[Tutor] working with multiple sets

[kevin parks]

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?

cheers,

-kp--




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
>
>
> 0 - x
> 1 - x
> 2 - x
> 3 - x
> 4 - x
> 5 - x, y
> 6 - x, y
> 7 - x, y
> 8 - x, y, z
> 9 - x, y, z
> 10 - y, x
>
> etc.
>
> Of course the whole point of this is that the sets will be more  
> complicated than 0-9, 5-14, and 8-21 and additionally, the sets may  
> not be a list of numbers but eventually a list of strings.
>
> So the steps would be to create the superset
> then test for membership for each list?
>
> I kinda get it, the thing that warps my brain is the idea that there  
> are more than 2 lists now to test against.... eventually my script  
> needs to accommodate 4, 5, 6 sets.. but i would just like to see if  
> i can get 3 sets to work first.
> -- 
> Bob Gailer
> Chapel Hill NC
> 919-636-4239