To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Why can't sunlight reach the very deep parts of an ocean? So unless you are using something like multiprocessing.Manager or some shared-memory construct, I dont think your current logic would work. Your email address will not be published. 2) The for loop will skip items in your list. Sorted by: 145. So 54 got deleted, but 55 was skipped. Yeah I do it in reverse order, works without copying: x = len(system_list) # remove all null entries for i in range(x-1, -1, -1): if not system_list[i]: del system_list[i]. Now we can utilize the even() method to remove all odd numbers from the numbers list while iterating over it: This will alter the numbers list so it now contains only even numbers: Advice: To solve the problem of removing elements from a list while iterating, we've used two key concepts - list comprehension and slice notation. The second list is the one from which we want to remove elements. You're modifying the list while you iterate over it. Is it proper grammar to use a single adjective to refer to two nouns of different genders? It's much easier to build a new list based on the old one: hymloth and sven's answers work, but they do not modify the list (the create a new one). Why cant we just iterate over the original list and delete elements while iterating? Therefore while deleting an element from the list while iterating, we need to make sure that we are iterating over the copy and removing elements from the original list to avoid iterator invalidation. Lets understand by an example. Iterate Through List in Python Using List Comprehension 6. In python, I noticed if I'm iterating through a list with for x in y, and I remove an element of y in the loop, the last element will be "skipped" - I'm assuming this is because len (y) has changed. May I reveal my identity as an author during peer review? For example: You can loop through your list from right to left. Learn how your comment data is processed. Don't modify a sequence being iterated over. Then the for loop goes to the second item in the list, which is not 2, but 3! When laying trominos on an 8x8, where must the empty square be? Does this definition of an epimorphism work? Find centralized, trusted content and collaborate around the technologies you use most. Remove elements from a list while iterating using list comprehension, Remove elements from a list while iterating using filter() function, Check if all elements in List are False in Python. @mikerodent no it's not. When we delete an element from a list using the remove() function in Python, it changes the remaining elements indexing. wait, so .remove() can remove instances from "b" even though technically the instance being passed is an instance of "a"? what to do about some popcorn ceiling that's left in some closet railing. Removing an item will shift all following items one place to the left, thus in the next iteration one item will be skipped. Then for each element, we will check if we want to delete this element or not. Removing Item From List - during iteration - what's wrong with this idiom? Can consciousness simply be a brute fact connected to some physical processes that dont need explanation? To learn more, see our tips on writing great answers. Remove elements from a list while iterating using filter () function Summary Remove elements from list in for loop Suppose we have a list of numbers, Copy to clipboard list_of_num = [51, 52, 53, 54, 55, 56, 57, 58, 59] We want to delete elements from the list while iterating over it, based on some conditions like all occurrences of 54 and 55. Of course, I'm expecting the numbers below 20 to not appear in the results. A comment would be good form to preclude future developers from "tidying up" your code and breaking it mysteriously. or slowly? (25 answers) I also wanted to add my special case using while and for using [:], but it might be out of the scope, so I'll keep it. Introducing another list object introduces another variable to remember when reading the code. That means that the first time through the loop, i == 1, so 1 is removed from the list. 593), Stack Overflow at WeAreDevelopers World Congress in Berlin, Temporary policy: Generative AI (e.g., ChatGPT) is banned. All rights reserved. For example: "Tigers (plural) are a wild animal (singular)". Can a Rogue Inquisitive use their passive Insight with Insightful Fighting? Removing items from list while iterating over it - Stack Overflow How can I remove a key from a Python dictionary? So in terms of time, hymloth's and your solutions seem to be the best AFAICT How to remove item from a python list in a loop? 11 Powerful Methods to Iterate Through List in Python So what's the problem? The best you can do is something like this: or this, for in-place alteration (the thing in parens is a generator expression, which is implicitly converted into a tuple before slice-assignment): If you want to perform an operation on n before removing it, one trick you could try is this: Begin at the list's end and go backwards: Having said that to further illustrate even a bit more your problem, if you think about it, you will always want to remove the index 0 twenty times: So you could actually go with something like this: The ones below are not bad practices AFAIK. This code outputs a count of all the .txt files, and the length of the list is one less than that (because the element a.txt was removed) - this is the desired behaviour. Best is usually to proceed constructively -- build the new list of the items you want instead of removing those you don't. