Chapter 2: Basic Abstract Data Types. Problem 2.20

Implement a circular queue where --
1. A pointer to the first element is kept.
2. Length of the queue is kept.

//problem 2.20 - circular queue - keep pointer to first element + length.

#include <iostream>
using namespace std;

const int MAXLENGTH = 10; 

class CIRQUEUE
{
    int list[MAXLENGTH]; 
    int front; 
    int length;

public: 
    //Constructor
    CIRQUEUE(); 
    bool QEmpty() { return !length; }
    bool QFull() { return length == MAXLENGTH ? true : false; }  
    bool EnQueue(int val); 
    bool DeQueue(int& val); //OUTPUT = val
    void PrintQ(); 
}; 

CIRQUEUE::CIRQUEUE()
{
    memset(list, 0, sizeof(list)); 
    front = length = 0; 
}

bool CIRQUEUE::EnQueue(int val)
{
    if( QFull() )
    {
        cout << "ERROR: Queue is full" << endl;
        return false;
    }

    list[(front+length)%MAXLENGTH] = val; 
    length++; 
    return true; 
}

bool CIRQUEUE::DeQueue(int& val) //OUTPUT = val
{
    if( QEmpty() )
    {
        cout << "ERROR: Queue is empty" << endl;
        return false;
    }

    val = list[front]; 
    front = (front+1)%MAXLENGTH; 
    length--; 
    return true; 
}

void CIRQUEUE::PrintQ()
{
    if( QEmpty() )
    {
        cout << "EMPTY CIRQUEUE." << endl;
        return; 
    }

    cout << "CIRQUEUE is ---->" << endl;

    int curr = front; 
    for ( int n = 0; n < length; n++ ) 
    {
        cout << list[curr] << " ";  
        curr = (curr+1)%MAXLENGTH; 
    }
    cout << endl << "<---- END of CIRQUEUE" << endl;
}

//Little driver program to test this
void main()
{
    CIRQUEUE q; 
    q.PrintQ(); 
    int val = 0;

    cout << endl << "EnQueuing 10 elements.. " << endl;
    for( val = 0; q.EnQueue(val); val++) 
        q.PrintQ();

    cout << endl << "DeQueuing 5 elements.. " << endl;
    q.DeQueue(val);
    q.PrintQ(); 
    q.DeQueue(val);
    q.PrintQ(); 
    q.DeQueue(val);
    q.PrintQ(); 
    q.DeQueue(val);
    q.PrintQ(); 
    q.DeQueue(val);
    q.PrintQ(); 

    cout << endl << "EnQueuing 10 elements.. " << endl;
    for( val = 0; q.EnQueue(val); val++) 
        q.PrintQ();
}