Array Implementation of Queue: Strange Output

Question

I am learning about queues and I wrote the following program which implements the queue as a linear array (NOT a circular one).

   #include<iostream>
using namespace std;

class queue {
    int front, max_queue, rear, count = 0;
    int *items;
public:
    queue(int);
    ~queue();
    void enqueue(int);
    void dequeue();
    bool isEmpty();
    int size();
    void display();
};

queue::~queue() {
    delete []items;
}

queue::queue(int max) {
    front = -1;
    rear = -1;
    max_queue = max;

    items = new int[max_queue];
}

void queue::enqueue(int n) {
    if (count == max_queue)
        cout << "queue is full, no enqueue possible";
    else {
        items[++rear] = n;
        count++;
    }
}

void queue::dequeue() {
    if (count == 0)
        cout << "no dequeue possible, queue already empty";
    else {
        front--;
        count--;
    }
}

bool queue::isEmpty() {
    return ((count == 0) ? 1 : 0);
}

int queue::size() {
    return count;
}

void queue::display() {
    if (count == 0)
        cout << "nothing to display";
    else {
        for (int i = front; i <= rear;)
            cout << items[i++] << endl;
    }

}

int main() {

    queue *qe = new queue(10);

    qe->enqueue(1);
    qe->enqueue(2);
    qe->enqueue(3);
    qe->enqueue(4);
    qe->display();


    return 0;
}

I get the following output

49
1
2
3
4

RUN FINISHED; exit value 0; real time: 10ms; user: 0ms; system: 0ms

Why is there a 49 in my output.? Is it a garbage value? Does not using a circular array implementation,a probable cause? I have no idea. Any help appreciated.

mark · Accepted Answer · 2013-08-14 15:21:04Z

3

You're starting the display loop i with front which is -1. You're pointing to a spot before your queue.

answered Aug 14, 2013 at 15:21

mark

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2 Comments

fts Over a year ago

But wont front be -1 only when the queue is empty? If count!=0 then shouldn't front be equal to 0 or any higher value?

mark Over a year ago

your enqueue function isn't updating front though

wjmolinam · Accepted Answer · 2013-08-14 15:25:59Z

1

If a queue is empty and you enqueue an element x, then x becomes both the front and the rear.

answered Aug 14, 2013 at 15:25

wjmolinam

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Comments

IanPudney · Accepted Answer · 2013-08-14 15:24:23Z

0

The issue is in your display function:

for (int i = front; i <= rear;)
    cout << items[i++] << endl;

You're setting i=front, but you've previously set front=-1. Thus, you're attempting to access items[-1]. You can either set i=front+1:

for (int i = front + 1; i <= rear;)
    cout << items[i++] << endl;

or continue until i<rear and use ++i instead of i++:

for (int i = front; i < rear;)
    cout << items[++i] << endl;

answered Aug 14, 2013 at 15:24

IanPudney

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