根据不同字符的数量对字符串进行排序

Question

I am confused why the code below, which is looking to sort strings based on their number of distinct alphabets, requires the set() and list() portions.

strings = ['foo', 'card', 'bar', 'aaaa', 'abab']

strings.sort(key = lambda x: len(set(list(x))))
print(strings)

Thanks

Answer 1

In fact, the key of that code is the set() function. Why? Because it will return a set with not-repeated elements. For example:

set('foo') -> ['f', 'o']
set('aaaa') -> ['a']
set('abab') -> ['a', 'b']

Then, in order to sort based on the number of distinct alphabets, the len() function is used.

Answer 2

Nice question! Let's peel the layers off the sort() call.

According to the Python docs on sort and sorted ,

key specifies a function of one argument that is used to extract a comparison key from each list element: key=str.lower. The default value is None (compare the elements directly).

That is, sort takes a keyword argument key and expects it to be a function. Specifically, it wants a key(x) function that will be used to generate a key value for each string in strings list, instead of the usual lexical ordering. In the Python shell:

>>> key = lambda x: len(set(list(x)))
>>> ordering = [key(x) for x in strings]
>>> ordering
[2, 3, 1, 2, 2, 4]

This could be any ordering scheme you like. Here, we want to order by the number of unique letters . That's where set and list come in. list("foo") will result in ['f', 'o', 'o'] . Then we get len(list('foo')) == 3 -- the length of the word. Not the number of unique characters.

>>> key2 = lambda x: len(list(x))
>>> ordering2 = [key2(x) for x in strings]
>>> ordering2
[3, 3, 4, 4, 4, 4]

So we use set and list to get a set of characters. A set is like a list , except they only include the unique elements of a list . For instance we can make a list of characters for any word like this:

>>> list(strings[0])
['f', 'o', 'o']

And a set:

>>> set(list(strings[0]))
set(['o', 'f'])

The len() of that set is 2, so when sort goes to compare the "foo" in strings[0] to all the other strings[x] in strings , it uses this list. For example:

>>> (len(set(strings[0][:])) < len(set(strings[1][:])))
True

Which gives us the ordering we want.

EDIT: @PeterGibson pointed out above that list(string[i]) isn't needed. This is true because strings are iterable in Python, just like lists:

>>> set("foo")
set(['o', 'f'])