How can I check if a Queue is empty?

Question

In C#, how can I check if a Queue is empty?

I want to iterate through the Queue's elements, and I need to know when to stop. How can I accomplish this?

Brickwork

which queue you use? please post the queue definition. – Daryl

Answer 1

Assuming you mean Queue<T> you could just use:

if (queue.Count != 0)

But why bother? Just iterate over it anyway, and if it's empty you'll never get into the body:

Queue<string> queue = new Queue<string>();

// It's fine to use foreach...
foreach (string x in queue)
{
    // We just won't get in here...
}

Slothful

Note, the foreach approach doesn't work if you need to add any items to the queue as you are processing the queue (e.g. if you are implementing BFS algorithm) because then the enumerator would be invalidated. Using the Count property is the right way for this scenario. – Dashiell

@DSO wouldn't the enumerator also be invalidated if you are removing items from the queue? – Glabrous

So what's the point of using Queue if you are not removing the elements by Dequeue? – Delayedaction

@Jakotheshadows, in OP's question, he isn't removing items from the queue, he is just iterating over what is in the queue. Which is a rather strange use of Queue, IMHO. (As titol comments, if you don't have a situation that needs Dequeue, then why are you using a queue?) – Partlet

@ToolmakerSteve: It was the OP who said they wanted to iterate over the queue, and never mentioned removing anything from it. I was only answering the question that was asked... – Slothful

Sorry, I realized that a minute later, and edited my answer just as you were pointing that out! – Partlet

Why guard against empty and not use foreach? recursion. – Esbenshade

@maxpleaner: Sorry, I don't understand your point at all. Please clarify where recursion is relevant here - in particular why it affects whether or not it's reasonable to iterate over an empty sequence (so you never enter the body of the loop). – Slothful

@JonSkeet i'm pretty new to C# admittedly. But say there's a method which takes a queue as a parameter. It dequeues an element and then calls itself with the modified queue. It needs to treat an empty queue parameter as a base case. Since it seems that Dequeue cannot be optimistically run (it raised an error when I ran it on an empty queue) i don't see another way to do it than to check the Count property. – Esbenshade

Answer 2

I would suggest using the Any() method, as this will not do a count on the entire queue, which will be better in terms of performance.

Queue myQueue = new Queue();
    if(myQueue.Any()){
      //queue not empty
    }

Egin

@GregoryBad: myQueue.Count will not do a count over the entire queue. The size is stored in a private variable in the Queue class which the Count property just returns see Queue<T>.Count Property. The Count you mean is Count() in the linq namespace. Enumerable.Count – Eliezer

@Eliezer Even the Enumerable.Count() method is optimized to check if it can cast to ICollection and return the .Count property without enumerating. – Pleuron

@AndrewPalmer It is now indeed ;). I don't know if that optimization was already in place when I commented 3 years ago :). – Eliezer

Even if the two approaches are now equivalent, in terms of performance, it's good to know that Queue.Any() is available for use. – Greave

Answer 3

Assuming you meant System.Collections.Generic.Queue<T>

if(yourQueue.Count != 0) { /* Whatever */ }

should do the trick.

Ernaline

Answer 4

    Queue test = new Queue();
    if(test.Count > 0){
      //queue not empty
    }

Interweave

Answer 5

There is an extension method .Count() that is available because Queue implements IEnumerable.

You can also do _queue.Any() to see if there are any elements in it.

Rhizopod

The second is bad, the first is horrible. Use the queue.Count property. – Carborundum

Answer 6

You can check if its Count property equals 0.

Overzealous

Answer 7

YourQueue.GetEnumerator().MoveNext() == true

Mitzvah

Answer 8

TryPeek() will let you check whether Queue has an element or not.

Queue q = new Queue();
if (!q.TryPeek(out object i)) {
    /* Queue is empty */
    ....
}

Fulgurite