@CiroSantilli : The pop(i) operation is still O(n). If you really need to do it in one pass withou Pros and cons of retrofitting a pedelec vs. buying a built-in pedelec, Keep a fixed distance between two bevelled surfaces. Best method for changing a list while iterating over it, Delete item from list in Python while iterating over it, Removing things from Python list during for loop. All Rights Reserved. WebRemoving elements from a list is a common operation when working with data, and it can be done efficiently using the built-in functions and methods available in Python. 25 Answers Sorted by: 1079 You can use a list comprehension to create a new list containing only the elements you don't want to remove: somelist = [x for x in remove_element_from_list Download fluidIter.py from here https://github.com/alanbacon/FluidIterator, it is just a single file so no need to install git. There are various inbuilt functions to achieve this. How to remove element from list without using for loop? 1) When using remove(), you attempt to remove integers whereas you need to remove a tuple. for i in range(len(somelist) - 1, -1, -1): How to remove an element from a list in Python - A list in Python is a linear data structure where elements are stored in contiguous memory locations and elements This doesn't make sense as far as I can tell. It might be smart to also just create a new list if the current list item meets the desired criteria. iteration. We need to figure out what each element of your list is. so the 4th iteration done pointer moved onto the 5th Thats why your loop doesnt cover 65 since its moved into the previous index. list comprehension. I wrote a library that allows you to do this: It's best to use another method if possible that doesn't require modifying your iterable while iterating over it, but for some algorithms it might not be that straight forward. The last step is to convert the filter object to a list using the on removing items from a list while iterating in python, remove items when looping/iterating a list. This deletes or removes the element at the index passed as an argument in pop(). If you aren't removing or adding new items to the list when iterating over it, I took this low-level approach. But what if the result of the testFunc depends on the elements that have been added to newList already? Not the answer you're looking for? You need to take a copy of the list and iterate over it first, or the iteration will fail with what may be unexpected results. Instead of a comprehension, you could also use itertools. As an example, I will create a random list of tuples somelist = [(1,2,3), (4,5,6), (3,6,6), (7,8,9), (15,0,0), (10,11,12)]. From what I measured, NumPy starts to be faster for lists of more than 20 elements, and reaches >12x faster filtering for big lists of 1000 elements and more. How to remove an element from a list while traversing it? So how can we get remove() to work properly with your list? One possible solution, useful if you want not only remove some things, but also do something with all elements in a single loop: A for loop will be iterate through an index You have used a list variable called lis. Java Program to remove an element from List with ListIterator. Likewise, if the suite inserts an item in the sequence before the current item, the current item will be treated again the next time through the loop. The example checks if the current list item is less than or equal to 10 and if That said, the choice of, Note that this is likely time inefficient: if. How to remove an object from a list in Python? use del with an index as mentioned at: https://stackoverflow.com/a/1207485/895245. You can make a generator that returns everything that isn't removed: def newlist(somelist): How do I do the same sliced assignment with a dict? An internal counter is used to keep track of which item is used next, and this is incremented on each iteration. elements 2,3,4) from the list. You can learn more about the related topics by checking out the following Removing coordinates from list on python. How to remove an element from a list in Python? - Online Remove In my case having the objects in a dictionary instead of a list worked fine: The most effective method is list comprehension, many people show their case, of course, it is also a good way to get an iterator through filter. You could use a while loop instead. You should NEVER delete an element from a list while iterating over it in a for loop. The code below is one example of an algorithm that suffers from the above problem. del your_list[i] What if my list is huge and can't afford making a copy? The original object can be accessed as a property of the fluid object like so: More examples / tests can be found in the if __name__ is "__main__": section at the bottom of fluidIter.py. Therefore other solutions such as the list comprehensions mentioned in David Raznick's answer should be considered first. You can also use the range() class to The list.copy method returns a shallow I needed to do this with a huge list, and duplicating the list seemed expensive, especially since in my case the number of deletions would be few c How to remove list elements in a for loop in Python? It's simple and just another way to look at a problem @MarkAmery. So that's what we need to be passing to remove(). returns an enumerate object containing tuples where the first element is the @MarkAmery don't think you can alter the list this way. Removing items from a list while iterating over the list Instead, it merely referenced L2 to the same object as L1. