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C Program To Implement Singly Linked List.

A linked list is a data structure consisting of a group of nodes which together represent a sequence. Under the simplest form, each node contains one or more data fields and a reference to the next node. The last node points to a null reference to indicate the end of the list. 

 The entry point into a linked list is the head of the list. Head is not a separate node, but a reference to the first Node in the list. If the list is empty, then the head has the value null. Linked list allows for efficient insertion, deletion, and traversal of elements in the sequence.

Example: A singly linked list whose nodes contain two fields: an integer value and a link to the next node. The last node is linked to a terminator used to signify the end of the list.



Source code of Singly Linked List:
# include<stdio.h>
# include<stdlib.h>

struct node {
int data;
struct node *link;
};

void append(struct node **,int);
void in_begin(struct node **,int);
void del(struct node **,int);

void in_middle(struct node **,int,int);
int count(struct node *);
void display(struct node *);

int main() {
struct node *p; /* p can be said as the head or a start ptr */
p=NULL;
/* Printing the menu */
int num,loc;
char choice;

while(1) {
fflush(stdin);
printf("\nPROGRAM TO IMPLEMENT SINGLY LINKED LIST ");
printf("\n=======================================");
printf("\n\n1. Create / Appending The List");
printf("\n2. Insert Node At Begining");
printf("\n3. Insert Node In Middle");
printf("\n4. Deleting A Node");
printf("\n5. Counting The No. Of Nodes");
printf("\n6. Displaying the List");
printf("\n7. Exit");
printf("\n\nEnter Your Choice : ");
scanf("%c",&choice);

switch(choice) {
case '1': {
char ans;
int a=1;

while(a) {
printf("Enter Any Number : ");
scanf("%d",&num);

append(&p,num);
fflush(stdin);
printf("Enter More (y/n) : ");
scanf("%c",&ans);

if(ans=='n')
a=0;
}
break;
}

case '2':
printf("Enter The Data : ");
scanf("%d",&num);
in_begin(&p,num);
break;

case '3':
printf("\nEnter The Position : ");
scanf("%d",&loc);
printf("\nEnter The Data : ");
scanf("%d",&num);
in_middle(&p,loc,num);
break;

case '4':
printf("\nEnter The Data You Want To Delete : ");
scanf("%d",&num);
del(&p,num);
break;

case '5':
printf("\nThe No Of Nodes Are %d \n",count(p));
break;

case '6':
display(p);
break;

case '7':
return 0;

default:
printf("Invalid choice. Please Enter Correct Choice. \n\n");
break;
}}
return 0;
}

void append(struct node **q,int num) {
struct node *temp,*r;
temp = *q;

if(*q==NULL) {
temp = (struct node *)malloc(sizeof(struct node));
temp->data=num;
temp->link=NULL;
*q=temp;
}

else {
temp = *q;

while(temp->link !=NULL) {
temp=temp->link;
}

r = (struct node *)malloc(sizeof(struct node));
r->data=num;
r->link=NULL;
temp->link=r;
}}

void display(struct node *q) {
if(q==NULL) {
printf("\nEmpty Linked List. Can't Display The Data\n");
}

else
while(q!=NULL) {
printf("\t%d",q->data);
q=q->link;
}
printf("\n");
}

int count(struct node *q) {
int c=0;

if(q==NULL) {
printf("Empty Linked List.\n");
goto last;
}

while(q!=NULL) {
c++;
q=q->link;
}
last:
return c;
}

void in_begin(struct node **q,int num) {
struct node *temp;

if(*q==NULL) {
printf("\nLinked List Is Empty. Can't Insert.\n\n");
}

else {
temp=(struct node *)malloc(sizeof(struct node));
temp->data=num;
temp->link=*q;
*q=temp; /* pointing to the first node */
}}

void in_middle(struct node **q,int loc,int num) {
struct node *temp,*n;
int c=1,flag=0;
temp=*q;

if(*q==NULL) { printf("\nLinked List Is Empty. Can't Insert.\n\n");
}

else
while(temp!=NULL) {

if(c==loc) {
n = (struct node *)malloc(sizeof(struct node));
n->data=num;
n->link=temp->link;
temp->link=n;
flag=1;
}
c++;
temp=temp->link;
}

if(flag==0) {
printf("\nNode Specified Doesn't Exist. Can't Enter The Data.\n\n");
}

else { printf("\nData Inserted.\n");
}}

void del(struct node**q,int num) {
if(*q==NULL) {
printf("\nEmpty Linked List. Can't Delete The Data.\n\n");
}

else {
struct node *old,*temp;

old=(struct node*)malloc(sizeof(struct node));
int flag=0;
temp=*q;

while(temp!=NULL) {
if(temp->data==num) {
if(temp==*q) /* First Node case */
*q=temp->link; /* shifted the header node */

else old->link=temp->link;
free(temp);
flag=1;
break;
}

else {
old=temp;
temp=temp->link;
}}

if(flag==0)
printf("\nData Not Found...\n");
else
printf("\nData Deleted...\n");
}}
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