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Java Forum / General / August 2007How do you iterate over a List and remove elements?
I've got an ArrayList and I want to iterate over the list and remove the occasional element. If I use the new for loop it throws a ConcurrentModificationException. An Iterator is going to have the same problem. There must be some simple way to do this that I'm not thinking of tonight. Thanks,  SignatureKnute Johnson email s/nospam/knute/ > I've got an ArrayList and I want to iterate over the list and remove the > occasional element. If I use the new for loop it throws a > ConcurrentModificationException. An Iterator is going to have the same > problem. There must be some simple way to do this that I'm not thinking > of tonight. Use an explicit Iterator-based loop, and the Iterator's remove method, not the underlying collection's remove. Patricia > > I've got an ArrayList and I want to iterate over the list and remove the > > occasional element. If I use the new for loop it throws a [quoted text clipped - 6 lines] > > Patricia In other words: Iterator i = l.iterator(); while (i.hasNext()) { if (shouldRemove(i.next()) { i.remove(); } } You can remove as many elements as you need to in the same iteration. Fred > I've got an ArrayList and I want to iterate over the list and remove > the occasional element. If I use the new for loop it throws a > ConcurrentModificationException. An Iterator is going to have the > same problem. The ConcurrentModificationException occurs because you're modifying the List behind the Iterator's back, so to speak. If you use the Iterator to do the removes you'll be fine. > I've got an ArrayList and I want to iterate over the list and remove the > occasional element. If I use the new for loop it throws a > ConcurrentModificationException. An Iterator is going to have the same The API docs say that ConcurrentModificationException is thrown when you try to delete an elment in the array list while the iterator is busy traversing the elements inside it. > problem. There must be some simple way to do this that I'm not thinking > of tonight. Yes indeed there is. One way to do it is if you can find an element that you'd like to delete then check it first, delete it and then break out. Iterator i = l.iterator(); while(i.hasNext()) { if (i.next().equals("a")) { //the key point is breaking out here l.remove("a");break; } } If you want to delete the whole list, why traverse it in the first place? ;) hth SignatureCheers, Ishwor Gurung /* humpty dumpty */ >> I've got an ArrayList and I want to iterate over the list and remove the >> occasional element. If I use the new for loop it throws a [quoted text clipped - 9 lines] > Yes indeed there is. One way to do it is if you can find an element that > you'd like to delete then check it first, delete it and then break out. The ConcurrentModificationException is caused by the Iterator "noticing" that you've changed something under its feet. The proper solution is to "play nice" with the iterator, and remove the element AND keep the iterator informed. http://java.sun.com/j2se/1.4.2/docs/api/java/util/Iterator.html#remove() BugBear On Aug 23, 11:13 pm, Knute Johnson <nos...@rabbitbrush.frazmtn.com> wrote: > I've got an ArrayList and I want to iterate over the list and remove the > occasional element. If I use the new for loop it throws a [quoted text clipped - 8 lines] > Knute Johnson > email s/nospam/knute/ If you are removing one occassional element, you can just remove it without iterating (or may be I did not get the situation). Something like: synchornize(list){ if(list.contains(obj){ list.remove(obj); } } -cheers, Manish > if(list.contains(obj){ > list.remove(obj); > } Collections let you do coll.remove(obj) whether or not it's definitely in there to remove, and return a boolean in case you want to do something different depending on whether it was there to remove or it wasn't. This is more efficient as the linked list, array, hash table, tree, or other backing data structure gets searched only once instead of twice. > On Aug 23, 11:13 pm, Knute Johnson <nos...@rabbitbrush.frazmtn.com> > wrote: [quoted text clipped - 20 lines] > } > } That technique works if you want to remove an item with a specified value. Suppose, for example, you want to remove every string shorter than 3 characters from a List<String>. It can be done easily using the Iterator remove method. What is the purpose of the list.contains(obj) pre-check? Patricia > What is the purpose of the list.contains(obj) pre-check? My bad..did not realize contains returns a boolean. Been thinking about this for a while, and I was wondering if it makes sense to use an array (or another list..a clone) as an intermediatory. I know this has scalability and performance numbers against it though. Here is a little snippet: ArrayList<Integer> list = new ArrayList<Integer>(); for(int i=0; i<10; i++){ list.add(i); } //remove all the even numbers Integer[] array = new Integer[list.size()]; list.toArray(array); for( int i:array ){ if( i%2==0 ) { list.remove(new Integer(i)); } } System.out.println(list); Ofcourse this needs to be synchronized, to make sure the array and the list do not go out of sync. -cheers, Manish >> What is the purpose of the list.contains(obj) pre-check? > [quoted text clipped - 23 lines] > -cheers, > Manish What is the advantage of this, compared to the really dumb, simple, obvious approach using the Iterator remove() method? //remove all the even numbers Iterator<Integer> it = list.iterator(); while(it.hasNext()){ if(it.next() % 2 == 0){ it.remove(); } } Patricia > What is the advantage of this, compared to the really dumb, simple, > obvious approach using the Iterator remove() method? [quoted text clipped - 6 lines] > } > } Not a whole lot except that it is not guaranteed that the Iterator will support remove(). -cheers, Manish >> What is the advantage of this, compared to the really dumb, simple, >> obvious approach using the Iterator remove() method? [quoted text clipped - 9 lines] > Not a whole lot except that it is not guaranteed that the Iterator > will support remove(). It is for ArrayList, and any other List that inherits the AbstractList iterator method and itself supports remove(int): "This implementation returns a straightforward implementation of the iterator interface, relying on the backing list's size(), get(int), and remove(int) methods." Patricia >> What is the advantage of this, compared to the really dumb, simple, >> obvious approach using the Iterator remove() method? [quoted text clipped - 9 lines] > Not a whole lot except that it is not guaranteed that the Iterator > will support remove(). Nor is it guaranteed that a Collection will support remove() (e.g., unmodifiableList). The number of implementations where the list supports remove() and the iterator doesn't is probably about zero. > -cheers, > Manish SignatureBeware of bugs in the above code; I have only proved it correct, not tried it. -- Donald E. Knuth > I've got an ArrayList and I want to iterate over the list and remove the > occasional element. If I use the new for loop it throws a [quoted text clipped - 3 lines] > > Thanks, Thanks everybody. That's what I was looking for. SignatureKnute Johnson email s/nospam/knute/ On Thu, 23 Aug 2007 23:13:08 -0700, Knute Johnson <nospam@rabbitbrush.frazmtn.com> wrote, quoted or indirectly quoted someone who said : >I've got an ArrayList and I want to iterate over the list and remove the >occasional element. If I use the new for loop it throws a >ConcurrentModificationException see http://mindprod.com/jgloss/jcheat.html#LOOPS SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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