Queue Data Structure in C++

The queue data structure is similar to the stack, where there are restrictions on adding and removing elements. likewise, queue follows the first-in-first-out (FIFO) principle where the data item stored first will be accessed first.

Diagram of a Queue data structure

 

Operations of a Queue data structure

 

Enqueue – This will add an element to the queue

The steps of the Enqueue operation are:

1. Check to see if the queue is full
2. If it’s full, show an error message and end
3. If the queue is not full, increment the last pointer to point to the next empty slot
4. Add the data element where the queue location

Dequeue – This will remove an element in the queue

The steps of the Dequeue operation are:

1. Check to see if the queue is empty
2. If it’s empty, show an error message and end
3. If the queue is not empty, access the element where the front is pointing
4. Increment the front pointer to point to the next available data element

Functions to add which will make a queue more efficient

• isFull() – This will check to see if the queue is full
• isEmpty() – This will check to see if the queue is empty
• Peek() – Retrieve the element at the front of the queue without removing it

C++ Implementation of a circular queue

//----------------------------------------------------------------------------
// Circular Queue using Arrays
// By Sahil Bora
//----------------------------------------------------------------------------
//---------------------------------------------
// Header files
//---------------------------------------------
#include <iostream>
#include <conio.h>
#include "queue.h"
 
using namespace std;
 
//----------------------------------------------------
// Queue class
// This can be used as a header file
// File:cqueue.h
//----------------------------------------------------
 
const int MAX = 5;
class cqueue
{
    int a[MAX],front,rear; 
    public:
    cqueue()
    {
        front = rear = ?1;
    }	 
    void insert(int);
    int deletion();
    void display();
};
//---------------------------------------------
// Queue insert constructor
//---------------------------------------------
void cqueue::insert(int val)
{
    if((front==0 &amp;&amp; rear==MAX?1) || (rear+1==front))
    cout<<" Circular Queue is Full";
    else
    {
        if(rear==MAX-1)
        rear=0;
        else
        rear++;
        a[rear]=val;
    }
    if(front==-1)
    front=0;
}
//---------------------------------------------
// Queue deletion constructor
//---------------------------------------------
int cqueue :: deletion()
{
int k;
if(front==-1)
cout<<"Circular Queue is Empty";
else
{
k=a[front];
if(front==rear)
front=rear=-1;
else
{
if(front==MAX-1)
front=0;
else
front++;
}
}
return k;
}
//---------------------------------------------
// Queue display constructor
//---------------------------------------------
void cqueue :: display()
{
    int i;
    if(front==-1)
    cout<<"Circular Queue is Empty";
    else
    {
    if(rear < front)
    {
    for(i=front;i<=MAX?1;i++)
    cout<<a[i]<<" ";
    for(i=0;i<=rear;i++)
    cout<<a[i]<<" ";
    }
    else
    {
    for(i=front;i<=rear;i++)
    cout<<a[i]<<" ";
    cout<<endl;
    }
    }
}
 
//--------------------------------------------------------
// Main Menu display code
//--------------------------------------------------------
void main()
{
    cqueue c1;
     
    int ch,val;
    char op;
    do
    {
    cout<<"CIRCULAR QUEUE IMPLEMENTATION IN C++\n";
     
    cout<<"----------Menu----------\n";
    cout<<"1.Insertion\n";
    cout<<"2.Deletion\n";
    cout<<"3.Display\n";
    cout<<"4.Exit\n";
     
    cout<<"Enter Your Choice <1..4> ?";
    cin>>ch;
    switch(ch)
    {
    case 1 : cout<<"Enter Element to Insert ?";
    cin>>val;
    c1.insert(val);
    break;
    case 2 : val=c1.deletion();
    cout<<"Deleted Element :"<<val<<endl;
    break;
    case 3 : c1.display();
    break;
    }
     
    cout<<"Do you want to continue<Y/N> ?";
    cin>>op;
    }
    while(op=='Y' || op=='y');
    getch();
}