Python find duplicates array operations

Question

How can I form an array (c) composed of elements of b which are not in a?

a=[1,2,"ID123","ID126","ID124","ID125"]
b=[1,"ID123","ID124","ID125","343434","fffgfgf"]
c= []

Can this be done without using a list comprehension?

Is this what you are looking for: http://stackoverflow.com/questions/5640630/array-filter-in-python — Justin Ethier, Jun 12 '12 at 17:52
[Does this answer your question?](http://stackoverflow.com/questions/3697432/python-how-to-find-list-intersection) — Nadir Sampaoli, Jun 12 '12 at 17:54
@Rajeev: why do you want to do it without a list comprehension? We can't guess at your requirements, you need to explain these things. — Ned Batchelder, Jun 12 '12 at 17:55
I am expecting simple things like a-b and i tried list comptrehensiom — Rajeev, Jun 12 '12 at 17:56

Ned Batchelder · Accepted Answer · 2012-06-12T18:00:39.810

6

If the lists are long, you want to make a set of a first:

a_set = set(a)
c = [x for x in b if x not in a_set]

If the order of the elements don't matter, then just use sets:

c = list(set(b) - set(a))

Python lists don't offer a direct - operator, as Ruby arrays do.

edited Jun 12 '12 at 18:00

answered Jun 12 '12 at 17:54

Ned Batchelder

Levon · Answer 2 · 2012-06-12T18:23:49.487

1

Using list comprehension is most straight forward:

[i for i in b if i not in a]
c
['343434', 'fffgfgf']

However, if you really did not want to use list comprehension you could use a generator expression:

c = (i for i in b if i not in a)

This will also not generate the result list all at once in memory (in case that would be a concern).

edited Jun 12 '12 at 18:23

answered Jun 12 '12 at 17:52

Levon

score 0 · Answer 3 · answered Jun 12 '12 at 17:55

0

The following will do it:

c = [v for v in b if v not in a]

If a is long, it might improve performance to turn it into a set:

a_set = set(a)
c = [v for v in b if v not in a_set]

answered Jun 12 '12 at 17:55

NPE

3 Answers3