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c#queue
Alex
Alex

Reputation: 3159

c# Adding a Remove(int index) method to the .NET Queue class

I would like to use the generic queue class as described in the .NET framework (3.5) but I will need a Remove(int index) method to remove items from the queue. Can I achieve this functionality with an extension method? Anyone care to point me in the right direction?

Upvotes: 32

Views: 58067

Answers (12)

Kind Contributor
Kind Contributor

Reputation: 18583

Although there isn't a built-in way, you shouldn't use a List structure or other structure, IFF RemoveAt isn't a frequent operation.

If you are normally enqueuing and dequeuing but only occasionally removing, then you should be able to afford a queue rebuild when removing.

public static void Remove<T>(this Queue<T> queue, T itemToRemove) where T : class
{
    var list = queue.ToList(); //Needs to be copy, so we can clear the queue
    queue.Clear();
    foreach (var item in list)
    {
        if (item == itemToRemove)
            continue;

        queue.Enqueue(item);
    }
}

public static void RemoveAt<T>(this Queue<T> queue, int itemIndex) 
{
    var list = queue.ToList(); //Needs to be copy, so we can clear the queue
    queue.Clear();

    for (int i = 0; i < list.Count; i++)
    {
        if (i == itemIndex)
            continue;

        queue.Enqueue(list[i]);
    }
}

The following approach might be more efficient, using less memory, and thus less GC:

public static void RemoveAt<T>(this Queue<T> queue, int itemIndex)
{
    var cycleAmount = queue.Count;

    for (int i = 0; i < cycleAmount; i++)
    {
        T item = queue.Dequeue();
        if (i == itemIndex)
            continue;

        queue.Enqueue(item);
    }
}

Upvotes: 8

Steve Greene
Steve Greene

Reputation: 502

Note that with a list you can make the "removal" process more efficient if you don't actually remove the item but merely "mark" it as "removed". Yes, you have to add a bit of code to deal with how you've done it, but the payoff is the efficiency.

Just as one example - Say you have a List<string>. Then you can, for example, just set that particular item to null and be done with it.

Upvotes: 0

Alex from Jitbit
Alex from Jitbit

Reputation: 60702

Here's how you remove a specific item from the queue with one line of Linq (it's recreating the queue, BUT for the lack of a better method...)

//replace "<string>" with your actual underlying type
myqueue = new Queue<string>(myqueue.Where(s => s != itemToBeRemoved));

I know it's not removing by index, but still, someone might find this useful (this question ranks in Google for "remove specific item from a c# queue" so I decided to add this answer, sorry)

Upvotes: 18

Adriano Repetti
Adriano Repetti

Reputation: 67090

It's a pretty late answer but I write it for future readers

List<T> is exactly what you need but it has a big disadvantage when compared to Queue<T>: it's implemented with an array then Dequeue() is pretty expansive (in terms of time) because all items must be shifted one step back with Array.Copy. Even Queue<T> uses an array but together with two indices (for head and tail).

In your case you also need Remove/RemoveAt and its performance won't be good (for the same reason: if you're not removing from list tail then another array must be allocated and items copied).

A better data structure to have quick Dequeue/Remove time is a linked list (you'll sacrifice - a little bit - performance for Enqueue but assuming your queue has an equal number of Enqueue/Dequeue operations you'll have a great gain in performance, especially when its size will grow).

Let's see a simple skeleton for its implementation (I'll skip implementation for IEnumerable<T>, IList<T> and other helper methods).

class LinkedQueue<T>
{
    public int Count
    {
        get { return _items.Count; }
    }

    public void Enqueue(T item)
    {
        _items.AddLast(item);
    }

    public T Dequeue()
    {
        if (_items.First == null)
           throw new InvalidOperationException("...");

        var item = _items.First.Value;
        _items.RemoveFirst();

        return item;
    }

    public void Remove(T item)
    {
        _items.Remove(item);
    }

    public void RemoveAt(int index)
    {
        Remove(_items.Skip(index).First());
    }

    private LinkedList<T> _items = new LinkedList<T>();
}

For a quick comparison:

           Queue            List              LinkedList
Enqueue    O(1)/O(n)*       O(1)/O(n)*        O(1)
Dequeue    O(1)             O(n)              O(1)
Remove     n/a              O(n)              O(n)

* O(1) is typical case but sometimes it'll be O(n) (when internal array need to be resized).

Of course you'll pay something for what you gain: memory usage is bigger (especially for small T overhead will be great). Right implementation (List<T> vs LinkedList<T>) must be chosen carefully according to your usage scenario, you may also convert that code to use a single linked list to reduce 50% of memory overhead.

Upvotes: 12

Lukazoid
Lukazoid

Reputation: 19426

I do not believe we should be using List<T> to emulate a queue, for a queue, the enqueue and dequeue operations should be very highly performant, which they would not be when using a List<T>. For the RemoveAt method however, it is acceptable to be non-performant, as it is not the primary purpose of a Queue<T>.

My approach at implementing RemoveAt is O(n) but the queue still maintains a largely O(1) enqueue (sometimes the internal array needs reallocating which makes the operations O(n)) and always O(1) dequeue.

