dict comprehension question.

Mitya Sirenef msirenef at lightbird.net
Sat Dec 29 22:40:04 CET 2012


On 12/29/2012 03:15 PM, Joel Goldstick wrote:
>
>
>
> On Sat, Dec 29, 2012 at 3:09 PM, Mitya Sirenef <msirenef at lightbird.net 
> <mailto:msirenef at lightbird.net>> wrote:
>
>     On 12/29/2012 03:01 PM, Mitya Sirenef wrote:
>
>         On 12/29/2012 02:48 PM, Quint  Rankid wrote:
>
>     >> Newbie question. I've googled a little and haven't found the
>     answer.
>     >>
>     >> Given a list like:
>     >> w = [1, 2, 3, 1, 2, 4, 4, 5, 6, 1]
>     >> I would like to be able to do the following as a dict
>     comprehension.
>     >> a = {}
>     >> for x in w:
>     >> a[x] = a.get(x,0) + 1
>     >> results in a having the value:
>     >> {1: 3, 2: 2, 3: 1, 4: 2, 5: 1, 6: 1}
>     >>
>     >> I've tried a few things
>     >> eg
>     >> a1 = {x:self.get(x,0)+1 for x in w}
>     >> results in error messages.
>     >>
>     >> And
>     >> a2 = {x:a2.get(x,0)+1 for x in w}
>     >> also results in error messages.
>     >>
>     >> Trying to set a variable to a dict before doing the comprehension
>     >> a3 = {}
>     >> a3 = {x:a3.get(x,0)+1 for x in w}
>     >> gets this result, which isn't what I wanted.
>     >> {1: 1, 2: 1, 3: 1, 4: 1, 5: 1, 6: 1}
>     >>
>     >> I'm not sure that it's possible to do this, and if not, perhaps the
>     >> most obvious question is what instance does the get method bind to?
>     >>
>     >> TIA
>     >
>     > Will this do?:
>     >
>     > >>> w = [1, 2, 3, 1, 2, 4, 4, 5, 6, 1]
>     > >>> {x: w.count(x) for x in w}
>     > {1: 3, 2: 2, 3: 1, 4: 2, 5: 1, 6: 1}
>     >
>     >
>     > - mitya
>     >
>
>     I should probably add that this might be inefficient for large
>     lists as
>     it repeats count for each item. If you need it for large lists,
>     profile
>     against the 'for loop' version and decide if performance is good
>     enough
>     for you, for small lists it's a nice and compact solution.
>
>     In a more general case, you can't refer to the list/dict/etc
>     comprehension as it's being constructed, that's just not a design goal
>     of comprehensions.
>
>
> Would this help:
>
>      >>> 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..


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