linked list spring 2013programming and data structure1
TRANSCRIPT
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Linked List
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Introduction• A linked list is a data structure which can change
during execution.– Successive elements are connected by pointers.– Last element points to NULL.– It can grow or shrink in size during execution of a
program.– It can be made just as long as required.– It does not waste memory space.
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A B C
head
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• Keeping track of a linked list:– Must know the pointer to the first element of the
list (called start, head, etc.).
• Linked lists provide flexibility in allowing the items to be rearranged efficiently.– Insert an element.– Delete an element.
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Illustration: Insertion
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AA
Item to be inserted
X
X
A B C
B C
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Illustration: Deletion
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A B
A B C
C
Item to be deleted
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In essence ...• For insertion:– A record is created holding the new item.– The next pointer of the new record is set to link it to
the item which is to follow it in the list.– The next pointer of the item which is to precede it
must be modified to point to the new item.
• For deletion:– The next pointer of the item immediately preceding
the one to be deleted is altered, and made to point to the item following the deleted item.
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Array versus Linked Lists• Arrays are suitable for:– Inserting/deleting an element at the end.– Randomly accessing any element.– Searching the list for a particular value.
• Linked lists are suitable for:– Inserting an element.– Deleting an element.– Applications where sequential access is required.– In situations where the number of elements cannot
be predicted beforehand.
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Types of Lists
• Depending on the way in which the links are used to maintain adjacency, several different types of linked lists are possible.
– Linear singly-linked list (or simply linear list)• One we have discussed so far.
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A B C
head
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– Circular linked list• The pointer from the last element in the list points back
to the first element.
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A B C
head
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– Doubly linked list• Pointers exist between adjacent nodes in both
directions.• The list can be traversed either forward or backward.• Usually two pointers are maintained to keep track of
the list, head and tail.
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A B C
head tail
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Basic Operations on a List
• Creating a list• Traversing the list• Inserting an item in the list• Deleting an item from the list• Concatenating two lists into one
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List is an Abstract Data Type• What is an abstract data type?– It is a data type defined by the user.– Typically more complex than simple data types like
int, float, etc.
• Why abstract?– Because details of the implementation are hidden.– When you do some operation on the list, say insert
an element, you just call a function.– Details of how the list is implemented or how the
insert function is written is no longer required.
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Conceptual Idea
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List implementation
and therelated functions
Insert
Delete
Traverse
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Example: Working with linked list• Consider the structure of a node as follows:
struct stud { int roll; char name[25]; int age; struct stud *next; };
/* A user-defined data type called “node” */typedef struct stud node;node *head;
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Creating a List
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How to begin?• To start with, we have to create a node (the
first node), and make head point to it.
head = (node *) malloc(sizeof(node));
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head
age
name
roll
next
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Contd.
• If there are n number of nodes in the initial linked list:– Allocate n records, one by one.– Read in the fields of the records.– Modify the links of the records so that the chain is
formed.
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A B C
head
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node *create_list() { int k, n; node *p, *head;
printf ("\n How many elements to enter?"); scanf ("%d", &n);
for (k=0; k<n; k++) { if (k == 0) { head = (node *) malloc(sizeof(node)); p = head;
} else {
p->next = (node *) malloc(sizeof(node)); p = p->next;
}
scanf ("%d %s %d", &p->roll, p->name, &p->age); }
p->next = NULL; return (head);}
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• To be called from main() function as:
node *head;
………
head = create_list();
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Traversing the List
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What is to be done?
• Once the linked list has been constructed and head points to the first node of the list,– Follow the pointers.– Display the contents of the nodes as they are
traversed.– Stop when the next pointer points to NULL.
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void display (node *head){ int count = 1; node *p; p = head; while (p != NULL) { printf ("\nNode %d: %d %s %d", count, p->roll, p->name, p->age); count++; p = p->next; } printf ("\n");}
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• To be called from main() function as:
node *head;
………
display (head);
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Inserting a Node in a List
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How to do?
• The problem is to insert a node before a specified node.– Specified means some value is given for the node
(called key).– In this example, we consider it to be roll.
• Convention followed:– If the value of roll is given as negative, the node
will be inserted at the end of the list.
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Contd.• When a node is added at the beginning,– Only one next pointer needs to be modified.
• head is made to point to the new node.• New node points to the previously first element.