Here is my implementation of a RemoveAt(int) extension method for a Queue<T>:

public static void RemoveAt<T>(this Queue<T> queue, int index)
{
    Contract.Requires(queue != null);
    Contract.Requires(index >= 0);
    Contract.Requires(index < queue.Count);

    var i = 0;

    // Move all the items before the one to remove to the back
    for (; i < index; ++i)
    {
        queue.MoveHeadToTail();
    }

    // Remove the item at the index
    queue.Dequeue();

    // Move all subsequent items to the tail end of the queue.
    var queueCount = queue.Count;
    for (; i < queueCount; ++i)
    {
        queue.MoveHeadToTail();
    }
}

Where MoveHeadToTail is defined as follows:

private static void MoveHeadToTail<T>(this Queue<T> queue)
{
    Contract.Requires(queue != null);

    var dequed = queue.Dequeue();
    queue.Enqueue(dequed);
}

This implementation also modifies the actual Queue<T> rather than returning a new Queue<T> (which I think is more in-keeping with other RemoveAt implementations).

Upvotes: 3

sfuqua
sfuqua

Reputation: 5853

If the Queue is being used to preserve the order of the items in the collection, and if you will not have duplicate items, then a SortedSet might be what you are looking for. The SortedSet acts much like a List<T>, but stays ordered. Great for things like drop down selections.

Upvotes: 2

casperOne
casperOne

Reputation: 74530

What you want is a List<T> where you always call RemoveAt(0) when you want to get the item from the Queue. Everything else is the same, really (calling Add would add an item to the end of the Queue).

Upvotes: 26

rollercodester
rollercodester

Reputation: 161

Combining both casperOne's and David Anderson's suggestions to the next level. The following class inherits from List and hides the methods that would be detrimental to the FIFO concept while adding the three Queue methods (Equeue, Dequeu, Peek).

public class ListQueue<T> : List<T>
{
    new public void Add(T item) { throw new NotSupportedException(); }
    new public void AddRange(IEnumerable<T> collection) { throw new NotSupportedException(); }
    new public void Insert(int index, T item) { throw new NotSupportedException(); }
    new public void InsertRange(int index, IEnumerable<T> collection) { throw new NotSupportedException(); }
    new public void Reverse() { throw new NotSupportedException(); }
    new public void Reverse(int index, int count) { throw new NotSupportedException(); }
    new public void Sort() { throw new NotSupportedException(); }
    new public void Sort(Comparison<T> comparison) { throw new NotSupportedException(); }
    new public void Sort(IComparer<T> comparer) { throw new NotSupportedException(); }
    new public void Sort(int index, int count, IComparer<T> comparer) { throw new NotSupportedException(); }

    public void Enqueue(T item)
    {
        base.Add(item);
    }

    public T Dequeue()
    {
        var t = base[0]; 
        base.RemoveAt(0);
        return t;
    }

    public T Peek()
    {
        return base[0];
    }
}

Test code:

class Program
{
    static void Main(string[] args)
    {
        ListQueue<string> queue = new ListQueue<string>();

        Console.WriteLine("Item count in ListQueue: {0}", queue.Count);
        Console.WriteLine();

        for (int i = 1; i <= 10; i++)
        {
            var text = String.Format("Test{0}", i);
            queue.Enqueue(text);
            Console.WriteLine("Just enqueued: {0}", text);
        }

        Console.WriteLine();
        Console.WriteLine("Item count in ListQueue: {0}", queue.Count);
        Console.WriteLine();

        var peekText = queue.Peek();
        Console.WriteLine("Just peeked at: {0}", peekText);
        Console.WriteLine();

        var textToRemove = "Test5";
        queue.Remove(textToRemove);
        Console.WriteLine("Just removed: {0}", textToRemove);
        Console.WriteLine();

        var queueCount = queue.Count;
        for (int i = 0; i < queueCount; i++)
        {
            var text = queue.Dequeue();
            Console.WriteLine("Just dequeued: {0}", text);
        }

        Console.WriteLine();
        Console.WriteLine("Item count in ListQueue: {0}", queue.Count);

        Console.WriteLine();
        Console.WriteLine("Now try to ADD an item...should cause an exception.");
        queue.Add("shouldFail");

    }
}

Upvotes: 16

wwe
wwe

Reputation: 1

The queue class is so difficult to understand. Use a generic list instead.

Upvotes: -3

RvdK
RvdK

Reputation: 19790

David Anderson's solution is probably the best but has some overhead. Are you using custom objects in the queue? if so, add a boolean like cancel

Check with your workers that process the queue if that boolean is set and then skip it.

Upvotes: 1

Anton Gogolev
Anton Gogolev

Reputation: 115769

In fact, this defeats the whole purpose of Queue and the class you'll eventually come up with the will violate the FIFO semantics altogether.

Upvotes: 2

David Anderson
David Anderson

Reputation: 13670

Someone will probably develop a better solution, but from what I see you will need to return a new Queue object in your Remove method. You'll want to check if the index is out of bounds and I may have got the ordering of the items being added wrong, but here's a quick and dirty example that could be made into an extension quite easily.

public class MyQueue<T> : Queue<T> {
    public MyQueue() 
        : base() {
        // Default constructor
    }

    public MyQueue(Int32 capacity)
        : base(capacity) {
        // Default constructor
    }

    /// <summary>
    /// Removes the item at the specified index and returns a new Queue
    /// </summary>
    public MyQueue<T> RemoveAt(Int32 index) {
        MyQueue<T> retVal = new MyQueue<T>(Count - 1);

        for (Int32 i = 0; i < this.Count - 1; i++) {
            if (i != index) {
                retVal.Enqueue(this.ElementAt(i));
            }
        }

        return retVal;
    }
}

Upvotes: 7

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