linked list using double pointers - c

Question

Using double pointers first time to create and display linked list
#include "stdio.h"
#include "stdlib.h"
struct node
{
int data;
struct node * next;
};
void Insert(struct node **, int , int );
void display(struct node *);
int main()
{
int c, data, position;
struct node* head;
do{
printf("Enter a choice :\n");
printf("1. Add an element.\n");
printf("2. Del an element.\n3.Display List.\n");
printf("4.Delete linked list.\n5.Exit.\n");
printf("Your Choice :");
scanf("%d",&c);
switch(c){
case 1 :
printf("\nEnter data and position :\n");
scanf("%d %d",&data,&position);
Insert(&head,data,position);
break;
case 2 :
break;
case 3 :
printf("Linked List : \n");
display(head);
break;
case 4 :
break;
case 5 :
exit(0);
default :
printf("Invalid Choice.\n");
break;
}
}while(1);
return 0;
}
void Insert(struct node **ptrhead, int item, int position){
struct node *p,*newnode;
//node creation.
newnode = (struct node *)malloc(sizeof(struct node));
if (!newnode)
{
printf("Memory Error.\n");
return;
}
newnode->next = NULL;
newnode->data = item;
p = *ptrhead;
// Creates initial node
if (!(p->data))
{
p = newnode;
}
// insertion at beginning
if (position==1)
{
newnode->next = p;
p = newnode;
free(newnode);
}
// insertionn at middle or end.
else
{
int i=1;
while(p->next!=NULL && i<position-1){
p=p->next;
i++;
}
newnode->next = p->next;
p->next = newnode;
}
*ptrhead = p;
};
// Display Linked list
void display(struct node *head){
if (head)
{
do{
printf("%d\n", head->data);
head = head->next;
}while(head->next);
}
};
I will add functions for deletion and other operations later. Right now , I just want to insert and display fns to work . But output comes as infinitely running loop with wrong values. I cannot figure out what's wrong in my code , please help ?
Thanks in advance.

Answer 1

Not sure why somebody would be writing this type of C today, looks like maybe I'm doing your homework for you... In any case, you asked to fix your code, not rewrite it, so here's the minimum set of changes.
head should be initialized to NULL.
if (!(p->data)) is not right. That if statement should just be:
// Creates initial node
if (!p)
{
*ptrhead = newnode;
return;
}
Remove free(newnode);.
The insert at middle/end code could be
int i = 1;
struct node *n = p;
while (n->next != NULL && i<position - 1){
n = n->next;
i++;
}
newnode->next = n->next;
n->next = newnode;
The final insert function:
void Insert(struct node **ptrhead, int item, int position)
{
struct node *p, *newnode;
//node creation.
newnode = (struct node *)malloc(sizeof(struct node));
if (!newnode)
{
printf("Memory Error.\n");
return;
}
newnode->next = NULL;
newnode->data = item;
p = *ptrhead;
// Creates initial node
if (!p)
{
*ptrhead = newnode;
return;
}
// insertion at beginning
if (position == 1)
{
newnode->next = p;
p = newnode;
}
// insertionn at middle or end.
else
{
int i = 1;
struct node *n = p;
while (n->next != NULL && i<position - 1){
n = n->next;
i++;
}
newnode->next = n->next;
n->next = newnode;
}
*ptrhead = p;
}
Your print function isn't quite right, just make it:
// Display Linked list
void display(struct node *head)
{
while (head)
{
printf("%d\n", head->data);
head = head->next;
}
}