• When a node is added at the end,– Two next pointers need to be modified.
• Last node now points to the new node.• New node points to NULL.
• When a node is added in the middle,– Two next pointers need to be modified.
• Previous node now points to the new node.• New node points to the next node.
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void insert (node **head) { int k = 0, rno; node *p, *q, *new; new = (node *) malloc(sizeof(node));
printf ("\nData to be inserted: "); scanf ("%d %s %d", &new->roll, new->name, &new->age); printf ("\nInsert before roll (-ve for end):"); scanf ("%d", &rno);
p = *head;
if (p->roll == rno) /* At the beginning */ { new->next = p; *head = new; }
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else {
while ((p != NULL) && (p->roll != rno)) { q = p; p = p->next; } if (p == NULL) /* At the end */ { q->next = new; new->next = NULL; }
else if (p->roll == rno) /* In the middle */ { q->next = new; new->next = p; } }}
The pointers q and p always point to consecutive nodes.
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• To be called from main() function as:
node *head;
………
insert (&head);
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Deleting a node from the list
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What is to be done?
• Here also we are required to delete a specified node.– Say, the node whose roll field is given.
• Here also three conditions arise:– Deleting the first node.– Deleting the last node.– Deleting an intermediate node.
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void delete (node **head) { int rno; node *p, *q; printf ("\nDelete for roll :"); scanf ("%d", &rno);
p = *head; if (p->roll == rno) /* Delete the first element */ { *head = p->next; free (p); }
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else { while ((p != NULL) && (p->roll != rno)) { q = p; p = p->next; }
if (p == NULL) /* Element not found */ printf ("\nNo match :: deletion failed"); else if (p->roll == rno) /* Delete any other element */ { q->next = p->next; free (p); } }}
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Few Exercises to Try Out• Write a function to:– Concatenate two given list into one big list.
node *concatenate (node *head1, node *head2);– Insert an element in a linked list in sorted order.
The function will be called for every element to be inserted.
void insert_sorted (node **head, node *element);– Always insert elements at one end, and delete
elements from the other end (first-in first-out QUEUE).
void insert_q (node **head, node *element) node *delete_q (node **head) /* Return the deleted node */
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A First-in First-out (FIFO) List
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Also called a QUEUE
In Out
AC BAB
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A Last-in First-out (LIFO) List
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In Out
ABC CB
Also called a
STACK
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Abstract Data Types
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Example 1 :: Complex numbers struct cplx { float re; float im; } typedef struct cplx complex;
complex *add (complex a, complex b); complex *sub (complex a, complex b); complex *mul (complex a, complex b); complex *div (complex a, complex b); complex *read(); void print (complex a);
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Structure definition
Function prototypes
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ComplexNumber
add
mul
sub
read
div
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Example 2 :: Set manipulation struct node { int element; struct node *next; } typedef struct node set;
set *union (set a, set b); set *intersect (set a, set b); set *minus (set a, set b); void insert (set a, int x); void delete (set a, int x); int size (set a);
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Structure definition
Function prototypes
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Set
union
size
minus
intersect
delete
insert
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Example 3 :: Last-In-First-Out STACK
Assume:: stack contains integer elements
void push (stack *s, int element); /* Insert an element in the stack */ int pop (stack *s); /* Remove and return the top element */ void create (stack *s); /* Create a new stack */ int isempty (stack *s); /* Check if stack is empty */ int isfull (stack *s); /* Check if stack is full */
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STACK
push
create
pop
isfull
isempty
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Contd.
• We shall look into two different ways of implementing stack:– Using arrays– Using linked list
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Example 4 :: First-In-First-Out QUEUE
Assume:: queue contains integer elements
void enqueue (queue *q, int element); /* Insert an element in the queue */ int dequeue (queue *q); /* Remove an element from the queue */ queue *create(); /* Create a new queue */ int isempty (queue *q); /* Check if queue is empty */ int size (queue *q); /* Return the no. of elements in queue */
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QUEUE
enqueue
create
dequeue
size
isempty
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Stack Implementations: Using Array and Linked List
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STACK USING ARRAY
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top
top
PUSH
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STACK USING ARRAY
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top
top
POP
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Stack: Linked List Structure
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top
PUSH OPERATION
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Stack: Linked List Structure
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top
POP OPERATION
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Basic Idea• In the array implementation, we would:
– Declare an array of fixed size (which determines the maximum size of the stack).
