklin', 'ms'], ['University of Georgia', 'Sara Dean', 'ms'], ['University of Georgia', 'Beth Johnson', 'bs']]
 After: [['University of Florida', 'Nate Franklin', 'ms'], ['University of Georgia', 'Anne   Greene', 'ba'], ['University of Georgia', 'Sara Dean', 'ms'], ['University of Georgia', 'Beth Johnson', 'bs'], ['University of Michigan', 'James Jones', 'phd'], ['University of Michigan', 'Frank Kimball', 'ma']]

Python: How to sort a list of lists by the most common first element?

2 Answers2