Am I in trouble? 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. But now the main question that comes to mind is why we need to create a copy of the list initially. Just as powerful are list compressions. 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. Can somebody be charged for having another person physically assault someone for them? Release my children from my debts at the time of my death, Cartoon in which the protagonist used a portal in a theater to travel to other worlds, where he captured monsters, minimalistic ext4 filesystem without journal and other advanced features. Do the subject and object have to agree in number? 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. There are two separate processes running in Python script. And processes dont share data, threads do. How can I delete a file or folder in Python? There was no need to copy the in-place assignment from the other answers, it's not better. Are Python built-in containers thread-safe? How to remove an element from a list by index. For example: "Tigers (plural) are a wild animal (singular)". How to remove element from a list while iterating? What its like to be on the Python Steering Council (Ep. Not the answer you're looking for? It really helped me in my case. How to remove elements from a generic list while iterating over it? Also why can't I even print the element? Both interact with a global variable POST_QUEUE = [] Process 1 ( P1) adds items to POST_QUEUE every 60 seconds. Your email address will not be published. Is there a simple way to delete a list element by value? 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? How can I define a sequence of Integers which only contains the first k integers, then doesnt contain the next j integers, and so on. Let's, therefore, create a method that checks whether the passed number is even and return True if it is: No spam ever. What are the pitfalls of indirect implicit casting? What should I do after I found a coding mistake in my masters thesis? May I reveal my identity as an author during peer review? Stepping through the debugger, I can see it's skipping the last iteration of the loop (I'm assuming because len(fileList) is now -=1 w.r.t. Asking for help, clarification, or responding to other answers. So unless you are using something like multiprocessing.Manager or some shared-memory construct, I dont think your current logic would work. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing, This asking is not very good or generic, because it only asks about removing the first element matching a specific value, for which Python already has. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. 593), Stack Overflow at WeAreDevelopers World Congress in Berlin, Temporary policy: Generative AI (e.g., ChatGPT) is banned. Find centralized, trusted content and collaborate around the technologies you use most. How to remove an element from a list by index, How to iterate over rows in a DataFrame in Pandas. finally, the best implementation IMO is to create or import a queue module It's not a good idea in general to remove items from list while looping, but if you need to do it, going from right to left is the best option: Since you do not really need the indexes of the elements I would suggest something like this as an easy solution: However this solution has a runtime of O(n^2) for every use of the loop since python needs to move every element in the list on every iteration. I'm iterating over a list of elements in Python, do some action on it, and then remove them if they meet certain criteria. | list.sort() Tutorial & Examples, Python: How to sort a list of tuples by 2nd Item using Lambda Function or Comparator. Is the issue resolved? Cold water swimming - go in quickly? rev2023.7.24.43543. This specific situation occurs when you try to remove items from a list while iterating over it. 593), Stack Overflow at WeAreDevelopers World Congress in Berlin, Temporary policy: Generative AI (e.g., ChatGPT) is banned. 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. There are two separate processes running in Python script. What's the DC of a Devourer's "trap essence" attack? >>> d= {1:'a',2:'b',3:'c'} >>> for i in d.keys (): if i==2: del d [2] >>> d {1: 'a', 3: 'c'} Similarly, we can also delete items using values, during iteration. One problem with this and the OP's approaches is that they can break down if there are multiple elements that compare equal. Could ChatGPT etcetera undermine community by making statements less significant for us? I still don't understand your comment then because it doesn't matter whether the list is shuffled or ordered, this code will still work. Find centralized, trusted content and collaborate around the technologies you use most. When laying trominos on an 8x8, where must the empty square be? (But iterating from back to front while removing might work as expected?). If a crystal has alternating layers of different atoms, will it display different properties depending on which layer is exposed? Why can't I remove character from the list? rev2023.7.24.43543. 