[Tutor] working with multiple sets

[bob gailer]

kevin parks wrote:
>
>
> I am looking at this and wondering:
>
> Why does this use collections.defaultdict ?
>
> In fact i guess since collections.defaultdict is new to me i am not 
> even sure why it exists
> and why someone would use this as opposed to using Python's built-in 
> dictionary? and why was it
> used in this instance?

It simplifies coding, as it takes care of initializing each new entry to 
a list.

> On Sep 6, 2009, at 3:06 AM, bob gailer wrote:
>
>>
>>
>> I want to be able to look at a number/item and see which lists it is 
>> in so that i could maybe have a master list of all the data, a 
>> superset, and then an indication of which lists that data  was in, as 
>> some items will only be in one list, some will appear in two lists (x 
>> & y, or x & z or y & z) and a small handful will be in all three lists.
>>
>> I think you mean "set" rather than "list"
>>
>> To enable processing of an arbitrary number of sets, put them in a 
>> collection (list or dictionary). Use a list if it is sufficient to 
>> identify sets by number, else use a dictionary.
>>
>> Use a dictionary to relate items to their set(s).
>>
>> import collections
>> lookup = collections.defaultdict(list)
>> sets = {'x': set((1,2,3)), 'y': set((2,3))}
>> for key, value in sets.items():
>>   for element in value:
>>     lookup[element].append(key)
>> print lookup
>>
>>
-- 
Bob Gailer
Chapel Hill NC
919-636-4239