– Keep a variable which always points to the “top” of the stack.
• Contains the array index of the “top” element.
• In the linked list implementation, we would:– Maintain the stack as a linked list.
– A pointer variable top points to the start of the list.– The first element of the linked list is considered as the stack top.
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Declaration#define MAXSIZE 100
struct lifo { int st[MAXSIZE]; int top;};typedef struct lifo stack;stack s;
struct lifo { int value; struct lifo *next;};typedef struct lifo stack;
stack *top;
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ARRAY LINKED LIST
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Stack Creationvoid create (stack *s){ s->top = -1; /* s->top points to last element pushed in; initially -1 */}
void create (stack **top){ *top = NULL;
/* top points to NULL, indicating empty stack */}
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ARRAYLINKED LIST
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Pushing an element into the stackvoid push (stack *s, int element) { if (s->top == (MAXSIZE-1)) { printf (“\n Stack overflow”); exit(-1); } else { s->top ++; s->st[s->top] = element; } }
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ARRAY
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void push (stack **top, int element){ stack *new;
new = (stack *) malloc(sizeof(stack)); if (new == NULL) { printf (“\n Stack is full”); exit(-1); }
new->value = element; new->next = *top; *top = new;}
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LINKED LIST
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Popping an element from the stackint pop (stack *s) { if (s->top == -1) { printf (“\n Stack underflow”); exit(-1); } else { return (s->st[s->top--]); } }
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ARRAY
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int pop (stack **top){ int t; stack *p;
if (*top == NULL) { printf (“\n Stack is empty”); exit(-1); } else { t = (*top)->value; p = *top; *top = (*top)->next; free (p); return t; }}
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LINKED LIST
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Checking for stack emptyint isempty (stack *s){ if (s->top == -1) return 1; else return (0);}
int isempty (stack *top){ if (top == NULL) return (1); else return (0);}
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ARRAY LINKED LIST
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Checking for stack fullint isfull (stack *s){ if (s->top == (MAXSIZE–1))
return 1; else return (0);}
• Not required for linked list implementation.
• In the push() function, we can check the return value of malloc().– If -1, then memory cannot be
allocated.
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ARRAY LINKED LIST
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Example main function :: array#include <stdio.h>#define MAXSIZE 100
struct lifo { int st[MAXSIZE]; int top;};typedef struct lifo stack;
main(){ stack A, B; create(&A); create(&B); push(&A,10); push(&A,20);
push(&A,30); push(&B,100); push(&B,5);
printf (“%d %d”, pop(&A), pop(&B));
push (&A, pop(&B));
if (isempty(&B)) printf (“\n B is empty”);}
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Example main function :: linked list#include <stdio.h>struct lifo { int value; struct lifo *next;};typedef struct lifo stack;
main(){ stack *A, *B; create(&A); create(&B); push(&A,10); push(&A,20);
push(&A,30); push(&B,100);
push(&B,5);
printf (“%d %d”, pop(&A), pop(&B));
push (&A, pop(&B));
if (isempty(B)) printf (“\n B is
empty”);}
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Queue Implementation using Linked List
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Basic Idea• Basic idea:– Create a linked list to which items would be added
to one end and deleted from the other end.– Two pointers will be maintained:• One pointing to the beginning of the list (point from
where elements will be deleted).• Another pointing to the end of the list (point where
new elements will be inserted).