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing, Most answers on this page don't really explain why removing elements while iterating over a list produces strange results, but the, Comments disabled on deleted / locked posts / reviews. Remove items from a Python list while iterating it | Techie so What I don't know is how efficient a couple of deletes are compared to copying a large list. The lambda function we passed to filter gets called with each element of the How to remove index list from another list in python? If you want to delete elements from a list while iterating, use a while-loop so you can alter the current index and end index after each deletion. at index 1. 50 and remove the elements that meet the condition. You can still use filter, moving to an outside function the element modification (iterating just once) def do_the_magic(x): You can try for-looping in reverse so for some_list you'll do something like: This way the index is aligned and doesn't suffer from the list updates (regardless whether you pop cur element or not). Why not rewrite it to be for element in somelist: How to remove an element from ArrayList in Java? Python: Removing list element while iterating over list The new list will only contain values that are greater than 100. This can lead to nasty bugs that can be avoided by making a temporary copy using a slice of the whole sequence, e.g.. The filter object only contains the elements for which the condition is met. The algorithm will reduce a list so that no element is a multiple of any other element. ChatGPT) is banned, Testing native, sponsored banner ads on Stack Overflow (starting July 6), Python: Removing list element while iterating over list. Big O time doesn't matter when dealing with lists of only a dozen items. For practical purposes, you can think of it as creating a reversed copy of its argument. The most important thing to note when removing items from a list in a for loop The range class is commonly used for looping a Wouldn't the original list. Above we have used the pop method on the fluid list object. Can you make it faster if you know only a few will be deleted, i.e., only delete those and leave the others in-place rather than re-writing them? The original fluidL object would still be used by the for loop but would become out of scope for us to modify. somelist[:] = (x for x in somelist if not check(x)) if check(element): @Paul: Since dicts are unordered, slices are meaningless for dicts. You need to take a copy of the list and iterate over it first, or the iteration will fail with what may be unexpected results. For example (depends The problem is you removed a value from a list during iteration and then your list index will collapse. I needed to do something similar and in my case the problem was memory - I needed to merge multiple dataset objects within a list, after doing some stuff with them, as a new object, and needed to get rid of each entry I was merging to avoid duplicating all of them and blowing up memory. In the final list we will only have those tuples whose sum is not equal to 15. Would it be possible for a civilization to create machines before wheels? Less memory usage, but it can be. The above code snippet shows that the remove(2) removes the first occurrence of element 2 ,i.e. @SamWatkins Yeah, this answer is for when you're removing a couple of elements from a very large array. In general, the list.copy() method is a little more readable than using a This is necessary because we aren't allowed to remove items from a list while iterating over it. The only condition is that you must only modify the list in place, if at any point fluidL or l were reassigned to a different list object the code would not work. The list comprehension in the example doesn't mutate the original list. Such as: Replacing a large sections of the list using a slice. Second is the list from which we want to delete elements It Were Patton's and/or other generals' vehicles prominently flagged with stars (and if so, why)? There is no installer so you will need to make sure that the file is in the python path your self. You should really just use comprehensions. Python Program to Remove All Occurrences of an Element in an Array/List, Python Remove Tuples from a List having every element as None, How to remove an element from an array in Java, Python Program to remove a specific digit from every element of the list. We may sometimes need to remove an element from a list in Python. The method returns a list containing the items for which the function returned Is religious confession legally privileged? That should be significantly faster than anything else. 