Thread: Issues and questions about queues

Hi there,
Recently, on some class I'm having at university, the teacher talked about "queues" using the well known linked-list format but with another structure added to the problem. I bet you are already familiar with this stuff so you probably know more than me (that's why I'm posting this here in the first place). Anyway, the structures are these:

Code:

typedef struct sNode {
	int elem;
	struct sNode *next;
} Node;

typedef struct {
	Node *first;
	Node *last;
} Queue;

Then, I have the "enqueue" function which was coded like this:

Code:

void enQueue(Queue *q, int elem) {
	if(!q) return;
	
	Node *x = (Node*)malloc(sizeof(Node));
	
	if(!x) return;
	
	x->elem = elem;
	x->next = NULL;
	
	if(q->last) q->last->next = x; // This line is important to my post!
	else q->first = x; // This line is important to my post!
	
	q->last = x; // This line is important to my post!
}

Please note the lines with a comment. Then, on main, I randomize 5 numbers and call the function enQueue with a pre-initialized (with NULL values on 'first' and 'last') queue and the number generated. I also have a function that lists the values on the queue, separated by the linked lists 'first' and 'last'.

After adding 5 numbers to the queue and calling the function that prints the list, the output is this:

Code:

FIRST: 52 -> 38 -> 93 -> 65 -> 27
LAST: 27

Remember the commented lines in the second snippet of code posted? Well, those are the lines that I don't get how they work. I don't understand why the linked-list 'first' has all the numbers generated and why 'last' as only one number which is the last one.

In case it's important, here's the code that prints the "queue":

Code:

void listQueue(Queue q) {
	printf("\n\nFIRST: ");

	do {
		printf("%d", q.first->elem);
		q.first = q.first->next;
		
		if(q.first) printf(" -> ");
	} while(q.first);
	
	printf("\nLAST: ");

	do {
		printf("%d", q.last->elem);
		q.last = q.last->next;
		
		if(q.last) printf(" -> ");
	} while(q.last);
}

Can somebody explain me as easy as possible how those [commented] lines work and what are they doing exactly?

I also have another question but I'll leave that after I understand this bit of code otherwise I'll be much more confused. Home my post is detailed and not confusing...