Spring 2013 Programming and Data Structure 65
Front
Rear
DELETION INSERTION
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QUEUE: LINKED LIST STRUCTURE
Spring 2013 Programming and Data Structure 66
front rear
ENQUEUE
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QUEUE: LINKED LIST STRUCTURE
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front rear
DEQUEUE
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QUEUE using Linked List
Spring 2013 Programming and Data Structure 68
#include <stdio.h>#include <stdlib.h>#include <string.h>
struct node{ char name[30]; struct node *next; };
typedef struct node _QNODE;
typedef struct { _QNODE *queue_front, *queue_rear; } _QUEUE;
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Spring 2013 Programming and Data Structure 69
_QNODE *enqueue (_QUEUE *q, char x[]){_QNODE *temp;temp= (_QNODE *) malloc (sizeof(_QNODE));if (temp==NULL){printf(“Bad allocation \n");return NULL;}strcpy(temp->name,x);temp->next=NULL;
if(q->queue_rear==NULL){q->queue_rear=temp;q->queue_front= q->queue_rear;}else{q->queue_rear->next=temp;q->queue_rear=temp;}return(q->queue_rear);}
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Spring 2013 Programming and Data Structure 70
char *dequeue(_QUEUE *q,char x[]){_QNODE *temp_pnt;
if(q->queue_front==NULL){q->queue_rear=NULL;printf("Queue is empty \n");return(NULL);}
else{strcpy(x,q->queue_front->name);temp_pnt=q->queue_front;q->queue_front= q->queue_front->next;free(temp_pnt);if(q->queue_front==NULL) q->queue_rear=NULL;return(x); }}
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Spring 2013 Programming and Data Structure 71
void init_queue(_QUEUE *q){ q->queue_front= q->queue_rear=NULL;}
int isEmpty(_QUEUE *q){ if(q==NULL) return 1; else return 0;}
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Spring 2013 Programming and Data Structure 72
main(){int i,j;char command[5],val[30];_QUEUE q;
init_queue(&q);
command[0]='\0';printf("For entering a name use 'enter <name>'\n");printf("For deleting use 'delete' \n"); printf("To end the session use 'bye' \n");while(strcmp(command,"bye")){scanf("%s",command);
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Spring 2013 Programming and Data Structure 73
if(!strcmp(command,"enter")) {scanf("%s",val);if((enqueue(&q,val)==NULL)) printf("No more pushing please \n");else printf("Name entered %s \n",val);}
if(!strcmp(command,"delete")) {if(!isEmpty(&q))printf("%s \n",dequeue(&q,val)); else printf("Name deleted %s \n",val);}} /* while */printf("End session \n");}
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Problem With Array Implementation
Spring 2013 Programming and Data Structure 74
front rearrear
ENQUEUE
front
DEQUEUE
Effective queuing storage area of array gets reduced.
Use of circular array indexing
0 N
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Queue: Example with Array Implementation
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typedef struct { char name[30]; } _ELEMENT;
typedef struct { _ELEMENT q_elem[MAX_SIZE]; int rear; int front; int full,empty; } _QUEUE;
#define MAX_SIZE 100
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Queue Example: Contd.
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void init_queue(_QUEUE *q) {q->rear= q->front= 0; q->full=0; q->empty=1;}
int IsFull(_QUEUE *q) {return(q->full);}
int IsEmpty(_QUEUE *q) {return(q->empty);}
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Queue Example: Contd.
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void AddQ(_QUEUE *q, _ELEMENT ob) { if(IsFull(q)) {printf("Queue is Full \n"); return;}
q->rear=(q->rear+1)%(MAX_SIZE); q->q_elem[q->rear]=ob;
if(q->front==q->rear) q->full=1; else q->full=0; q->empty=0;
return; }
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Queue Example: Contd.
Spring 2013 Programming and Data Structure 78
_ELEMENT DeleteQ(_QUEUE *q) { _ELEMENT temp; temp.name[0]='\0';
if(IsEmpty(q)) {printf("Queue is EMPTY\n");return(temp);}
q->front=(q->front+1)%(MAX_SIZE); temp=q->q_elem[q->front];
if(q->rear==q->front) q->empty=1; else q->empty=0; q->full=0;
return(temp); }
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Queue Example: Contd.
Spring 2013 Programming and Data Structure 79
main(){int i,j;char command[5];_ELEMENT ob;_QUEUE A;
init_queue(&A);
command[0]='\0'; printf("For adding a name use 'add [name]'\n"); printf("For deleting use 'delete' \n"); printf("To end the session use 'bye' \n");
#include <stdio.h>#include <stdlib.h>#include <string.h>
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Queue Example: Contd.
Spring 2013 Programming and Data Structure 80
while (strcmp(command,"bye")!=0){ scanf("%s",command); if(strcmp(command,"add")==0) { scanf("%s",ob.name); if (IsFull(&A)) printf("No more insertion please \n"); else { AddQ(&A,ob); printf("Name inserted %s \n",ob.name); } }
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Queue Example: Contd.
Spring 2013 Programming and Data Structure 81
if (strcmp(command,"delete")==0) { if (IsEmpty(&A)) printf("Queue is empty \n"); else { ob=DeleteQ(&A); printf("Name deleted %s \n",ob.name); } } } /* End of while */ printf("End session \n");}