We help IT Professionals succeed at work.
Troubleshooting Question

deleting contents of one list from another

Europa MacDonald
on
61 Views
Last Modified: 2020-08-11
Is it possible in python 3 to delete the contents of one list from another list ? and what is the best way to do it ?

eg
a = [1,2,3,4]
b = [1,2,3,4,5,6]
delete a from b to get c = [5,6]

thankyou
Comment
Watch Question

nociSoftware Engineer
CERTIFIED EXPERT
Distinguished Expert 2019

Commented:
Try this:
a = [1,2,3,4]
b = [1,2,3,4,5,6]
for e in a:
   b.remove(e)
print(b)
Principal: IT
CERTIFIED EXPERT
Commented:
This one is on us!
(Get your first solution completely free - no credit card required)
UNLOCK SOLUTION
CERTIFIED EXPERT

Commented:
@Brent: This is not a good solution. Fistly, the set does not preserve the order of items. Say, if 1 was at the end of the second list, it would also be removed. Secondly, the set absorbs multiple instances of the same value. Say, if the second list contained multiple 5, then only one would be present after the actions.

Noci's solution is correct if one occurence of the element in the first list is to be removed from the second list. The list.remove() removes only the the first occurence. So, it depends on what is needed.

There is one more possibility that leads to another kind of solution. If the first list is the prefix of the second list, then the second list can be cut using the slicing operator:
a = [1,2,3,4]
b = [1,2,3,4,5,6]
if len(a) <= len(b) and b[0:len(a)] == a:
    b = b[len(a):]
print(b)

Open in new window

The if just show testing whether a is the prefix of b. The b[len(a):] meas get the part of b from index len(a) to the end.
ste5anSenior Developer
CERTIFIED EXPERT

Commented:
hm, as the OP's question is not complete. I would have expected "remove all occurances":

elementsToRemove = [1,2,3,4]
sourceList = [1,2,3,4,5,6,4,5]

resultList = list(
                  filter(
                         lambda sourceItem : (sourceItem not in elementsToRemove),
                         sourceList)
                  )

print(resultList)

Open in new window

gives us

[5,6,5]

Open in new window

Gain unlimited access to on-demand training courses with an Experts Exchange subscription.

Get Access
Why Experts Exchange?

Experts Exchange always has the answer, or at the least points me in the correct direction! It is like having another employee that is extremely experienced.

Jim Murphy
Programmer at Smart IT Solutions

When asked, what has been your best career decision?

Deciding to stick with EE.

Mohamed Asif
Technical Department Head

Being involved with EE helped me to grow personally and professionally.

Carl Webster
CTP, Sr Infrastructure Consultant
Empower Your Career
Did You Know?

We've partnered with two important charities to provide clean water and computer science education to those who need it most. READ MORE

Ask ANY Question

Connect with Certified Experts to gain insight and support on specific technology challenges including:

  • Troubleshooting
  • Research
  • Professional Opinions
Unlock the solution to this question.
Join our community and discover your potential

Experts Exchange is the only place where you can interact directly with leading experts in the technology field. Become a member today and access the collective knowledge of thousands of technology experts.

*This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.

OR

Please enter a first name

Please enter a last name

8+ characters (letters, numbers, and a symbol)

By clicking, you agree to the Terms of Use and Privacy Policy.