C Linked-List Queue : What should I return when trying to dequeue an empty queue?

Question

What should I return when trying to dequeue an empty queue?

My textbook tells be to return a 'key' that signals an error.

I tried :

Element queueEmpty()
{
    Element elem = {'!'};
    printf("Queue is empty");
    return elem;
}

but figured that it's not a good design since '!' is not a unique representation of an error.

(Assuming that any char data can be a valid input)

My full code for Queue ADT & dequeue function :

typedef struct
{
    char data;
} Element;

typedef struct Node *nodePtr;
typedef struct Node
{
    Element elem;
    nodePtr next;
} Node;

nodePtr front[MAX_QUEUES];
nodePtr rear[MAX_QUEUES];





Element dequeue(int i)
{
    Element elem;
    nodePtr temp = front[i];
    if(!front[i])
    {
        return queueEmpty();
    }
    elem = temp->elem;
    front[i] = temp->next;
    free(temp);
    return elem;
}

FYI : My textbook is : "Fundamentals of Data Structures in C" by Horowitz, Sahni, Anderson-Freed

If ...:'!' is not a unique representation of an error.", then perhaps you know something that is. How would we know what that would be, when we didn't even know ! was unacceptable until you said so? — WhozCraig
– WhozCraig, Commented Apr 11, 2016 at 6:03

Schafwolle · Accepted Answer · 2016-04-11 06:55:42Z

1

Why not returning a integer and using an pointer as additional output like this:

int dequeue (int i, Element *elem)
{
nodePtr temp = front[i];
if(!front[i])
{
    return -1; // Error parttern
}
*elem = temp->elem;
front[i] = temp->next;
free(temp);
return elem;
}

Calling:

Element elem;
if(dequeue(i,&elem) == -1)
{
 // Queue was empty
}
else
{
 // do what you want to do
}

answered Apr 11, 2016 at 6:55

Schafwolle

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Comments

deeiip · Accepted Answer · 2016-04-11 06:26:03Z

0

There are two things you can try:

Return an invalid Element. In this case the user needs to check the returned element is valid or not.
Throw an exception, when the user is trying to call dequeue operation on empty queue.

Personally I'll recommend the second way.

answered Apr 11, 2016 at 6:26

deeiip

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