Question regarding queue codes

Hey all. I'm making a simple program to enqueue and dequeue a string of characters as entered into an array. I'm fairly close to completing the code but I've run into a bit of difficulty. After a certain number of enqueues/dequeues the operation seems to break down to the point where it doesn't display some of the members of the queue. Here's the code... and then with an example of what I mean.

Code:

void enqueue (char queue[], int &in, int &full)
{
    char ch;
    if (in == 4)
        { in = 0; }
    if (full == 5)
        { cout << "Enqueue: Queue is full.\n\n"; }
    else
        {
        cout << "Enqueue: ";
        cin >> ch;
        cout << endl;
        queue[in] = ch;
        in++, full++;
        }
}
void dequeue (char queue[], int &out, int &full)
{
    char ch;
    if (out == 4)
        { out = 0; }
    if (full == 1)
        { cout << "Dequeue: Queue is empty.\n"; }
    else
        {
        ch = queue[out];
        cout << "Dequeue: " << ch << endl;
        out++, full--;
        }
}
void display (char queue[], int &out, int &full)
{
    cout << "Queue status:";
    if (full == 1)
        { cout << " Empty"; }
    for (int i = out; i <= (full+1); i++)
        { cout << " " << queue[i]; }
    cout << endl << endl;
}
main ()
{
   char selection ;
   char mainqueue[4] ;
   int n = 0, d = 0;
   int full = 1;
   /*  Declare and initialize various counters.  */

   cout << endl ;
   cout << "Enter e to enqueue, d to dequeue or q to quit." ;
   cout << endl << endl << "e, d or q: " ;
   cin >> selection ;
   while ( ( selection == 'e' ) || ( selection == 'd' ) )
      {
      switch ( selection )
         {
         case 'e' :  enqueue (mainqueue, n, full);
            break ;
         case 'd' :  dequeue (mainqueue, d, full);
            break ;
         }
      display (mainqueue, d, full) ;
      cout << "e, d or q: " ;
      cin >> selection ;
      }
}

A sample output after several enqueue/dequeue progressions (my "notes" are in bold):

Code:

Enter e to enqueue, d to dequeue or q to quit.
FIRST RUN
e, d or q: e

Enqueue: a

Queue status: a

SECOND RUN
e, d or q: e

Enqueue: b

Queue status: a b

THIRD RUN
e, d or q: e

Enqueue: c

Queue status: a b c

FOURTH RUN
e, d or q: e

Enqueue: d

Queue status: a b c d

FIFTH RUN
e, d or q: e

Enqueue: Queue is full.

Queue status: a b c d

SIXTH RUN
e, d or q: d

Dequeue: a

Queue status: b c d

SEVENTH RUN
e, d or q: d

Dequeue: b

Queue status: c d

EIGHTH RUN
e, d or q: d

Dequeue: c

Queue status: d

NINTH RUN
e, d or q: d

Dequeue: d

Queue status: Empty

TENTH RUN -- HERE'S WHERE IT BREAKS DOWN
e, d or q: e

Enqueue: g

Queue status: Should output a "g" here...

ELEVENTH RUN
e, d or q: e

Enqueue: g

Queue status: Should output "g g" here

TWELFTH RUN
e, d or q: e

Enqueue: k

Queue status: Should show "g g k"...

THIRTEENTH RUN
e, d or q: e

Enqueue: l

Queue status: Should show "g g k l"...

RUN 14
e, d or q: e

Enqueue: Queue is full.

Queue status: Doesn't display queue again.

RUN 15
e, d or q: d

Dequeue: g

Queue status: g k l

e, d or q: d

Dequeue: g

Queue status: k l

e, d or q: d

Dequeue: k

Queue status: l

e, d or q: d

Dequeue: l

Queue status: Empty

e, d or q: q It runs fine for the dequeue as far as displaying the queue.

Basically, it doesn't display the queue after I've enqueued/dequeued a full (read: 4-element) queue. It was deemed to be a "circular" queue too. Any advice?

How can n decrease, when you pass d to dequeue?

Again, it's supposed to be a circular queue. Once it reaches the end of the queue (in this case, when in/n = 4), it should revert back to 0... I'm not quite sure what you're getting at here. The problem seems to be in the display function.

The print statement is using out as a starter -- and out is still 4 until you call dequeue again. You need to do the circle right when you do in++ or out++, not the next time through the function.

