In Python program: please explain why 3 is not there in the out put [2, 4, 6, 1]

Question

0.00/5 (No votes)

See more:

Python

numbers = [2,2,4,6,3,4,6,1]
for num in numbers:
    if num in numbers:
       numbers.remove(num)
print(numbers)

# why output shows

[2, 4, 6, 1]

here in this output 3 is missing. Please can anybody explain it?

What I have tried:

i wrote a program in python and getting output as

[2, 4, 6, 1]

and in this 3 is missing why?

Posted 14-Jan-22 20:02pm

pnr58

Updated 14-Jan-22 22:19pm

Wendelius

v3

Add a Solution

3 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

RickZeeland · Answer 1 · 2022-01-14T20:12:00

Solution 1

I'm not a Python programmer, but in general removing elements in a for loop is a bad idea, this will confuse the for loop.
You can do it, but start at the end of the array, see: Backward iteration in Python - GeeksforGeeks[^]

Posted 14-Jan-22 20:12pm

RickZeeland

Wendelius · Answer 2 · 2022-01-14T20:22:00

The best way to understand what is happening is to get to know the debugger. Depending on the tools you use you most likely have debugger but if not, there are plenty available online.

If you use the debugger and go through the execution line-by-line you'll notice that you always check the existence of the number at hand against the whole array, not just the remaining portion of it. For example if you're in the first number 2 it exists, actually twice, so you remove the first element.

The next question you'll bump into is, why isn't every number eliminated? The reason is that when you're in the first element, you find it in the array but when you remove the first element, the second element becomes the first and then your loop is advanced to the next element, in this case to number 4, which previously was element 3 but now is element 2.

This explains why 3 is eliminated from the array even though other numbers look like they would be behaving correctly.

Richard MacCutchan · Answer 3 · 2022-01-14T22:19:00

Solution 3

You cannot modify a list while iterating over it. You should create a new list and copy the numbers over checking first if it already exists.

Python

numbers = [2,2,4,6,3,4,6,1]
newnums = []
for num in numbers:
    if num not in newnums:
        newnums.append(num)
print(sorted(newnums))

Posted 14-Jan-22 22:19pm

Richard MacCutchan