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Queue is an abstract data structure, somewhat similar to Stacks. Unlike stacks, a queue is open at both its ends. One end is always used to insert data (enqueue) and the other is used to remove data (dequeue). Queue follows First-In-First-Out methodology, i.e., the data item stored first will be accessed first.

A real-world example of queue can be a single-lane one-way road, where the vehicle enters first, exits first. More real-world examples can be seen as queues at the ticket windows and bus-stops.

As we now understand that in queue, we access both ends for different reasons. The following diagram given below tries to explain queue representation as data structure −

As in stacks, a queue can also be implemented using Arrays, Linked-lists, Pointers and Structures. For the sake of simplicity, we shall implement queues using one-dimensional array.

Queue operations may involve initializing or defining the queue, utilizing it, and then completely erasing it from the memory. Here we shall try to understand the basic operations associated with queues −

**enqueue()**− add (store) an item to the queue.**dequeue()**− remove (access) an item from the queue.

Few more functions are required to make the above-mentioned queue operation efficient. These are −

**peek()**− Gets the element at the front of the queue without removing it.**isfull()**− Checks if the queue is full.**isempty()**− Checks if the queue is empty.

In queue, we always dequeue (or access) data, pointed by **front** pointer and while enqueing (or storing) data in the queue we take help of **rear** pointer.

Let's first learn about supportive functions of a queue −

This function helps to see the data at the **front** of the queue. The algorithm of peek() function is as follows −

**Algorithm**

begin procedure peek

return queue[front]

end procedure

Implementation of peek() function in C programming language −

**Example**

int peek() {

return queue[front];

}

As we are using single dimension array to implement queue, we just check for the rear pointer to reach at MAXSIZE to determine that the queue is full. In case we maintain the queue in a circular linked-list, the algorithm will differ. Algorithm of isfull() function −

**Algorithm**

begin procedure isfull

if rear equals to MAXSIZE

return true

else

return false

endif

end procedure

Implementation of isfull() function in C programming language −

**Example**

bool isfull() {

if(rear == MAXSIZE - 1)

return true;

else

return false;

}

Algorithm of isempty() function −

**Algorithm**

begin procedure isempty

if front is less than MIN OR front is greater than rear

return true

else

return false

endif

end procedure

If the value of **front** is less than MIN or 0, it tells that the queue is not yet initialized, hence empty.

Here's the C programming code −

**Example**

bool isempty() {

if(front < 0 || front > rear)

return true;

else

return false;

}

Queues maintain two data pointers, **front** and **rear**. Therefore, its operations are comparatively difficult to implement than that of stacks.

The following steps should be taken to enqueue (insert) data into a queue −

**Step 1**− Check if the queue is full.**Step 2**− If the queue is full, produce overflow error and exit.**Step 3**− If the queue is not full, increment**rear**pointer to point the next empty space.**Step 4**− Add data element to the queue location, where the rear is pointing.**Step 5**− return success.

Sometimes, we also check to see if a queue is initialized or not, to handle any unforeseen situations.

**Algorithm for enqueue operation**

procedure enqueue(data)

if queue is full

return overflow

endif

rear ← rear + 1

queue[rear] ← data

return true

end procedure

Implementation of enqueue() in C programming language −

**Example**

int enqueue(int data)

if(isfull())

return 0;

rear = rear + 1;

queue[rear] = data;

return 1;

end procedure

Accessing data from the queue is a process of two tasks − access the data where **front** is pointing and remove the data after access. The following steps are taken to perform **dequeue** operation −

**Step 1**− Check if the queue is empty.**Step 2**− If the queue is empty, produce underflow error and exit.**Step 3**− If the queue is not empty, access the data where**front**is pointing.**Step 4**− Increment**front**pointer to point to the next available data element.**Step 5**− Return success.

**Algorithm for dequeue operation**

procedure dequeue

if queue is empty

return underflow

end if

data = queue[front]

front ← front + 1

return true

end procedure

Implementation of dequeue() in C programming language −

**Example**

int dequeue() {

if(isempty())

return 0;

int data = queue[front];

front = front + 1;

return data;

}

**Applications of Queue:**

Queue is used when things don’t have to be processed immediatly, but have to be processed in **F**irst **I**n**F**irst **O**ut order. This property of Queue makes it also useful in following kind of scenarios.

**1)** When a resource is shared among multiple consumers. Examples include CPU scheduling, Disk Scheduling.**2) **When data is transferred asynchronously (data not necessarily received at same rate as sent) between two processes. Examples include IO Buffers, pipes, file IO, etc.

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