Why a specific value is not removed from a list?

Question

I'm trying to complete the following short exercise: Iterate a given list and Check if a given element already exists in a dictionary as a key’s value if not delete it from the list.

My solutions seem to work and remove all items from the list, which doesn't exist in the dictionary as values, beside one value... 95. How can it be?

When I'm doing debug, during the iteration of the loop, it seems that Python skips that value (95):

rollNumber = [47, 64, 69, 37, 76, 83, 95, 96, 97]
sampleDict = {"Jhon": 47, "Emma": 69, "Kelly": 76, "Jason": 97}

for current_item in rollNumber:
    if not current_item in sampleDict.values():
        rollNumber.remove(current_item)

print(rollNumber)

Actual results:

[47, 69, 76, 95, 97]

Expected results:

[47, 69, 76, 97]

Do not modify the list you are iterating on. Instead iterate on a copy of the list `for current_item in rollNumber[:]: ` — Devesh Kumar Singh, Jun 17 '19 at 07:17
@Devesh. There's absolutely nothing wrong with modifying a list in place, if you do it properly. — Mad Physicist, Jun 17 '19 at 07:21
Yes, but doing it properly is something not so obvious to a newbie, so it's better to stick to something foolproof like iterating on a copy and modifying the original IMO @MadPhysicist — Devesh Kumar Singh, Jun 17 '19 at 07:23
@Devesh. Ah yes, the "foolproof" way of doing it. Right. Just kidding. It really is much safer that way. — Mad Physicist, Jun 17 '19 at 07:26
Hah @MadPhysicist I am not quite sure you agreed with me or disagreed — Devesh Kumar Singh, Jun 17 '19 at 07:26
The program should have worked fine it is funny why it doesn't — Amartya Gaur, Jun 17 '19 at 07:27
@Devesh. I agree that your way is safer. I don't agree that doing things safely is the best way to learn every time. So mostly, I agree with you. — Mad Physicist, Jun 17 '19 at 07:30
@Amartya. Care to explain? I'm looking at it and seeing how it clearly shouldn't work. The duplicates should help clarify why not. — Mad Physicist, Jun 17 '19 at 07:31
@MadPhysicist got it, just that this is one of the python gotcha which I see almost every day on SO :) — Devesh Kumar Singh, Jun 17 '19 at 07:32
Thank you all for your answers. I used Devesh Kumar Singh's solution and it worked. — Tal, Jun 18 '19 at 09:17

score 0 · Answer 1 · answered Jun 17 '19 at 07:19

0

try this

rollNumber = [47, 64, 69, 37, 76, 83, 95, 96, 97]
sampleDict = {"Jhon": 47, "Emma": 69, "Kelly": 76, "Jason": 97}

rollNumber = [i for i in rollNumber if i in sampleDict.values()]

print(rollNumber)

result: [47, 69, 76, 97]

answered Jun 17 '19 at 07:19

LinPy

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Why a specific value is not removed from a list?

1 Answers1