list comprehention

Bryan Olson fakeaddress at
Fri Jan 20 17:58:23 EST 2006

Duncan Booth wrote:
> Here's the way I would do it:
>>>>def occurrences(it):
>     res = {}
>     for item in it:
>         if item in res:
>             res[item] += 1
>         else:
>             res[item] = 1
>     return res

I slightly prefer:

def occurrences(it):
     res = {}
     res[item] = res.get(item, 0) + 1
     return res

> Or in other words, define a function to return a dictionary containing 
> a count of the number of occurrences of each element in the list (this 
> assumes that the list elements are hashable). Then you just add up the 
> values in the test list making sure each count is limited to no higher than 
> the reference count.

Resulting in a linear-time average case, where the posted
list-comprehension-based solutions are quadratic. The title
of the thread is unfortunate.

The generalized problem is multiset (AKA "bag") intersection:


More information about the Python-list mailing list