Problem with a simple queue class

[JoshuaS]

Hi!

For an assignment I have to implement a simple queue using a class.

I'm having problem with this assignment though. The queue I wrote actually handles the data in first-in-last-out ( FILO ) order. So it's more a stack then a queue.

The problem is that I can't figure out why it works in this way; If I look at my program I would really be sure that I'm doing things correctly. But for some reason I don't.

Here is the code for my class:

const unsigned int MAX_QUEUE_SIZE ( 100 );

class queue
{
    private:
        int items[MAX_QUEUE_SIZE];
        unsigned int count;
    public:
        queue ( );
        queue ( queue & old_queue );
        void put ( int item );
        int get ( );
};

queue::queue ( )
{
    count = 0;
}

queue::queue ( queue & old_queue )
{
    unsigned int index;
    
    for ( index = 0; index < old_queue.count; ++index )
    {
        items[index] = old_queue.items[index];
    }
    
    count = old_queue.count;
}

void queue::put ( int item )
{
    items [ count ] = item;
    ++count;
}

int queue::get ( )
{
    int item_to_return = items[0];
    unsigned int index;
    
    for ( index = 0; index < count; ++index )
    {
        items [ index ] = items [ index + 1 ];
    }
    --count;
    
    return ( item_to_return );
}

Here is the code I test my class with:

    queue a_queue;
    
    a_queue.put ( 6 );
    a_queue.put ( 8 );
    
    cout << "6-> " << a_queue.get ( ) << '\n'
         << "8-> " << a_queue.get ( ) << "\n\n";

To give you a better idea of the situation, the output of these lines of code should be:

6-> 6
8-> 8

But it actually is:

6-> 8
8-> 6

Can someone help me solve this problem?

Thanks in advance :)

[coder777]

Hello JoshuaS,

That is because of the order of evaluation. Means the last 'get()' is called first. So don't modify the contents of a class within a get() function. better break it in 2 function like top() and pop().

do you know that stl already provides stack and queue?

[JoshuaS]

Thanks for the answer.

And your answer was the solution. I rewrote the testing code to the following and it works:

    queue a_queue;
    
    a_queue.put ( 6 );
    a_queue.put ( 8 );
    
    cout << "6-> " << a_queue.get ( ) << '\n';
    cout << "8-> " << a_queue.get ( ) << "\n\n";

Nevertheless, this behaviour is extremely odd and I wonder where this behaviour would be useful.

I also know about the queue class the STL provides. But this isn't production code, it's just an exercise we needed to do to practise writing useful classes :)

[ne555]

You may want to modify your queue so get() an put() work in constant time.
Just keep two indices (top, bottom) and use modulo operator to recycle the cells