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Connect and share knowledge within a single location that is structured and easy to search. A question on Demailly's proof to the cannonical isomorphism of tangent bundle of Grassmannian. if statement not removing element from list Python3. You can change your settings at any time, including withdrawing your consent, by using the toggles on the Cookie Policy, or by clicking on the manage consent button at the bottom of the screen. Python Delete Dictionary Key While Iterating - Fedingo By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Python: How To Remove Items From a List While Iterating Dimitrije Stamenic Iterating over a list of objects in Python to access and change them is a common thing to do while coding. How do I remove duplicates from a list, while preserving order? Is it appropriate to try to contact the referee of a paper after it has been accepted and published? What is the smallest audience for a communication that has been deemed capable of defamation? How to remove items from a list while iterating? Each event is regarded as a distinct element. Can a Rogue Inquisitive use their passive Insight with Insightful Fighting? Why I get the list unchanged when using this code? The output for counter is one less than the total number of .txt files. Depending on the intent, either of the linked duplicates would be appropriate. So whether or not you use list_object[:] = depends on if you want any other names bound to your list to see the changes. To learn more, see our tips on writing great answers. How do I split a list into equally-sized chunks? 592), How the Python team is adapting the language for an AI future (Ep. How did this hand from the 2008 WSOP eliminate Scott Montgomery? python - How to remove items from a list while iterating? L1.remove (a,b) . Instead of manually filtering on files ending in .txt, you can glob for files matching this pattern, And you want to count the nuber of *.txt files, except for a.txt, Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Why? [duplicate]. Using del function In this example, we will iterate through dictionary key and use del function to delete the desired item. Are there any practical use cases for subtyping primitive types? Three different Python examples to remove items from a list while iterating 1 Dumb observation: Your whole loop is kind of pointless if you're just looking for a specific key. Warning: Generally speaking, removing elements from a list while iterating over it in Python is not a great idea and good practice advises against it. That means that the first time through the loop, i == 1, so 1 is removed from the list. python - Best way to remove elements from a list - Stack Overflow 592), How the Python team is adapting the language for an AI future (Ep. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. How do I get the number of elements in a list (length of a list) in Python? Iterate Through List in Python Using Loop and Range 7. Suppose we need to record the ages of 5 students. Can I spin 3753 Cruithne and keep it spinning? The problem you will encounter is the same as before. How do you manage the impact of deep immersion in RPGs on players' real-life? The new list can then be assigned to the same reference variable that was part of the original list. Not consenting or withdrawing consent, may adversely affect certain features and functions. Was the release of "Barbie" intentionally coordinated to be on the same day as "Oppenheimer"? : L [:] = [el for el in L if el != 3] the list comprehension builds the desired list and the assignment to the "whole-list slice", L [:], ensure you're not just rebinding a name, but fully replacing the contents, so the . 593), Stack Overflow at WeAreDevelopers World Congress in Berlin, Temporary policy: Generative AI (e.g., ChatGPT) is banned. Using list comprehension, we can iterate over the list and choose which elements to keep in the new list. 593), Stack Overflow at WeAreDevelopers World Congress in Berlin, Temporary policy: Generative AI (e.g., ChatGPT) is banned. .remove() in for loop wasn't working as expected, Iterating over the elements of a list and perform 'list.remove(x)', but the list is not empty after the iteration, python 3 'for' command iteration looses order of sequence, My .remove() isn't removing all the numbers from the list even though I'm passing it through an iterator. I've commented out the line I originally had, which caused the "skipping" of iteration(s). That way, you are effectively changing the length of the list while accessing its elements, therefore risking facing unexpected behavior. Nope, this will behave in really weird ways indeed -- removing L's "third item" (possibly more than once) if completely unrelated items equal, How to delete an element from a list while iterating over it in Python? You're trying to remove something that you don't know is in the list (the index from, That is not the correct way to use slices to reverse a list, try, @agf: No, unfortunately -- my main point was refuting your claim that. Iterating through list and using remove() doesn't produce desired result. This implies that if you iterate the list back to front, if you remove an item at the current index, everything to it's right shifts left - but that doesn't matter, since you've already dealt with all the elements to the right of the current position, and you're moving left - the next element to the left is unaffected by the change, and so the iterator gives you the element you expect. There was no need to copy the in-place assignment from the other answers, it's not better. Do I have a misconception about probability? I'm writing a round robin algorithm for a tournament app. Why do capacitors have less energy density than batteries? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. The best way to rewrite the code depends on what it is you're trying to do. 