Message Automatically Removed

Brisingr Aerowing

Andrew Rissing

I'm not sure if you were showcasing a weird or a wonderful here, because the second method won't perform as expected.

Brisingr Aerowing wrote:

public static void RemoveAllBut(this ICollection source, params object[] notRemove) { RemoveAllBut<object>(source.OfType<object>().ToArray(), notRemove); }

Richard Deeming

With any decent implementation of ICollection<T> your method will produce an InvalidOperationException with the message: "Collection was modified; enumeration operation may not execute." You'll need to store the items to remove in a separate list, and remove them outside of any enumeration of the source list:

public static void RemoveAllBut(this ICollection source, params T[] notRemove)
{
var itemsToRemove = source.Except(notRemove).ToList();
itemsToRemove.ForEach(item => source.Remove(item));
}

As Andrew pointed out, your non-generic version won't work. It would need to be:

public static void RemoveAllBut(this IList source, params object[] notRemove)
{
var itemsToRemove = source.Cast().Except(notRemove).ToList();
itemsToRemove.ForEach(source.Remove);
}

"These people looked deep within my soul and assigned me a number based on the order in which I joined."
** - Homer**

Gary Wheeler

At least in .NET 3.5, your first function will throw an exception, since you are modifying the collection being enumerated.

Software Zen: delete this;

TnTinMn

In addition to the comments above, each of those Remove statements will incur an array copy cost. Count the occurrence of each item in notRemove, clear the collection, then add the items in notRemove by their respective counts. Possibly something like this:

public static void RemoveAllBut<T>(this ICollection<T> source, params T[] notRemove)
{
Int32[] counts = new Int32[notRemove.Length];
EqualityComparer<T> comparer = EqualityComparer<T>.Default;

foreach (T itm in source)
{
    for (Int32 i = 0; i <= counts.Length - 1; i++)
    {
        if (comparer.Equals(notRemove\[i\], itm))
        {
            counts\[i\] += 1;
            break;
        }
    }
}
source.Clear();
for (Int32 i = 0; i <= counts.Length - 1; i++)
{
    for (Int32 j = 1; j <= counts\[i\]; j++)
    {
        source.Add(notRemove\[i\]);
    }
}

}

Lutoslaw

Posting in this particular forum triggers a find-what-is-bad thinking. Sorry. ;) 1. It's O(n^3) which is not good. Did you consider working on sorted collections? Then it would be just O(n). (more precisely, O(max{n,m})). Why O(n^3): 1. foreach loop 2. Contains method which searches notRemove 3. Remove(itm) method which has to find index of itm (in worst case, n calls to Equals method) and, if it's an array list, it has to shift all elements by one which makes it even worse. Besides, I though that you cannot edit a collection inside foreach. 2. I method 2: Why copy all elements to a new array? Without it, it would be memory complexity of Ω(1), in situ operation. Now it's Ω(n) because it allocates a new array in the process. 3. Formatting horror, but maybe it was screwed up during pastying*.

   if (notRemove.Contains(itm)) continue;
       source.Remove(itm);

Shouldn't it be:

   if (notRemove.Contains(itm))
        continue;
   source.Remove(itm);

IF it is known that all items in notRemoves appear in the source exactly once, then what about:

public static void RemoveAllBut<T>(this ICollection<T> source, params T[] notRemove)
{
source.Clear();
foreach(var x in notRemove)
source.Add(x);
// because we don't have source.AddAll, unfortunately
}

which is O(m) and Ω(1). * -- "pastying" - is it a correct spelling? (derived from a verb "paste").

Greetings - Jacek

PIEBALDconsult

Yeah, what they said. Plus I'd avoid ToArray. And see whether or not a HashSet might be of use: HashSet<T>.IntersectWith Method[^] Or not, whatever. :sigh:

Richard Deeming

Jacek Gajek wrote:

* -- "pastying" - is it a correct spelling? (derived from a verb "paste").

The word you're looking for is "pasting". However, I like the idea of using "pastying" when referring to anything connected to the Cornish foodstuff[^]. :)

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

Lutoslaw

Thanks. Finally I know it. I didn't write "pasting" because it seemed to be related to "the past" and made no sense to me (or did it?).

Greetings - Jacek

lewax00

Brisingr Aerowing wrote:

Message Automatically Manually Removed

FTFY ;P

ZurdoDev

There was no need to remove your message. Your idea was a good one but I am guessing you didn't test it. And with all of the egos on this site (not saying anyone who responded necessarily has one) if you say something wrong or that can be perceived as wrong there are many who jump at the opportunity to attack. Lots of vultures. :)

There are only 10 types of people in the world, those who understand binary and those who don't.

lewax00

And in this case, most of the posts looked like they were trying to help (except mine ;P ).

pkfox

As CM says "Leave the egos at the door" When the going gets weird the weird turn pro - Hunter S Thompson RIP