Velocity Reviews > Standard ways to get union, intersection, difference of lists?

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

=?ISO-8859-1?Q?Mickel_Gr=F6nroos?=
Guest
Posts: n/a

 06-26-2003
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

Paul Rudin
Guest
Posts: n/a

 06-26-2003
>>>>> "Mickel" == Mickel Grönroos <(E-Mail Removed)> 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.

Erik Max Francis
Guest
Posts: n/a

 06-26-2003
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])

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