Thread: dequeueing issues

The last part of my program is supposed to be able to remove one item from a queue at the top of a stack, but stop if there is only one item left in the queue. I can't figure out how to make it stop exactly. And also, after I remove the item and I go to print it out, the item I removed still prints as part of the queue (I know there is another post on this problem, but the fix would not work for my code). Here are my functions:

First the dequeue function:

Code:

void remove_car(struct station* s, struct train* q)
{   
    if(s->top == 0)
    {
        printf("Car not removed because there are no trains!\n");
        return;
    }

    s->top --;
    q = s->store[s->top];
    
    if(q->front + 1 == q->back)
    {
        printf("Car not removed because there is only one car on the train!\n");
        return;
    }
    printf("Car %d removed!\n", q->store[q->front]);
    q->front = (q->front + 1) % q->size;

    q = s->store[s->top];
    s->top ++;
    
    return;
}

And my printing function:

Code:

void train_departs(struct station* s, struct train* q)
{
    int i;
    
    if(s->top == 0)
    {
        printf("Station empty, no train departed!\n");
        return;
    }
    
    s->top--;
    q = s->store[s->top];
    
    printf("Train Departed: ");
    
    for(i = q->front; i < q->size; i++)
    {
        printf("%d ", q->store[i]);
        q->front = (q->front + 1) % q->size;
    }
    printf("\n");
}

Even if I cut it down, I still can't get it to work right. I made it a little shorter here:

Code:

int remove_car(struct train* q)
{
    int val = q->store[q->front];
    
    if(q->front + 1 == q->back)
    {
        printf("Car not removed because there is only one car on the train!\n");
        return;
    }
    q->front = (q->front + 1) % q->size;
    
    return val;
}

And I have it returning the value it supposedly deleted. However, it's still not removing it from the list when I print it out.

My edit was slightly wrong: q->size isn't supposed to change -- but your print function had better stop at q->back, unless you want all the extra deleted stuff to still print.

hm.. alright. Thanks. I'll have to play with it because when I changed it to stop at q->back, it didn't print anything. Just a new line.

I changed my loop in my print function to this:

Code:

for(i = q->front; i < q->back; i++)
    {
        printf("%d ", q->store[i]);
        q->front = (q->front + 1) % q->size;
    }
    printf("\n");

But now my function does no print anything at all. Does anyone know what might be causing this problem?

Print out q->front and q->back before you start to see what they are. Are you using wrap-around? If so, you'll need to check !=, not <. Also, you are dequeueing as you print, so at the end of this print function you'd have an empty queue