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# Standard ways to get union, intersection, difference of lists?

Hi!

Are there any standard list methods for getting the intersection and
difference of two lists? (The union is easy ("list1.extend(list2)"),
unless you want it to contain unique values.)

Here is what I would like to have:

list1 = [1,2,3]
list2 = [3,4]

list3 = list1.intersection(list2)
(list3 is now [3])

list3 = list1.difference(list2)
(list3 is now [1,2])

list3 = list2.difference(list1)
(list3 is now [4])

I realize I could quite easily implement this myself, but I was hoping for
a built-in solution.

Cheers,

/Mickel

--
Mickel Grönroos, application specialist, linguistics, Research support,CSC
PL 405 (Tekniikantie 15 a D), 02101 Espoo, Finland, phone +358-9-4572237
CSC is the Finnish IT center for science, www.csc.fi
Jul 18 '05 #1
3 80516
Mickel Grönroos wrote:
Are there any standard list methods for getting the intersection and
difference of two lists? (The union is easy ("list1.extend(list2)"),
unless you want it to contain unique values.)

If you have Python 2.3 available, why not use a set?
import sets
s1 = sets.Set([1, 2, 3])
s2 = sets.Set([3, 4])
s1.intersection(s2) Set([3]) s1.difference(s2) Set([1, 2]) s2.difference(s1)

Set([4])

--
Erik Max Francis && ma*@alcyone.com && http://www.alcyone.com/max/
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Jul 18 '05 #2
>>>>> "Mickel" == Mickel Grönroos <mi****@csc.fi> writes:
Hi! Are there any standard list methods for getting the
intersection and difference of two lists? (The union is easy
("list1.extend(list2)"), unless you want it to contain unique
values.)
[snip]
I realize I could quite easily implement this myself, but I was
hoping for a built-in solution.

You might want to take a look at the sets module in Python 2.3. It's
not exactly what you ask for since sets.Sets are not lists - but they
do support these operations and can easily be converted to or from
lists.
Jul 18 '05 #3
I know lots of people hate using filter and lambda calls but this will give you a union between 2 lists, example:

[COLOR=Green]list1=range(5) # [0,1,2,3,4]
list2=range(3,7) # [3,4,5,6]
union=list1+filter(lambda x:x not in list1,list2)
# union = [0,1,2,3,4,5,6]
[/COLOR]

Intersection is just as easy

[COLOR=Green]
intersection=filter(lambda x:x in list1,list2)
# intersection=[3,4]
[/COLOR]

Difference is the last thing you wanted...
[COLOR=Green]
difference=filter(lambda x:x not in list2,list1)
# difference=[0,1,2]
[/COLOR]

And my set vocabulary is rusty but the distinct elements or those from both lists that are NOT in common (un-intersection?) would be:

[COLOR=Green]
distinct=filter(lamba x:x not in list2,list1)+filter(lambda x:x not in list1,list2)
# distinct=[0,1,2,5,6]
[/COLOR]

. -. -.. - .-. .- -. ... -- .. ... ... .. --- -.
Xoanan
Aug 9 '05 #4

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