tallying occurrences in list

[Lie Ryan]

On 06/05/10 04:38, Magdoll wrote:
> On Jun 4, 11:33 am, Peter Otten <__pete... at web.de> wrote:
>> kj wrote:
>>
>>> Task: given a list, produce a tally of all the distinct items in
>>> the list (for some suitable notion of "distinct").
>>
>>> Example: if the list is ['a', 'b', 'c', 'a', 'b', 'c', 'a', 'b',
>>> 'c', 'a'], then the desired tally would look something like this:
>>
>>> [('a', 4), ('b', 3), ('c', 3)]
>>
>>> I find myself needing this simple operation so often that I wonder:
>>
>>> 1. is there a standard name for it?
>>> 2. is there already a function to do it somewhere in the Python
>>>    standard library?
>>
>>> Granted, as long as the list consists only of items that can be
>>> used as dictionary keys (and Python's equality test for hashkeys
>>> agrees with the desired notion of "distinctness" for the tallying),
>>> then the following does the job passably well:
>>
>>> def tally(c):
>>>     t = dict()
>>>     for x in c:
>>>         t[x] = t.get(x, 0) + 1
>>>     return sorted(t.items(), key=lambda x: (-x[1], x[0]))
>>
>>> But, of course, if a standard library solution exists it would be
>>> preferable.  Otherwise I either cut-and-paste the above every time
>>> I need it, or I create a module just for it.  (I don't like either
>>> of these, though I suppose that the latter is much better than the
>>> former.)
>>
>>> So anyway, I thought I'd ask. :)
>>
>> Python 3.1 has, and 2.7 will have collections.Counter:
>>
>>>>> from collections import Counter
>>>>> c = Counter("abcabcabca")
>>>>> c.most_common()
>>
>> [('a', 4), ('c', 3), ('b', 3)]
>> 
>> Peter
>
> 
> Thanks Peter, I think you just answered my post :)

If you're using previous versions (2.4 and onwards) then:

[(o, len(list(g))) for o, g in itertools.groupby(sorted(myList))]