1. ECE 103 Engineering Programming Chapter 63 Queue Implementation
Herbert G. Mayer, PSU CS
Status 6/4/2014
Initial content copied verbatim from
ECE 103 material developed by
Professor Phillip PSU ECE

2. Syllabus
Pointer Version

3. Queue : Pointer Version
In this example, a bank queue is implemented as a singly-linked list using pointers.
Nodes are created at runtime using malloc(). A B C
head D
Queue
tail
“enqueue” operation
“dequeue” operation 2

4. Define structure of list element
struct list_node {
int id; /* Customer ID number */
float money; /* Amount of money */
bank_op op; /* Transaction type */
struct list_node *next; /* Pointer to next node */ };
typedef struct list_node node;
typedef enum { FALSE, TRUE } boolean;
typedef enum { DEPOSIT, WITHDRAWAL } bank_op; 3

5. Bank queue (singly-linked) 4 head id money op next id money op next
tail id
money op
next
NULL 4

6. Create the queue 5 int main (void) {
node *head = NULL; /* Pointer to head of queue */
node *tail = NULL; /* Pointer to tail of queue */
node *p; /* Work pointer */
/* Perform several bank transactions */
add_to_queue(&head, &tail, 1031, , DEPOSIT);
add_to_queue(&head, &tail, 2111, , DEPOSIT);
add_to_queue(&head, &tail, 3312, , WITHDRAWAL);
print_queue(head);
remove_from_queue(&head, &tail);
printf("Searching for customer ID 3312:\n");
if ((p = search_queue(head, 3312)) == NULL)
printf("ID was not found in the queue.\n");
else
print_node(p);
return 0; } 5

7. Create a new queue node
node * create_node (int id, float money, bank_op op) {
node *p; /* Work pointer */
/* Use run-time memory allocation to create new node */
if ((p = (node *) malloc(sizeof(node))) == NULL)
printf("ERROR: Unable to allocate memory for node\n");
exit(1); }
/* Initialize contents of new node with transaction information */
p->id = id;
p->money = money;
p->op = op;
p->next = NULL;
return p; /* Return pointer to newly created node */ 6

8. Check if the queue is empty
boolean is_empty (node *head) {
return (head == NULL) ? TRUE : FALSE; } 7

9. Add a new node to the tail of the queue
void add_to_queue (node **head, node **tail, int id, float money, bank_op op) {
node *new_node;
new_node = create_node(id, money, op);
if (!is_empty(*head))
/* Queue was not empty, so add node to the tail of the queue */
(*tail)->next = new_node;
*tail = new_node; }
else
/* Queue was empty, so node is now the entire queue */
*head = new_node; 8

10. Remove the node at the head of the queue
void remove_from_queue (node **head, node **tail) {
node *p = *head;
if (!is_empty(*head))
*head = (*head)->next;
if (is_empty(*head))
*tail = NULL;
free(p); /* Deallocate memory of removed node */ } 9

11. Print a single node in the queue
void print_node (node *p) {
if (!is_empty(p))
printf("ID=%4d Money= $%8.2f Op=%s\n",
p->id, p->money, p->op ? "DEP" : "WTH" ); }
else
printf("The node is NULL.\n"); 10

12. Print the contents of the entire queue
void print_queue (node *head) {
node *p = head;
if (!is_empty(head))
while (p != NULL)
printf("ID=%4d Money= $%8.2f Op=%s\n",
p->id, p->money, p->op ? "DEP" : "WTH" );
p = p->next; }
else
printf("List is empty\n");
printf("\n"); 11

13. Search the queue for a specific key value
node * search_queue (node *head, int search_key) {
node *p = head;
while (p != NULL)
if (p->id == search_key)
break; /* Found it */
p = p->next; }
return p; 12