By Safa Mulani and Anish Singh Walia
A queue in C is basically a linear data structure
to store and manipulate the data elements. It follows the order of First In First Out (FIFO).
In queues, the first element entered into the array is the first element to be removed from the array.
For example, let’s consider the scenario of a bus-ticket booking stall. Here, the fashion of a C programming queue is followed. The tickets are distributed on the first-come-first-serve basis i.e. the first one to enter is the first one to be served with the tickets.
A queue is open at both ends. One end is provided for the insertion of data and the other end for the deletion of data.
A queue can be implemented with any programming language such as C, Java, Python, etc.
A queue being an Abstract Data Structure provides the following operations for manipulation on the data elements:
isEmpty()
: To check if the queue is emptyisFull()
: To check whether the queue is full or notdequeue()
: Removes the element from the frontal side of the queueenqueue()
: It inserts elements to the end of the queueFront
: Pointer element responsible for fetching the first element from the queueRear
: Pointer element responsible for fetching the last element from the queueQueue follows the First-In-First-Out pattern. The first element is the first to be pulled out from the list of elements.
Front
and Rear
pointers keep the record of the first and last element in the queue.Front = -1
and Rear = -1
Queues in C can be implemented using Arrays, Lists, Structures, etc. Below here we have implemented queues using Arrays in C.
Example:
Output:
Stack Data Structure can be used to implement the operations of the queue. We’ll need two stacks to implement a queue using them. Before you work through the examples below, make sure you understand the functioning of stacks very well.
A queue can be implemented using Stacks by either of the following ways:
Let us assume two stacks: S1 and S2 to implement queue operations using the same.
Note: The time complexity of the enqueue operation would be O(n).
Note: The time complexity of the dequeue operation would be O(1).
If you analyze the time complexities of the Enqueue and Dequeue operations using Stack, you’ll find out that the Enqueue operation is much costlier than the Dequeue operation.
Thus, as mentioned above, we make the first entered or the oldest entered element to remain at the top of Stack S1 so that it gets removed when the Dequeue operation is invoked.
Let us again assume two Stacks: S1 and S2 to implement queue operations using the same.
Queues have numerous applications in various fields, including simulations, scheduling, and task management. Here are some examples:
In simulations, queues can be used to model real-world scenarios where entities are processed in a First-In-First-Out (FIFO) order. For instance, in a simulation of a bank, customers can be represented as elements in a queue, and the bank tellers can be thought of as the processing units that serve the customers in the order they arrive.
Example in C:
In scheduling, queues can be used to manage tasks or processes that need to be executed in a specific order. For example, in an operating system, processes can be placed in a queue and executed by the CPU in the order they were received. This ensures that each process gets a fair share of the CPU time and prevents any single process from monopolizing the CPU.
Example Implementation in C:
In task management, queues can be used to prioritize and manage tasks efficiently. For instance, in a project management scenario, tasks can be represented as elements in a queue, and the project manager can prioritize tasks based on their urgency and importance. This ensures that critical tasks are addressed first, and the project progresses in an orderly manner.
Example Implementation in C:
These are just a few examples of how queues can be applied in simulations, scheduling, and task management. The use of queues in these contexts helps to ensure that processes are executed efficiently, fairly, and in the correct order.
When implementing a fixed-size queue using an array, it’s essential to check for overflow and underflow conditions to prevent errors. Overflow occurs when trying to enqueue an element into a full queue, while underflow occurs when trying to dequeue from an empty queue. For example, if you have a queue of size 5 and you try to enqueue a sixth element, it will result in an overflow error.
Example Code:
dequeue()
without NULL checksWhen implementing the dequeue operation, it’s essential to check if the queue is empty before attempting to dequeue an element. Without this check, the dequeue operation can result in an infinite loop if the queue is empty, leading to a program crash or unexpected behavior.
Example Code:
Static memory queues are fixed in size and cannot be resized once allocated. This can lead to memory wastage if the queue is not used to its full capacity.
Dynamic memory queues, on the other hand, can be resized dynamically as needed, making them more flexible and efficient.
Queue Type | Memory Allocation | Space Complexity | Flexibility |
---|---|---|---|
Static | Fixed at compile time | O(n) | Low |
Dynamic | Allocated at runtime | O(1) | High |
Note: In the table above, ‘n’ represents the fixed size of the static queue, and ‘1’ represents the dynamic allocation of memory as needed.
To implement a queue in C, you can use either arrays or linked lists. Here’s an example implementation using linked lists:
No, C does not have a built-in queue data structure. However, you can implement a queue using arrays or linked lists as shown above.
No, there is no standard library in C that provides a queue data structure. You need to implement it yourself or use a third-party library.
To create a queue, you need to define a structure to represent the queue and its nodes. Then, implement functions for enqueue and dequeue operations. Here’s an example implementation in C:
The easiest way to implement a queue is using linked lists. This approach allows for dynamic memory allocation and easy insertion and deletion of elements. Here’s an example of how to build a queue in C using linked lists:
Creating a stack in C is similar to creating a queue. You can use arrays or linked lists to implement a stack. Here’s an example implementation using linked lists:
A queue in C is a linear data structure that follows the First-In-First-Out (FIFO) principle. It is used to implement scenarios where elements need to be processed in the order they were received, such as job scheduling, print queues, and network protocols.
Implementing a queue using arrays in C involves defining a structure to represent the queue and its elements. Here’s an example implementation:
A queue implemented using linked lists involves defining a structure to represent the queue and its nodes. Each node contains data and a pointer to the next node. The queue structure contains pointers to the front and rear nodes. Enqueue operation involves adding a new node at the rear, and dequeue operation involves removing a node from the front.
Here’s a simple example of how a queue can be implemented using linked lists in C:
The main difference between a queue and a stack is the order in which elements are added and removed. A queue follows the First-In-First-Out (FIFO) principle, whereas a stack follows the Last-In-First-Out (LIFO) principle.
Here’s a simple example in C to illustrate the difference:
The common queue operations in C are:
To check if a queue is full or empty, you can use the following conditions:
if (q->rear == SIZE - 1)
for full, if (q->front == -1 || q->front > q->rear)
for empty.if (q->rear == NULL)
for empty.Array-Based Queue Example in C:
Linked List-Based Queue Example in C:
A circular queue is a variant of the queue data structure where the last element points back to the first element, forming a circle. This allows for efficient use of memory and enables the queue to wrap around when it reaches its capacity. The main difference is that the rear element points back to the front element, allowing for circular traversal.
Here’s a simple example in C to illustrate the difference:
In this article, we have understood the working of queue data structure and have also shadowed over the implementation of queues using stack data structure. We have also discussed the applications of queues in simulations, scheduling, and task management.
Thanks for learning with the DigitalOcean Community. Check out our offerings for compute, storage, networking, and managed databases.
Hi, this code doesn’t run because you never defined exit()
- neil