0

I need to sort a list of words based on two criteria given. I need to return a list with the same words in order of length (longest to shortest) and the second sort criteria should be alphabetical.

Example list :

l = ['aa','aaa','aaaa','b','bb','z','ccc']

Desired output:

['aaaa', 'aaa', 'ccc', 'aa', 'bb', 'b', 'z']
Improve this question
3

5 Answers 5

Reset to default
11

You only need one call to sort, because Python automatically sorts tuples lexicographically. That is, if you ask Python to compare two tuples it will order them by their first element, except if those compare equal in which case it will order them by their second element, except if those compare equal in which case...

You want to sort the list of elements by minus their length and then alphabetically, so you want the key of a string s to be the tuple (-len(s), s). Hence:

>>> l = ['aa','aaa','aaaa','b','bb','z','ccc']
>>> sort_key = lambda s: (-len(s), s)
>>> l.sort(key=sort_key)
>>> l
['aaaa', 'aaa', 'ccc', 'aa', 'bb', 'b', 'z']
Improve this answer
6

Design

The keyword here is "stable sorting algorithm". Think of two stable sorting functions:

  • one sorting according to length (maintaining the relative order of entries with equal lengths),
  • the other sorting alphabetically.

In which order would you combine them in order to get the desired order?

Implementation

As others have mentioned, the first sorting function can be called like this:

list.sort(key=len, reverse=True)

The second sorting function can be called like this:

list.sort()

This should be enough to write a complete solution.

Result

If you combine the function in the right way, you should get the following:

>>> l = ['aaa', 'fff', 'bbb', 'ddd', 'e', 'cccc']
# ... sorting functions combined in the right way ...
>>> l
['cccc', 'aaa', 'bbb', 'ddd', 'fff', 'e']
Improve this answer
2

In Python, list's sort method has a key which can be used to specify the criteria for sorting. For the problem you describe I would use the key as well as do the normal sorting as follows.

>>> l = ['aa','aaa','aaaa','b','bb','z','ccc']
>>> l.sort(key=len,reverse=True)
>>> l
['aaaa', 'ccc', 'aaa', 'bb', 'aa', 'z', 'b']
>>> l.sort()
>>> l
['aa', 'aaa', 'aaaa', 'b', 'bb', 'ccc', 'z']
>>>
Improve this answer
4
  • 2
    l.sort(key=len, reverse=True) would be better than l.sort(...); l.reverse()
    – Chris Lutz
    Commented Jan 11, 2011 at 9:01
  • After the second sort(), the list is no longer displaying in order of length (longest to shortest).
    – John
    Commented Jan 11, 2011 at 9:10
  • Call: sortlist(['a','b','bb','ba']) Expected: [bb, ba, b, a] Result: ['a', 'b', 'ba', 'bb'] This is what happen after I tried the code
    – John
    Commented Jan 11, 2011 at 9:14
  • 1
    @John, the idea here is the use of key=len and reverse=True. What you need to do is call reverse first and then sort based on the key len.
    – Senthil Kumaran
    Commented Jan 11, 2011 at 9:23
0

Hints:

mylist = ['one', 'three', 'zero']

mylist.sort(key=len)

print mylist

mylist.reverse()

print mylist

mylist.sort()

print mylist

otherlist = [(2, 'a'), (1, 'a'), (3, 'b'), (3, 'a')]

otherlist.sort()

print otherlist
Improve this answer
0 
a = sorted([["foo", "o"], ["bar2", "yadda"], ["allo","as3r"]], key=len)
b = sorted(a)
Improve this answer
3
  • While this code may answer the question, providing additional context regarding how and/or why it solves the problem would improve the answer's long-term value.
    – Tomer Shetah
    Commented Jan 26, 2021 at 9:15
  • This is from over 10 years ago
    – tekknolagi
    Commented Jan 26, 2021 at 22:49
  • 1
    Probably someone flagged this answer, so I reviewed it as part of one of the queues. I don't mind you leave it as it is :)
    – Tomer Shetah
    Commented Jan 26, 2021 at 22:50

Not the answer you're looking for? Browse other questions tagged or ask your own question.