Question

I’m trying to implement a queue using an array. Here is my code:

#include <iostream.h>
#define SIZE 5

class queue
{
    int *Queue, front, rear;

    public:
    queue() {
        Queue = new int[SIZE];
        front = rear = -1;
    }

    void push() {
        if (rear == (SIZE-1)) {
            cout<<"\n Overflow!";
        } else {
            rear++;
            cout<<"\n Enter element: ";
            cin>>Queue[rear];   
        }
    }

    void pop() {
        if (front == rear) {
            cout<<"\n Underflow!";
        } else {
            cout<<"\nElement popped: "<<Queue[++front];
        }
    }

    void display() {
        if (front == rear) {
            cout<<"\n Queue Empty";
        } else {
            for(int i = (front+1); i<=rear; i++) {
                cout<<Queue[i]<<" ";
            }
        }
    }
};


int main()
{
    int choice;
    queue q;
    while(choice != 4)
    {
        cout<<"\n\n Enter your choice :"
            <<"\n 1. Push an element into Queue."
            <<"\n 2. Pop an element from Queue."
            <<"\n 3. Display the Queue."
            <<"\n 4. Exit the program.\n\n";
        cin>>choice;

        switch (choice) {
            case 1:
                q.push();
                break;
            case 2:
                q.pop();
                break;
            case 3:
                q.display();
                break;
            case 4:
                break;
        }
    }

    return 0;
}

The thing is that once the overflow is met, even after popping an element the rear remains the same and another element is not added when there is a vacant space where it can go.

The solution for this could be to shift every element one place ahead so that there is empty spot at the end but I am having trouble with the shift. Also, if I try inserting after popping 2-3 times before reaching overflow then it still gives overflow even when there are only 3 elements in the queue.
How can I solve this?

Answer 1

You never reset front or rear, they keep increasing forever.

the overlflow test is also wrong:

if (rear == (SIZE-1)) {
        cout<<"\n Overflow!";

what you should be testing is if your are about to overwrite front. I think you will benefit from keeping just front and the number of elements N instead. Then overflow and underflow becomes N>SIZE and N==0 instead. Then just increase and decrease N as you pop and push. Keep front as it is but also keep it modulo SIZE.

Also, as written in a comment. No need to move around the data.