Queue using Arrays

Question

Below is my implementation of a simple queue using arrays.

#include<stdio.h>
#include<stdlib.h>
#define QSIZE 5 //Limit size of queue to just 5 enteries

/*Beginning of prototype for queue functions: Insert, retrieve and display*/
void qdisp(); //Display to queue array contents
void qinsert(); //Insert a element into rear of queue
int qdelete(); //Remove an element from front of queue
/*End of prototyping*/

// Variables
int fe=0,re=0,q[QSIZE],item; //fe(front entry), re(rear entry), q[](queue array), item(element to i/p or delete)

void main()
{
  int choice;
  while(1)
  {
    printf("1.Insert element\n2.Remove element\n3.Display element(s)\n4.Exit program\nEnter number for appropriate choice:  ");
    scanf("%d",&choice);
    switch(choice)
    {
      case 1:   printf("Enter value of element: ");
            scanf("%d",&item);
            qinsert();
            break;
      case 2:   item=qdelete();
            printf("Removed \'%d\' from the queue\n",item);
            break;
      case 3:   qdisp();
            break;
      case 4:   exit(0);

      /*case default : printf("Wrong choice i/p!");
              break;*/
    }
  }
}
//End of main, beginning for function definitons

void qinsert()
{
  if(re==QSIZE-1)
  {
    printf("Queue Overflow\n");
    exit(0);
  }
  q[re]=item;
  re++;
}

int qdelete()
{
  if(fe>re)
  {
    printf("Queue is empty!\n");
    exit(0);
  }

  item=q[fe];
  fe++;
  return item;
}

void qdisp()
{
  int i; //i is loop counter variable
  if(fe>re)
  {
    printf("Queue is empty!\n");
    exit(0);
  }
  printf("Queue items are: \n");
  for(i=fe;i<=re;i++)
    printf("%d\n",q[i]);
}

I have used initial front and rear entry as 0 since initially in a queue any entry that goes first becomes the last entry as well. However my teacher says I should keep the rear entry as '-1' and while inserting an element into queue, first increment the rear entry index and then add opposing my code of first adding then incrementing. I looked into it and online and till now I don't find how I'm wrong.

Provide me information if I'm wrong or my teacher is?

Answer 1

in insert function when first to check queue is empty or not condition is:

if(r==0)

because when queue is empty rear is always at 0.. then check wheather queue has only 1 element or more than 1..

if(f==r-1)
 {f=0;r=0;}
 else
 {f++;}

in insert function when u insert last element your rear will incremented and set to 5 then when u delete element front is incremented and for last element front is 4 which is less than rear by 1...so condition to check whether only 1 element left or not is f==r-1 and if it is satisfied then array becomes completely empty so we set front and rear to 0..

Answer 2

Both pre-incrementing and post-incrementing can be used in a queue. What changes however is the full and empty conditions. With pre-increment the full condition is QSIZE-1, with post-incrementing the full condition is QSIZE. With pre-increment the empty condition is fe==re, with post-increment fe>re.

Using pre-increment can save a temporary variable in delete. Notice how you must save the current element into item, then increment the index, then return item.

item=q[fe];
fe++;
return item;

You can do away with the item variable by incrementing the index, then returning the element.

fe++;
return q[fe];

Just remember, if you pre-increment to insert, you need to pre-increment to delete.

Ok, consider this. What happens fe and re are both QSIZE-1? The queue is empty but your code will interpret it as full (because re==QSIZE-1).

Answer 3

Any teachers around? Okay, good.

Teachers aren't always right, but there's probably a good chance you simply misunderstood him. As long as you understand that when fe == re the queue is empty, both being 0 initially is just fine.

However, some of your functions need to change:

Delete

void qdisp(void)
{
    int i;
    if(fe == re)                     // 1
    {
        printf("Queue is empty!\n");
        return;                      // 2
    }
    printf("Queue items are:\n");
    for(i = fe; i < re; i++)         // 3
        printf("%d\n", q[i]);
}

1. == instead of >
2. return instead of exit(0)
3. < instead of <=

Insert

if(re == QSIZE)                 // 1
{
    printf("Queue Overflow\n");
    return;                     // 2
}

1. QSIZE instead of QSIZE-1
2. return instead of exit(0)

Delete

There's no reason to call exit(0). Nothing fatal with trying to delete from an empty queue.