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. if #condition : In this case, each one is a tuple. The value of i is not changed in the if block because you'll want to get value of the new item FROM THE SAME INDEX, once the old item is deleted. WebRemove items from a Python list while iterating it This post will discuss how to remove items from a Python List while iterating it. do_action(element) "The for statement", https://docs.python.org/2/reference/compound_stmts.html#for. How to iterate over a list, removing elements? The enumerate function takes an iterable and Find centralized, trusted content and collaborate around the technologies you use most. Agree It's assigning to a list slice that just happens to be the entire list, thereby replacing the list contents within the same Python list object, rather than just reseating one reference (from the previous list object to the new list object) like the other answers. Edit: The last code example in this answer gives a use case for why you might sometimes want to modify a list in place rather than use a list comprehension. First of all you'll need to replace foreach loop with while loop. use a linked list implementation/roll your own. This means that if the suite deletes the current (or a previous) item from the sequence, the next item will be skipped (since it gets the index of the current item which has already been treated). How to remove the last element from a set in Python? I had a use case where the list was quite long (110K items) and it was smarter to keep reducing the list instead. takes a function and an iterable as arguments and constructs an iterator from remove items from a list while iterating over it. def inplace(a): Here is an example where copying the list beforehand is incorrect, reverse iteration is impossible and a list comprehension is also not an option. Method 1.> Use the framework that you had suggested (where one fills in a code inside a for loop). You may be thinking of sorted(), which. This function is used to remove elements from index a(inclusive) to index b(not inclusive) in a list. The list can also be modified using slices (sort and reverse methods are not implemented). index, and the second is the item. Is there a linked list predefined library in Python? To remove list elements while iterating over it: Use a for loop to iterate over a copy of the list. Actually, I realized there's some cleverness here in that you make a copy of the list with an open slice (. You can use a list comprehension to create a new list containing only the elements you don't want to remove: Or, by assigning to the slice somelist[:], you can mutate the existing list to contain only the items you want: This approach could be useful if there are other references to somelist that need to reflect the changes. So, you delete the element I can think of three approaches to solve your problem. This can be ensured using a return or a break. Like enumerate(), you have to wrap it in list() to actually get a list out of it. You should definitely not do this for extremely large lists, since this involves first copying the entire list, and also performing an O(n) remove operation for each element being removed, making this an O(n^2) algorithm. specific number of times in for loops and takes the following arguments: We used a negative step to reverse the range. The above code removes the elements from index 2 to 5 (i.e. I probably think this is the most idiomatic way of removing the items from list. We used a for loop to I would like to see more community input on this algorithm. This behaviour will also be thread safe since your application is not mutating the variable. In my case I need to move those 'unwanted' elements into another list. If you want to do anything else during the iteration, it may be nice to get both the index (which guarantees you being able to reference it, for example if you have a list of dicts) and the actual list item contents. iterating over it. Method 1 and method 2 are faster than method 3. We used a list comprehension to remove the items from a list while iterating. There is an example (get the odds in the tuple): Caution: You can also not handle iterators. list. That being said, I have found times where this class has been useful to me and has been easier to use than keeping track of the indices of elements that need deleting. How to perfect forward variadic template args with default argument std::source_location? To meet these criteria: modify original list in situ, no list copies, only one pass, works , a traditional solution is to iterate backwards : for How to delete/remove an element from a C# array? The solution follows on from this answer (for a related question) from senderle. This is O(N*M) for arrays, it is very slow if you remove many items from a large list. So you shouldn't reference a list into another variable which still references the original instead of a copy. Official Python 2 tutorial 4.2. Make sure to import the filterfalse method as shown in the code sample. The output and the final reduced list are shown below. You might also see examples that use the my_list[:] syntax to get a shallow It turns out modifying the list while using a loop to iterate over it is a very bad idea without special care. false. Affordable solution to train a team and make them project ready. The first part of the answers serves as tutorial of how an array can be modified in place. Overview of workarounds Either: use a linked list implementation/roll your own. A linked list is the proper data structure to support efficient ite This is the right answer if performance is an issue (although same as @Alexey). The reason that (1, -2) remains in the list is that the locations of each item within the list changed between iterations of the for loop. It seems like that would only be a potential problem in multi-threaded applications, not single-threaded. Or using (and modifying) the same iterable in nested for loops. 