Queues

Queues

To help represent real-world data, C# provides another kind of data structure called a queue.

Much like List and Dictionary, Queue is named for its ability to append data to the end of a list and remove data from the beginning. You can think of this as a queue of people waiting to be served by a teller. Another term for this kind of data structure is "first-in, first-out."

Not random access

Unlike a Listor Dictionary, a Queue does not allow random access to its elements. That is we cannot use [] to access any element we want.

We may only use Enqueue and Dequeue to add and remove data from the queue.

Enqueue

To add elements to the end of a queue, we use the Enqueue method. Much like Add appends things to a List, Enqueue adds things to a Queue. However, we can only add to the end of the queue.

var students = new Queue<string>();
students.Enqueue("Mary");
students.Enqueue("Bill");
students.Enqueue("Paul");
students.Enqueue("Sandra");
students.Enqueue("Thomas");

Now our students queue contains five elements.

Looping through a queue

Much like our List and Dictionary we can use the foreach method to loop through the elements of a queue.

foreach (var student in students)
{
        Console.WriteLine($"Hello {student}");
}

But if we try to access a Queue element using [], we will get an error.

// This code will not work, it won't even compile.
var student = students[0];

Removing an element

We can use Dequeue to remove an element from the beginning of a queue. We can only remove the first element in a queue.

var student = students.Dequeue();
Console.WriteLine($"Goodbye {student}");
Console.WriteLine($"There are now {students.Count} students in the queue.");

This will print Goodbye Mary and There are now 4 students in the queue.

Peeking in on a queue

We can also see what element is at the beginning of a queue using the Peek method. While Dequeue removes an element from the queue, Peek will not.

var student = students.Peek();
Console.WriteLine($"You are up next {student}");

This will print You are up next Bill and There are now 4 students in the queue. since we didn't remove Bill.

Does a queue contain a specific element?

We can also use Contains to check if a queue contains a specific element.

var isThomasThere = students.Contains("Thomas");
Console.WriteLine($"Thomas is there? {isThomasThere}");

Clear out a queue

students.Clear();
Console.WriteLine($"There are now {students.Count} students in the queue.");

This will print There are now 0 students in the queue.