Possible Duplicate:
Python, compute list difference
I have two lists For example:
A = [1,3,5,7]
B = [1,2,3,4,5,6,7,8]
Now, A is always a subset of B I want to generate a third list C: which has elements which are present in B but absent in A like
C = [2,4..]
Thanks
List comprehensions are one way to do this:
[x for x in B if x not in A]
If you use Python, I recommend gaining familiarity with list comprehensions. They're a very powerful tool.
(Several people have suggested using set. While this is a very good idea if you only care about whether or not an element is in the set, note that it will not preserve the order of the elements; a list comprehension will.)
>>> set(B) - set(A)
set([8, 2, 4, 6])
or
>>> sorted(set(B) - set(A))
[2, 4, 6, 8]
An easy way to do this is
C = [x for x in B if x not in A]
This will become slow for big lists, so it would be better to use a set for A:
A = set(A)
C = [x for x in B if x not in A]
If you have multiple operations like this, using sets all the time might be the best option. If A and B are sets, you can simply do
C = B - A
C = sorted(list(set(B) - set(A)))
That should do it.