6 min. The range object is not dependent on the list in any way, so removing items Let's run through what happens when we execute your code: The first problem is that you are passing both 'a' and 'b' to remove(), but remove() only accepts a single argument. For those who like functional programming: somelist[:] = filter(lambda tup: not determine(tup), somelist) @tonysepia glad to see this solution is still helpful :). And then the next value (65) move on to the previous index. By using this website, you agree with our Cookies Policy. Fortunately, it's extremely easy to get both the speed of list comprehensions AND the required semantics of in-place alterationjust code: Note the subtle difference with other answers: this one is not assigning to a barename. Removing elements from a list containing specific characters, Remove elements as you traverse a list in Python, Removing from a list while iterating over it, problem Deleting list items in a for loop (python). When are complicated trig functions used? The condition that I choose is sum of elements of a tuple = 15. lists). Method2 and method3 are more efficient than method1. than 100. is to use the list.copy() method to iterate over a copy of the list. Return a deep copy of x. In some situations, where you're doing more than simply filtering a list one item at time, you want your iteration to change while iterating. All the elements are removed from the list, but an empty list is left. Remove Element I couldn't understand any answers before this one. you don't have to create a copy. Generally, if you are doing it quick and dirty and don't want to add a custom LinkedList class, you just want to go for the faster .append() option by default unless memory is a big concern. In Python 2.6? We used the my_list[:] syntax to get a slice that represents the entire list, Reverse iterating avoids some of the pitfalls, but it is much more difficult to follow code that does that, so usually you're better off using a list comprehension or filter. Connect and share knowledge within a single location that is structured and easy to search. This is more space efficient since it dispenses the array copy, but it is less time efficient, because removal from dynamic arrays requires shifting all following items back by one, which is O(N). This part of the docs makes it clear that: If you need to modify the sequence you are iterating over while inside the loop (for example to duplicate selected items), it is recommended that you first make a copy. run into difficult to locate bugs. Your best approach for such an example would be a list comprehension somelist = [tup for tup in somelist if determine(tup)] while len(a) > 0: Or the elements still in oldList that might be added next? or from itertools import ifilterfalse Most of the answers here want you to create a copy of the list. I had a use case where the list was quite long (110K items) and it was smarter to k How to remove element from a list while iterating? List comp: results = [x for x in (do_action(element) for element in somelist) if check(element)] For the aforementioned example, time(method1) : time(method2) : time(method3) = 1 : 1 : 1.7, If you will use the new list later, you can simply set the elem to None, and then judge it in the later loop, like this. The answers suggesting list comprehensions are almost correctexcept that they build a completely new list and then give it the same name the o A linked list is the proper data structure to support efficient item removal, and does not force you to make space/time tradeoffs. This approach achieves the same result as using the. However, I disagree with this implementation, since .remove() has to iterate the entire list to find the value. Finally someone pointed out the actual documentation. Should work on all mutable sequences not just lists. The above code snippet shows that the pop(2) removes the element at index 2. So we modify your code and run it again: This code runs without any error, but let's look at the list it outputs: Why is (1,-2) still in your list? and to avoid having to re-code the entire project with the new lists name: copy.copy(x) There doesn't seem to be an explicit linked list type in the Python stdlib either: Python Linked List, Your best approach for such an example would be a list comprehension, In cases where you're doing something more complex than calling a determine function, I prefer constructing a new list and simply appending to it as I go. The main use case for when this is when trying to determine if an item should be removed (or added or moved) based not on just the item itself, but on the state of another item in the list or the state of the list as a whole. Elegant way to remove items from sequence in Python? If your