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Queue implementation in C

In the C programming language, the queue is the abstract concept that is similar to stacks. However, it is the part of the data structures that work opposite to that of what stack does. A queue works in both ways; that is, it is open at both ends. One of the ends is always used to insert the data, and the other is used to remove the data. The end which is used to insert the data element is referred to as ‘en queue’, while the other, which is used to remove the data element, is termed as ‘de queue’.

Queue concept in the data structure follows the principle of First In First Out (FIFO) or Last In Last Out (LILO), i.e., the data element to be stored first is the first one to be accessed; on the other hand, the stack concept follows the principle of First In Last Out (FILO) or Last In First Out (LIFO).

Queues are followed in the real world, where a single lane one way road is present and the vehicle to enter first is the first one to be moved out from it. Similarly, the queues can be observed at ticket windows, and bus stops, and so on.

Basic operations in the queue:

Queue operations incur the initializing, defining, utilizing, and erasing it from the memory location permanently.

  • Enqueue (): enqueue lets the compiler add or store a data element into the queue.
  • Dequeue (): dequeue lets the compiler remove or access a data element from the queue.
  • Peek (): peek will let the programmer check the element at the front of the queue without removing or accessing it.
  • isfull(): isfull, as the name suggests, will let the compiler check if the queue is full or not.
  • isempty(): isempty, as the name suggests, will let the compiler check if the queue is empty or not.

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

E.g.:

#include <stdio.h>
#include<stdlib.h>
#define MAX 50
void insert();
void delete();
void display();
int queue_array[MAX];
int rear = - 1;
int front = - 1;
int main()
{
int choice;
while (1)
{
printf("1.Insert element to queue \n");
printf("2.Delete element from queue \n");
printf("3.Display all elements of queue \n");
printf("4.Quit \n");
printf("Enter your choice : ");
scanf("%d", &choice);
switch(choice)
{
case 1: insert();
break;
case 2: delete();
break;
case 3: display();
break;
case 4: exit(1);
default: printf("Wrong choice \n");
}
}
}


void insert()
{
int item;
if(rear == MAX - 1)
printf("Queue Overflow \n");
else
{
if(front== - 1)
front = 0;
printf("Inset the element in queue : ");
scanf("%d", &item);
rear = rear + 1;
queue_array[rear] = item;
}
}


void delete()
{
if(front == - 1 || front > rear)
{
printf("Queue Underflow \n");
Return;
}
else
{
printf("Element deleted from queue is : %d \n", queue_array[front]);
front = front + 1;
}
}


void display()
{
int i;
if(front == - 1)
printf("Queue is empty \n");
else
{
printf("Queue is : \n");
for(i = front; i <= rear; i++)
printf("%d ", queue_array[i]);
printf("n");
}
}

Output:

1.Insert an element to the queue 
2.Delete an element from the queue 
3.Display all elements of the queue 
4.Quit 
Enter your choice: 1
Insert an element to the queue: 3


1.Insert an element to the queue 
2.Delete an element from the queue 
3.Display all elements of the queue 
4.Quit 
Enter your choice: 1
Insert an element to the queue: 5


1.Insert an element to the queue 
2.Delete an element from the queue 
3.Display all elements of the queue 
4.Quit 
Enter your choice: 1
Insert an element to the queue: 2


1.Insert an element to the queue 
2.Delete an element from the queue 
3.Display all elements of the queue 
4.Quit 
Enter your choice: 3 
Display all elements of the queue: 2  5  3


1.Insert an element to the queue 
2.Delete an element from the queue 
3.Display all elements of the queue 
4.Quit 
Enter your choice: 2
Item deleted: 3


1.Insert an element to the queue 
2.Delete an element from the queue 
3.Display all elements of the queue 
4.Quit 
Enter your choice: 2
Item deleted: 5


1.Insert an element to the queue 
2.Delete an element from the queue 
3.Display all elements of the queue 
4.Quit 
Enter your choice: 3 
Display all elements of the queue: 2 

This code is a menu-driven implementation of a queue. First, define the size of the MAX variable to be 50. Then, the array called queue_array is declared of size MAX. Three functions are to be declared. The functions are, insert, display and delete functions. A menu-driven main function is used. The user is asked to enter his choice and call the appropriate function to perform the task.

There are pointers, the front is at the front of the queue, and the rear is at the back of the queue. We add elements from the back of the queue and remove them from the front of the queue.

Time complexity: O(1) is the complexity in all the cases.

Auxiliary space: O(N) where N is the size of the array to store the data elements.

Advantages:   

  1. Easy to implement.

Disadvantages: 

  1. Static Data Structure, fixed size.
  2. If the queue has a large number of enqueue and dequeue operations, at some point (in case of linear increment of front and rear indexes), we may not be able to insert elements in the queue even if the queue is empty (this problem is avoided by using circular queue).



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