When I put the in/out check after the in++ and out++, it only exacerbates the problem:

Code:

#include <iostream>
using namespace std ;

void enqueue (char queue[], int &in, int &full)
{
    char ch;
    if (full == 4)
        { cout << "Enqueue: Queue is full.\n"; }
    else
        {
        cout << "\nEnqueue: ";
        cin >> ch;
        cout << endl;
        queue[in] = ch;
        in++, full++;
        }
    if (in == 4)
        { in = 0; }
}
void dequeue (char queue[], int &out, int &full)
{
    char ch;
    if (full == 0)
        { cout << "Dequeue: Queue is empty.\n"; }
    else
        {
        ch = queue[out];
        cout << "Dequeue: " << ch << endl;
        out++, full--;
        }
    if (out == 4)
        { out = 0; }
}
void display (char queue[], int &out, int &full)
{
    cout << "Queue status:";
    if (full == 0)
        { cout << " Empty"; }
    for (int i = out; i <= full; i++)
        { cout << " " << queue[i]; }
    cout << endl << endl;
}
main ()
   {
   char selection ;
   char mainqueue[4] ;
   int n = 0, d = 0;
   int full = 0;

   cout << endl ;
   cout << "Enter e to enqueue, d to dequeue or q to quit." ;
   cout << endl << endl << "e, d or q: " ;
   cin >> selection ;
   while ( ( selection == 'e' ) || ( selection == 'd' ) )
      {
      switch ( selection )
         {
         case 'e' :  enqueue (mainqueue, n, full);
            break ;
         case 'd' :  dequeue (mainqueue, d, full);
            break ;
         }
      display (mainqueue, d, full) ;
      cout << "e, d or q: " ;
      cin >> selection ;
      }
   }

Sample output...

Code:

Enter e to enqueue, d to dequeue or q to quit.

e, d or q: e

Enqueue: a

Queue status: a

e, d or q: e

Enqueue: b

Queue status: a b

e, d or q: e

Enqueue: c

Queue status: a b c

e, d or q: e

Enqueue: d

Queue status: a b c d

e, d or q: e
Enqueue: Queue is full.
Queue status: a b c d

e, d or q: d
Dequeue: a
Queue status: b c d

e, d or q: d
Dequeue: b
Queue status: c

e, d or q: d
Dequeue: c
Queue status:

e, d or q: d
Dequeue: d
Queue status: Empty a

e, d or q: d
Dequeue: Queue is empty.
Queue status: Empty a

e, d or q: e

Enqueue: g

Queue status: g b

e, d or q: e

Enqueue: h

Queue status: g h c

e, d or q: e

Enqueue: i

Queue status: g h i d

e, d or q: e

Enqueue: j

Queue status: g h i j

e, d or q: e
Enqueue: Queue is full.
Queue status: g h i j

e, d or q: d
Dequeue: g
Queue status: h i j

e, d or q: d
Dequeue: h
Queue status: i

e, d or q: d
Dequeue: i
Queue status:

e, d or q: q

If by "exacerbated" you mean "exposed all the other errors that were hiding behind the first one", then yes. Your display is still broken, in that (a) it doesn't print the right number of things (when you have one element in the queue, full is then 2, which means you actually print three elements mainqueue[0], mainqueue[1], and mainqueue[2]) and (b) it has no way to "wrap around" -- for instance if you had enqueued four, dequeued two, and then enqueued one, you would have your queue in mainqueue[2], mainqueue[3], and mainqueue[0] -- but you would print 2, 3, 4 instead.

Originally Posted by tabstop

If by "exacerbated" you mean "exposed all the other errors that were hiding behind the first one", then yes.

Touche. I'm not even going to pretend I'm a great programmer... I'm going off hunches when I do these first off and then I have to go back and fix them once I see they're incorrect... and sometimes the errors aren't obvious to me. haha

Your display is still broken, in that (a) it doesn't print the right number of things (when you have one element in the queue, full is then 2, which means you actually print three elements mainqueue[0], mainqueue[1], and mainqueue[2]) and (b) it has no way to "wrap around" -- for instance if you had enqueued four, dequeued two, and then enqueued one, you would have your queue in mainqueue[2], mainqueue[3], and mainqueue[0] -- but you would print 2, 3, 4 instead.

Okay. In that case, a check for my "i" would be necessary to get it to wrap around, but I can see that such a check (inside the for-loop) would cause a never-ending loop. I get what you're saying (because I traced out the program myself a few times to see where the breakdown(s) was/were), but I'm unsure of how to really implement such a check.

Anybody able to give any other advice?

It seems like if I had a hybrid of my first and second code sets (the ones with outputs given) it'd be close to being correct. Of course, the problem with the display function is still nagging. How am I supposed to be able to get the [i] to reset to zero in such a case where components [2], [3] and [0] (for example) are supposed to be output? Without creating an infinite loop?

Why do you think the array index and the thing controlling the loop should be the same? If your loop control was j, your array index would have to be (out+j)%5.

The problem in that case is that after dequeuing just one item, it skips the second item in the queue.

a b c d

Dequeue a

Queue Status: c d

Dequeue b

Queue Status: [shows nothing]

j doesn't start at out; it starts at 0 and goes to (full-1) I guess.