why does this code produce a runtime error? - c++

Question

#include<stdio.h>
#include<conio.h>
#include<windows.h>
struct node
{
int a;
struct node *next;
};
void createlist(struct node **head)
{
struct node *p,*temp;
int n;
printf("enter the number\n");
scanf("%d",&n);
p=(struct node*)malloc(sizeof(struct node));
p->a=n;
if(*head==0) {
*head=p;
p->next=0;
temp=p;
}
else {
temp->next=p;
temp=p;
p->next=0;
}
}
void frontbacksplit(struct node **head,struct node **head1,struct node **head2)
{
int counter=0,i;
struct node *temp,*p;
temp=*head;
while(temp!=0) {
counter++;
temp=temp->next;
}
int n;
if(counter%2==0) {
n=counter/2;
} else {
n=(counter+1)/2;
}
temp=*head;
for(i=0;i<n-1;i++) {
if(*head1==0) {
*head1=temp;
}
temp=temp->next;
}
p=temp;
temp=temp->next;
p->next=0;
for(i=n+1;i<counter;i++) {
if(*head2==0) {
*head2=temp;
}
temp=temp->next;
}
}
void display(struct node **head)
{
struct node *temp;
temp=*head;
while(temp!=0) {
printf("%d\t",temp->a);
temp=temp->next;
}
printf("\n");
}
int main()
{
int n=1,i,k;
struct node *head3,*head1,*head2;
head3=0;
head1=0;
head2=0;
while(n==1) {
printf("enter \n1-To add the elements\n2-To split the list into front and the back\n3-To display the elements\n");
scanf("%d",&i);
switch(i)
{
case 1:
createlist(&head3);
break;
case 2:
frontbacksplit(&head3,&head1,&head2);
break;
case 3:
printf("enter\n1-To display front list\n2-To display rear list\n");
scanf("%d",&k);
switch(k)
{
case 1:
display(&head1);
break;
case 2:
display(&head2);
}
break;
default:
printf("please enter a valid option\n");
}
printf("enter\n1-To continue\nany other number to exit\n");
scanf("%d",&n);
}
getch();
return 0;
}
I have written this code for front / back splitting of the linked list. For example if the list is [1 2 3 4 5] then this program splits the list into the two lists: front part (1 2 3) and back part(4 5), if the number of elements are equal both the parts get equal number of elements.
Problem: When I try to add elements in the source list, the first elements get added as usual but when I try add other element my program shows a run time error. I think there is problem with the pointer variable temp but almost same code for the creation of linked list worked properly.
I am using dev c++ ide on windows 8.
Of course point out if you did't like the way this question is asked, as this is my first time.

Answer 1

On first look this line temp->next = p in createlist function is wrong, temp is a local variable, although you make it point to the element when the list is created for the first time the value is lost after the createlist function returns, so will you be accessing junk addresses in consecutive calls to createlist

Answer 2

The pointer variable 'temp' in createlist() function is being used without assignment on calls subsequent to the first call. You could benefit using -Wall options if you are using GCC.

Answer 3

Maybe you should change to this:
void createlist(struct node **head)
{
struct node *p,*temp;
int n;
printf("enter the number\n");
while(scanf("%d",&n) != EOF)
{
p=(struct node*)malloc(sizeof(struct node));
p->a=n;
if(*head==0)
{
*head=p;
p->next=0;
temp=p;
}
else
{
temp->next=p;
temp=p;
p->next=0;
}
}
}