dict comprehension question.
Tim Chase
python.list at tim.thechases.com
Sat Dec 29 19:26:37 EST 2012
On 12/29/12 15:40, Mitya Sirenef wrote:
>> >>> w = [1,2,3,1,2,4,4,5,6,1]
>> >>> s = set(w)
>> >>> s
>> set([1, 2, 3, 4, 5, 6])
>> >>> {x:w.count(x) for x in s}
>> {1: 3, 2: 2, 3: 1, 4: 2, 5: 1, 6: 1}
>
> Indeed, this is much better -- I didn't think of it..
Except that you're still overwhelmed by iterating over every element
in "w" for every distinct element. So you've gone from O(N**2) to
O(k*N).
The cleanest way to write it (IMHO) is MRAB's
>>> w = [1, 2, 3, 1, 2, 4, 4, 5, 6, 1]
>>> from collections import Counter
>>> results = dict(Counter(w))
which should gather all the statistics in one single pass across "w"
making it O(N), and it's Pythonically readable.
-tkc
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