[Tutor] finding non unique data in lists

Kent Johnson kent_johnson at skillsoft.com
Thu Oct 14 13:44:55 CEST 2004


You can make a Set from each list, then find the intersection of all the 
sets. That will contain just the elements in all the lists.

 >>> from sets import Set

First some raw data:
 >>> l1 = [1,2,3,4,5,6,7]
 >>> l2 = [2,3,4,7]
 >>> l3 = [1,2,3,5,6,7]

Sets can be constructed from lists:
 >>> s1 = Set(l1)
 >>> s1
Set([1, 2, 3, 4, 5, 6, 7])
 >>> s2 = Set(l2)
 >>> s3 = Set(l3)

Sets overload the & operator to mean set intersection:
 >>> all = s1 & s2 & s3
 >>> all
Set([2, 3, 7])

Sets allow iteration to process their elements one-by-one:
 >>> for i in all:
...   print i

For more information about sets see the library reference page at 


At 01:12 PM 10/14/2004 +0200, Scott Melnyk wrote:
>I have a collection of lists of different lengths (containing exons
>from genetic data) that I would like to compare and only keep the
>those that are found within each of the lists.  Any exon not present
>in all the lists is not important to me.
>I am new to python and was told that sets could be used easily to find
>the unique data but I am unsure the most efficient way how to pull out
>the non-unique data.
>All suggestions are very much appreciated and welcome.
>Tutor maillist  -  Tutor at python.org

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