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 can I randomly select an item from a list? Don't modify a sequence being iterated over. It's pretty common when you want to modify a list while iterating over it that going backwards works. Please take a look at the code -- the problem is simple and I suppose it's about lists; I just i can't see it. How do I remove duplicates from a list, while preserving order? I'm not sure how it's supposed to work, but it's got at least a couple of errors. @Velociraptors What does object identity matter? Do the subject and object have to agree in number? Not consenting or withdrawing consent, may adversely affect certain features and functions. rev2023.7.24.43543. The secret is "list(fileList)". I'm writing a round robin algorithm for a tournament app. Using del function In this example, we will iterate through dictionary key and use del function to delete the desired item. Removing an item will shift all following items one place to the left, thus in the next iteration one item will be skipped. There's much more to know. Is this mold/mildew? What information can you get with only a private IP address? Problem in deleting items from dictionary while iterating [Solved] - Remove key-value pairs from dictionary during Iteration Delete items from dictionary while iterating using comprehension Iterate over a dictionary and remove items using pop () function Can you actually change the size of dictionary during iteration in Python? Not the answer you're looking for? For example: "Tigers (plural) are a wild animal (singular)". 11 Powerful Methods to Iterate Through List in Python Connect and share knowledge within a single location that is structured and easy to search. 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? Most likely neither matters. Removing Item From List - during iteration - what's wrong with this idiom? Using robocopy on windows led to infinite subfolder duplication via a stray shortcut file. How can I avoid this? For this, we need first to create a copy of the list, and then we will iterate over that copied list. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Good idea. You could copy every element that doesn't match the condition into a second list: Alternatively, you could use filter or a list comprehension and assign the result back to a: where stands for the condition that you need to check. Anthology TV series, episodes include people forced to dance, waking up from a virtual reality and an acidic rain. The same value may appear multiple times in a list. +1 for using [:] to just modify the same list object. Connect and share knowledge within a single location that is structured and easy to search. (A modification to) Jon Prez Laraudogoitas "Beautiful Supertask" What assumptions of Noether's theorem fail? The second approach will delete the wrong elements when there's more than one element to delete, because the indices change after the first deletion. Why do capacitors have less energy density than batteries? There will be no reference to the dictionary during the iteration. Summary: To remove items from a list while iterating, use any of the following methods. Using for loop Using for loop and range () Using while loop Using list comprehension Using for loop with lambda Using for loop with enumerate () Using numpy library Using the iter & next functions Python loop through a list If yes, then delete that element from the original list using the remove() function. [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. Conclusions from title-drafting and question-content assistance experiments How do I get the number of elements in a list (length of a list) in Python? Removing from a list while iterating over it, I'm trying to learn why I can't seem to delete every index in a list with a loop, Deleting list elements whilst iterating over it, For-loop does not process all elements from the list, Iterating over dictionaries using 'for' loops. It created a new list, and then we assigned the new list back to the same reference variable. Note the "insert" inserts the list item number at position 0 and pushes the remainder along so when deleting the items, the list is deleted from the highest number back to the lowest number so the list stays in sequence. Release my children from my debts at the time of my death. python - iterating through a list removing items, some items are not A comment would be good form to preclude future developers from "tidying up" your code and breaking it mysteriously. Not the answer you're looking for? You could also use continue to ignore the values less than 20. (25 answers) Closed 8 years ago. If Phileas Fogg had a clock that showed the exact date and time, why didn't he realize that he had arrived a day early? It is generally not recommended to remove items from a list while iterating over it because it can cause unexpected behavior. Iterating through list and using remove() doesn't produce desired result. python - Why does removing items from list while iterating over it skip How to remove list elements in a for loop in Python? Python then picks the item at position 1 from the list (appearing to skip an item) python - How do I delete items from a dictionary while iterating over But I rely on items in the list when I iterate over loop. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. Line integral on implicit region that can't easily be transformed to parametric region, Do the subject and object have to agree in number? 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 to iterate a list while deleting items from list using range() function? Depending on the intent, either of the linked duplicates would be appropriate. Python: How to remove elements from list while iterating through it I am going to think if they are a more pythonic way. [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. How do I make a flat list out of a list of lists? Of course, I'm expecting the numbers below 20 to not appear in the results. However, removing things that are after the current index doesn't affect things. Can a Rogue Inquisitive use their passive Insight with Insightful Fighting? Can somebody be charged for having another person physically assault someone for them? English abbreviation : they're or they're not. 