want to replace the contents of dict. How to remove items from a list while iterating? "for Statements", https://docs.python.org/2/tutorial/controlflow.html#for-statements. We used the my_list[:] syntax to get a slice that represents the entire list. somelist.remove(item) Not exactly in-place, but some idea to do it: a = ['a', 'b'] There doesn't seem to be a linked list in the standard library however: start a new list() from scratch, and .append() back at the end as mentioned at: https://stackoverflow.com/a/1207460/895245. Learn more. the elements of the iterable for which the function returns a truthy value. I can't figure out how to remove the item in this fashion. Perhaps the underlying rationale is that Python lists are assumed to be dynamic array backed, and therefore any type of removal will be time inefficient anyways, while Java has a nicer interface hierarchy with both ArrayList and LinkedList implementations of ListIterator. Do you have any new comment about this solution? When this counter has reached the length of the sequence the loop terminates. (Ep. Why do keywords have to be reserved words? for element in somelist: This approach changes the contents of the original list. As I stated to start with: this is a complicated solution that will hurt the readability of your code and make it more difficult to debug. Enjoy unlimited access on 5500+ Hand Picked Quality Video Courses. In Python 2: The answers suggesting list comprehensions are almost correctexcept that they build a completely new list and then give it the same name the old list as, they do not modify the old list in place. This function removes the first occurrence of the element passed as argument in remove(). Most of the answers here want you to create a copy of the list. In recent versions of Python, you can do this even more cleanly by using the, reversed() does not create a new list, it creates a reverse iterator over the supplied sequence. Often clear and simple for future programmers to understand is far more valuable than performance. Python list remove and iterating over a list, Removing items for a list using a loop in Python, Twist? You can use a list comprehension to create a new list containing only the elements you don't want to remove: somelist = [x for x in somelist if n Tuples in python are very easy, they're simply made by enclosing values in parentheses. This function is used to remove all the elements from the list. Ok, I searched, what's this part on the inner part of the wing on a Cessna 152 - opposite of the thermometer, Calculating Triple Integral using Cylindrical Coordinates, If and When a Catholic Priest May Reveal Something from a Penitent's Confession. For example (depends on what type of list): You need to go backwards otherwise it's a bit like sawing off the tree-branch that you are sitting on :-), Python 2 users: replace range by xrange to avoid creating a hardcoded list. It returns a 'listreverseiterator' object when a list is passed to it. We aren't removing or adding items to the list while iterating, so creating a copy isn't necessary. represents the entire list. I prefer method2. Iterators are sometimes better than sequences. remove all None values from a list while do_action(element) Iterating over a sequence does not implicitly make a copy. if some_condition(somelist, i): Filter applies the passed function to each element in turn, and then decides whether to retain or discard the element depending on whether the function return value is True or False. Or, record the indices of all the elements you want to remove and then delete them after the iteration is complete inspectorG4dget May 16, https://stackoverflow.com/a/1207460/895245, https://stackoverflow.com/a/1207485/895245, Why on earth are people paying for digital real estate? 587), The Overflow #185: The hardest part of software is requirements, Starting the Prompt Design Site: A New Home in our Stack Exchange Neighborhood, Temporary policy: Generative AI (e.g. However, we can iterate over a copy of the list and remove items from the Following is the code for that: Method 3.> Find indices where the given condition is met, and then use remove elements (tuples) corresponding to those indices. What should I use in place of code_to_remove_tup? The other answers are correct that it is usually a bad idea to delete from a list that you're iterating. Alternatively, you can use a So you can try list comprehension instead. Copyright Tutorials Point (India) Private Limited. It seems like this particular Python API could be improved. It's easy to understand and appears to be overlooked by the contributors! Remove items from a List while iterating using a list comprehension, Remove items from a List while iterating using filter(), Removing items from a List while iterating over it with range(), Removing items from a list while iterating with filterfalse(), remove items from a list while iterating over it, For or While loop to print Numbers from 1 to 10 in Python, Using multiple variables in a For loop in Python, Using a For or While Loop to take user input in Python, An integer representing the start of the range (defaults to, Go up to, but not including the provided integer, Range will consist of every N numbers from.