Data Structures II Lesson 1 (Circular Queue)

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Richard Lyons
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1 Data Structures II Lesson 1 (Circular Queue) Introduction The circular queue: is a queue where the last location in it is linked to the first location As shown in figure 1, location (5) which is the last location in the queue is connected to location (0) which is the first location in the queue Figure 1. The circular queue The insertion of a new element is done at the very first location of the queue if the last location of the queue is reserved. It is possible to insert new elements if and only if there are empty locations in the circular queue. The circular queue overcomes the issue of unutilized space that occurs in the linear queue. 1

2 Pointers Rules The used pointers rules in a circular queue are exactly the same as in the case of a linear queue. These rules are: 1. Front pointer will always denote to the oldest inserted element in the queue. 2. Rear pointer will always denote to the newest inserted element in the queue. 3. Every time a new element is inserted into the queue Rear is incremented by one. 4. Every time an element is deleted from the queue front is incremented by one. Insertion Rules The used rules to insert a new element in a circular queue are similar to the rules in the case of linear queue, with some modifications. These rules are: 1. The first thing to do in the case of inserting new element, is to check if the circular queue is full or not. This is accomplished by performing the following equation: Front = (Rear + 1) % Max Size Where max size is the total number of slots in the circular queue. 2. The user will insert the new element. 3. If the inserted element is the first element in the circular queue, the front and rear pointers will denote to the first location which zero. The circular queue is empty if the front pointer is equal -1. 2

3 4. If the new inserted element is not the first element in the circular queue, the location of this element will be calculated as follows: Rear = (Rear + 1) % Max Size. Deletion Rules The deletion method for a circular queue also requires some modification as compared to a linear queue. The used rules are: 1. Check if value of the front pointer equals -1 or not. If yes, it the circular queue is empty and underflow situation existed. 2. Check if the value of front and rear pointers are the same. If yes, this means there are only one element in the circular queue and it is deleted by setting the value of front and rear pointers to Otherwise, the front pointer value will be modified as follows: Front = (Front + 1) % Max Size Example: Consider having a circular queue with six locations as shown below: Rear Front 3

4 Draw figures to represent the contents of the circular queue and the location of front and rear pointers after performing the following add and delete operations: ADD (30), ADD (40),,, ADD (50),,,, ADD (60) Solution: 30 ADD (30) 40 ADD (40) 4

5 ADD (50) 50 Rear 5

6 6

7 60 ADD (60) Practical Implementation of the Circular Queue #include<iostream> #include<conio.h> using namespace std; //First, initializing the queue with its size and pointers const int MaxSize = 5; int front=-1; int rear=-1; int cq[maxsize]; int choice; void cqinsert () int num; //First, check if the circular queue is full or not if(front==(rear+1)%maxsize) cout<<"the Queue is Full\n"; //If the circular queue is not full, allow the user to insert an element cout<<"enter the element to be inserted\n"; cin>>num; //If the element is the first element in the circular queue if(front==-1) front=rear=0; rear=(rear+1)%maxsize; cq[rear]=num; 7

8 int cqdelete() int num=0; //Check if the circular queue is empty or not if(front==-1) cout<<"queue is Empty\n"; num=cq[front]; cout<<"the deleted elemnt is: \n"<<cq[front]<<"\n"; //Check if the deleted element is the last element if(front==rear) front=rear=-1; front=(front+1)%maxsize; return(num); void cqdisplay() int i; if(front==-1) cout<<"the queue is empty \n"; cout<<"the contents of the queue are: \n"; for(i=front;i<=rear;i++) cout<<cq[i]<<"\n"; if(front>rear) for(i=front;i<maxsize;i++) cout<<cq[i]<<"\n"; for(i=0;i<=rear;i++) cout<<cq[i]<<"\n"; void main() do cout<<"1. Insert \n"; cout<<"2. Delete \n"; 8

9 cout<<"3. Display \n"; cout<<"4. Exit \n"; cout<<"enter your choice: \n"; cin>>choice; switch(choice) case 1:cqinsert(); break; case 2:cqdelete(); break; case 3:cqdisplay(); break; case 4:break; while(choice<4); system("pause"); 9

Circular Queue can be created in three ways they are: Using single linked list Using double linked list Using arrays

Circular Queue can be created in three ways they are: Using single linked list Using double linked list Using arrays Circular Queue: Implementation and Applications In linear queue, when we delete any element, only front increment by one but position is not used later. So, when we perform more add and delete operations,