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). Why is this Etruscan letter sometimes transliterated as "ch"? Cartoon in which the protagonist used a portal in a theater to travel to other worlds, where he captured monsters. How to iterate through this list to subtract any elements that are not a two-pair? minimalistic ext4 filesystem without journal and other advanced features. The output for counter is one less than the total number of .txt files. January 3, 2021 by Chris 3.8/5 - (5 votes) Modifying a sequence while iterating over it can cause undesired behavior due to the way the iterator is build. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing, Python: How to remove elements from list while iterating through it without skipping future iterations [duplicate]. List iteration works by keeping track of an index, incrementing it each time around the loop until it falls off the end of the list. Then, for each element, well decide whether or not to delete 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. How to remove an element from a list by index. How to remove items from a list while iterating? Understandably, I've run into an issue where when removing items from a list that a for-loop is iterating over, it screws up the indexing and terminates the loop earlier than expected (i.e., before it performs the necessary operation on each post and removes it from POST_QUEUE). Are there any practical use cases for subtyping primitive types? Catch multiple exceptions in one line (except block). Delete item from list in Python while iterating over it - Stack Overflow Delete item from list in Python while iterating over it [duplicate] Ask Question Asked 11 years, 9 months ago Modified 3 years, 8 months ago Viewed 18k times 5 This question already has answers here : How to remove items from a list while iterating? A car dealership sent a 8300 form after I paid $10k in cash for a car. Here's an in-place O(n) version that only needs O(1) additional memory: nice one. Suppose we need to record the ages of 5 students. Both interact with a global variable POST_QUEUE = []. Not the answer you're looking for? or slowly? 593), Stack Overflow at WeAreDevelopers World Congress in Berlin, Temporary policy: Generative AI (e.g., ChatGPT) is banned. What would naval warfare look like if Dreadnaughts never came to be? The Filter() function takes two arguments. Conclusions from title-drafting and question-content assistance experiments How to remove items from a list while iterating? or slowly? It may be a long way of doing it but seems to work well. Why can't sunlight reach the very deep parts of an ocean? How do I figure out what size drill bit I need to hang some ceiling hooks? Connect and share knowledge within a single location that is structured and easy to search. How to create and initialize a list of lists in python? I tried the following code but it didn't do it: Best is usually to proceed constructively -- build the new list of the items you want instead of removing those you don't. Then you copy b (the altered copy) back to a. 593), Stack Overflow at WeAreDevelopers World Congress in Berlin, Temporary policy: Generative AI (e.g., ChatGPT) is banned. Why strange list comprehension behavior with side effects? How do I concatenate two lists in Python? @Rodic: My pleasure. Is there a more elegant solution to this problem? When we delete an element from a list using the remove() function in Python, it changes the remaining elements indexing. Still, the listcomp I recommended at the start of this answer looks better and better as alternatives are examined, doesn't it?-). Is it proper grammar to use a single adjective to refer to two nouns of different genders? Can a Rogue Inquisitive use their passive Insight with Insightful Fighting? For example: "Tigers (plural) are a wild animal (singular)". (See also my comment on that question.). python - Best way to remove elements from a list - Stack Overflow The following code "works", but feels like a hack - I'm adding the files I'd like to remove from the list to a second list, then iterating over that after the fact. Your email address will not be published. Now, consider the following Python code: elems = ['a','b','c'] for e in elems: print e elems.remove (e) This prints: a c Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. This looks like a neat suggestion to a classic problem, but I'm not sure that in practice it will prove better than the 2-stage solution suggested at the end of the chosen answer: firstly, what is the cost of the, Strange result when removing item from a list while iterating over it in Python, What its like to be on the Python Steering Council (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. But the iterator doesn't know about this, and effectively skips the next element since it is now at the current index rather than the next one. But there is an easier solution. Example 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? Because instead of modifying the list object in place, it re-bound the name sched to a new list, leaving the original list still bound to the name schedule in the caller. The technical storage or access that is used exclusively for anonymous statistical purposes. Then that's removed from the list, and then the for loop goes on to the third item in the list, which is now 5. : 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 . to be honest i still don't know how to ask question about python properly or where to look for the answer. Why can't sunlight reach the very deep parts of an ocean? It uses itertools.compress which was introduced in python 2.7: This stems from the fact that on deletion, the iteration skips one element as it semms only to work on the index. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user. How do you manage the impact of deep immersion in RPGs on players' real-life? Elegant, easynot entirely obfuscated. 593), Stack Overflow at WeAreDevelopers World Congress in Berlin, Temporary policy: Generative AI (e.g., ChatGPT) is banned. For example, you'll obtain a copy of list_1 by using the slice notation with default values list_1 [:]. How did this hand from the 2008 WSOP eliminate Scott Montgomery? Can a simply connected manifold satisfy ? The already-mentioned list comprehension approach is probably your best bet. You creating an additional list and iterates over this. Is it better to use swiss pass or rent a car? @radtek: Since removing an item from a list is an O(n) operation, that approach has O(n) worst-case complexity. How to remove items from a list while iterating? A question on Demailly's proof to the cannonical isomorphism of tangent bundle of Grassmannian. E.g. 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). In the circuit below, assume ideal op-amp, find Vout? Thanks for contributing an answer to Stack Overflow! 592), How the Python team is adapting the language for an AI future (Ep. Removing items from list while iterating over it - Stack Overflow @Boris you haven't understood my comment. How do I remove the first item from a list? Connect and share knowledge within a single location that is structured and easy to search. When iterating over a list and removing each item, why is every other item skipped and not removed? If you start from the end and go backwards, removing items on-the-go won't matter, because when it steps to the "next" item (actually the prior item), the deletion does not affect the first half of the list. To provide the best experiences, we and our partners use technologies like cookies to store and/or access device information. i didn't know that removing items during iteration is different. Why does CNN's gravity hole in the Indian Ocean dip the sea level instead of raising it. Removing things from Python list during for loop, how to delete element in the list in a for loop, Removing items for a list using a loop in Python, Twist? Python : How to create a list and initialize with same values, Second is the list from which we want to delete elements. Does ECDH on secp256k produce a defined shared secret for two key pairs, or is it implementation defined? I have proposition to ignore the last loop : This solution works. Python picks the item at position 0 from the list. See the other questions about deleting from a list while iterating over it. The alternative. | append() vs extend(), Python : How to Sort a list of strings ? What's the DC of a Devourer's "trap essence" attack? If so, please mark the correct answer as accepted or post and accept a solution of your own. Because a single test may modify its list, we need N lists to modify for N tests; therefore I've created the set of lists before running a test. How do I make a flat list out of a list of lists? Why is this Etruscan letter sometimes transliterated as "ch"? Why elements are skipped if I delete from a list while iterating it? List values are referred to as list items or list elements. 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. Lets understand by an example. What its like to be on the Python Steering Council (Ep. Therefore you have to subtract 1 from every index in the index_arr after you just removed a number from the numbers arr, like in the following: It will work, but I guess it's not the intended way to do it. 1. what to do about some popcorn ceiling that's left in some closet railing. How to Download Instagram profile pic using Python. How can I remove a key from a Python dictionary? Can consciousness simply be a brute fact connected to some physical processes that dont need explanation? "Fleischessende" in German news - Meat-eating people? How to make a shallow copy of a list in Python, How to remove palindromic words from a string. (25 answers) For example. hmm you are right. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. The updated list now looks like: Works: to avoid changing the list you are iterating on, you make a copy of a, iterate over it and remove the items from b. For example: "Tigers (plural) are a wild animal (singular)". As the second argument, we provided a list from which we want to delete the elements. >>> some_list [2, 4] >>> another_list [2, 4] I'm aware that Python doesn't support modifying a list while iterating over it and the right way is to iterate over copy of list instead. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Then for each element, we will check if we want to delete this element or not. Airline refuses to issue proper receipt. A collection is an ordered list of values. Remember to increase the index by 1 after each iteration. Personally, I'd just use generators everywhere or write a algorithm that doesn't need the bogus entries, but that's not the question. @mikerodent no it's not. Making statements based on opinion; back them up with references or personal experience. This way, you iterate starting from the end. If you need the object modification you need to assign to a slice: However, for large lists in which you need to remove few elements, this is memory consuming, but it runs in O(n). Why? Can a simply connected manifold satisfy ? 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. Can I spin 3753 Cruithne and keep it spinning? By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Due to this, during the next iteration, it picks the element after 55 and skips the 55. (Python). The one-stop solution is to note down those unwanted elements and remove them in the next iteration.

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