Queue in C Programming

November 2, 2023

Queue Implementation

A Queue, generally implemented using structures. Generally, the implementation is FIFO i.e. First in First out. A good example would be people standing at a billing counter in a queue, the person who comes first, will be served first

Basic Information about Queues

Queue is FIFO type i.e. First in First out. In queues inserting an item is called as Enqueue and removing an item is called as Dequeue.

As given in the image the following about queues is true –

Terminologies

Enqueue means adding one item to the queue and happens at the rear end of queue
Dequeue means removal of one item from the queue and happens at the front end of the queue
If the max size of queue is reached then the queue is said to be in Overflow condition
If the queue has no items then queue is said to be in Underflow condition

Stack Representation in Java

Representation of a Stack as a Linked List

Infix to Postfix Conversion in java

Infix to prefix conversion in java

Run

#include<stdio.h>
#include<string.h>
#include<stdlib.h> 
#include<stdbool.h>


#define MAX 6

int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;

int peek() {
   return intArray[front];
}

bool isEmpty() {
   return itemCount == 0;
}

bool isFull() {
   return itemCount == MAX;
}

int size() {
   return itemCount;
}  

void insert(int data) {

   if(!isFull()) {
	
      if(rear == MAX-1) {
         rear = -1;            
      }       

      intArray[++rear] = data;
      itemCount++;
   }
}

int removeData() {
   int data = intArray[front++];
	
   if(front == MAX) {
      front = 0;
   }
	
   itemCount--;
   return data;  
}

int main() {
   
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);

   if(isFull()) {
      printf("Queue is full!\n");   
   }

   int num = removeData();
	
   printf("Element removed: %d\n",num);
   
   insert(16);
   insert(17);
   insert(18);

   printf("Element at front: %d\n",peek());

   printf("----------------------\n");
   printf("index : 5 4 3 2  1  0\n");
   printf("----------------------\n");
   printf("Queue:  ");
	
   while(!isEmpty()) {
      int n = removeData();           
      printf("%d ",n);
   }   
}

Limitations with Queues in C

This is an Alert Aftter multiple enqueing and dequeing, the size of the queue gets reduced and new elements can't be added

The following is demonstrated in the image below -

How to solve the above issue ?

The above issue can be solved in the following ways –

Either we can use Circular queues
Or modify the code as given below –

The modification in the below code is purely because we are applying remainder operation to size.

That is, for most operations we are doing (value)%size

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