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Cartoon in which the protagonist used a portal in a theater to travel to other worlds, where he captured monsters. Python doesn't complain at you, it just gives a slightly (read: completely) different result to what people might expect. Well, to nitpick: You "can", but you'll get useless results and other containers outright disallow it. Find centralized, trusted content and collaborate around the technologies you use most. What are the pitfalls of indirect implicit casting? Consenting to these technologies will allow us and our partners to process personal data such as browsing behavior or unique IDs on this site and show (non-) personalized ads. "/\v[\w]+" cannot match every word in Vim. It gave an effect that we have deleted elements from the list while iterating. What should I do after I found a coding mistake in my masters thesis? or slowly? Iterate Through List in Python Using Numpy Module 4. given_list = [1,2,3,4,4,5,6] new_list = list(given_list) for item in new_list: if item < 4: given_list.remove(item) print(given_list) It will print: The updated list now looks like: Click below to consent to the above or make granular choices. Iterate Through List in Python Using For Loop 2. Python: How To Remove Items From a List While Iterating - Stack Abuse 592), How the Python team is adapting the language for an AI future (Ep. Making statements based on opinion; back them up with references or personal experience. As other answers have said, the best way to do this involves making a new list - either iterate over a copy, or construct a list with only the elements you want and assign it back to the same variable. Am I in trouble? what to do about some popcorn ceiling that's left in some closet railing. I am going to think if they are a more pythonic way. I create a second list that only holds numbers that relate to the list item to delete. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing, This is a good answer, but with the final solution, "if you want to perform an operation", is slightly unsatisfactory because 1) in fact there is no need to include that qualification: it is just a waste of effort to try and remove elements while iterating in a single operation, so this 2-stage solution applies in all cases, and 2) because there should be a warning that setting to. Can a creature that "loses indestructible until end of turn" gain indestructible later that turn? Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. [1, 2, 3, 4, 5, 6, 7, 8, 9], Then, at the second iteration of the loop, you get the second item in the updated list. I'm not sure how it's supposed to work, but it's got at least a couple of errors. You need an additional list. To remove elements from a list while iterating over it, you need to go backwards: A better option is to use a list-comprehension: Now, you could just do schedule = instead of schedule[:] = -- what's the difference? 593), Stack Overflow at WeAreDevelopers World Congress in Berlin, Temporary policy: Generative AI (e.g., ChatGPT) is banned. Weird behavior removing elements from a list in a loop in Python. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. How can the language or tooling notify the user of infinite loops. Remove objects from an array without it overshooting, Append items that weren't removed into separate list, I can't delete a list of used numbers from another list of lists. Can a Rogue Inquisitive use their passive Insight with Insightful Fighting? Are there any practical use cases for subtyping primitive types? @radtek: Since removing an item from a list is an O(n) operation, that approach has O(n) worst-case complexity. This means that existing ones can be added to, deleted from, or changed. Anthology TV series, episodes include people forced to dance, waking up from a virtual reality and an acidic rain. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. It's not clear what OP wanted in this question. (A modification to) Jon Prez Laraudogoitas "Beautiful Supertask" What assumptions of Noether's theorem fail? How to iterate over a list, removing elements? There are several ways to remove elements from the list while iterating some of them are: To accomplish this, we must first make a copy of the list, and then iterate over that copied list. The problem is that python keeps an internal counter to remember the current elelement, which will refer to something unexpected when the list changes from under the iterator. Why is this Etruscan letter sometimes transliterated as "ch"? If so, please mark the correct answer as accepted or post and accept a solution of your own. For example. How to remove items from a list while iterating? what to do about some popcorn ceiling that's left in some closet railing. :). How to remove elements from a generic list while iterating over it? The chosen answer provides the former, which is infinitely more helpful, but also the latter, in the shape of a one-line list comprehension solution. Iterating over list but some values are skipped? A list is a mutable container. that way you keep the order (which I suppose is important to you throughout this answer) while only moving the elements of the list once and resulting in a runtime of O(n). Cold water swimming - go in quickly? Is there a better way to do this than just creating a copy of POST_QUEUE and having P2 iterate over that while removing items from the original POST_QUEUE object? Using robocopy on windows led to infinite subfolder duplication via a stray shortcut file. How can I avoid this? Find centralized, trusted content and collaborate around the technologies you use most. My bechamel takes over an hour to thicken, what am I doing wrong. In the circuit below, assume ideal op-amp, find Vout? for element in somelist: do_action (element) if check (element): remove_element_from_list What should I use in place of remove_element? Step 1: position 0 L = [1, 2, 3] x = 1 Step 2: remove x from L position is still 0 L = [2, 3] Conclusions from title-drafting and question-content assistance experiments Python - Why is this loop to remove items from list not removing all items from list? python - iterating through a list removing items, some items are not Thou Shalt Not Modify A List During Iteration Here's an in-place O(n) version that only needs O(1) additional memory: nice one. @NPE I guess it depends on what you expect to happen. [0, 1, 2, 3, 4, 5, 6, 7, 8, 9], Then, in the first iteration of the for loop, you get the first item of the list which is 0, and remove it from the list. [1, 3, 4, 5, 6, 7, 8, 9], And it goes on until you get: | Search by Value or Condition, Python : Check if all elements in a List are same or matches a condition, Python : Check if a list contains all the elements of another list, Python : How to add an element in list ?