For example, I have two lists
A = [6, 7, 8, 9, 10, 11, 12]
subset_of_A = [6, 9, 12]; # the subset of A
the result should be [7, 8, 10, 11]; the remaining elements
Is there a built-in function in python to do this?
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10 Answers
If the order is not important, you should use set.difference. However, if you want to retain order, a simple list comprehension is all it takes.
result = [a for a in A if a not in subset_of_A]
EDIT: As delnan says, performance will be substantially improved if subset_of_A is an actual set, since checking for membership in a set is O(1) as compared to O(n) for a list.
A = [6, 7, 8, 9, 10, 11, 12]
subset_of_A = set([6, 9, 12]) # the subset of A
result = [a for a in A if a not in subset_of_A]
Yes, the filter function:
filter(lambda x: x not in subset_of_A, A)
2 Comments
modulitos
Note that in Python 2,
filter returns the list itself, while in Python 3, it returns an iterator.
Paweł Kaczorowski
@modulitos
list(filter(...))set(A)-set(subset_of_A) gives your the intended result set, but it won't retain the original order. The following is order preserving:
[a for a in A if not a in subset_of_A]
answered Apr 12, 2011 at 19:43
Alexander Gessler
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1 Comment
Life after Guest
This is wrong, it should be "if a not in", NOT "if not a in"
No, there is no build in function in python to do this, because simply:
set(A)- set(subset_of_A)
will provide you the answer.
2 Comments
Alok Mysore
While this works for his example, there may be problems if elements are repeated in the list A.
vmonteco
@AlokMysore Yet, doesn't
set() remove duplicates by precisely making the array a set?tuple(set([6, 7, 8, 9, 10, 11, 12]).difference([6, 9, 12]))
Comments
How about
set(A).difference(subset_of_A)
Comments
This was just asked a couple of days ago (but I cannot find it):
>>> A = [6, 7, 8, 9, 10, 11, 12]
>>> subset_of_A = set([6, 9, 12])
>>> [i for i in A if i not in subset_of_A]
[7, 8, 10, 11]
It might be better to use sets from the beginning, depending on the context. Then you can use set operations like other answers show.
However, converting lists to sets and back only for these operations is slower than list comprehension.
Comments
Use the Set type:
A_set = Set([6,7,8,9,10,11,12])
subset_of_A_set = Set([6,9,12])
result = A_set - subset_of_A_set
Comments
>>> a = set([6, 7, 8, 9, 10, 11, 12])
>>> sub_a = set([6, 9, 12])
>>> a - sub_a
set([8, 10, 11, 7])
