Sort list of strings by length and alphabetically [duplicate]

This question already has answers here: 开发者_StackOverflow社区 How to sort a list by length of string followed by alphabetical order? (6 answers) Closed 7 months ago.

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']

---

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']

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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']

---

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']
>>> 

---

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

---

a = sorted([["foo", "o"], ["bar2", "yadda"], ["allo","as3r"]], key=len)
b = sorted(a)