#include <stdio.h>
#include <stdlib.h>

struct node{
    int data;
    struct node * next;
}*head=NULL;

void create(int A[],int n){
    struct node* last,*t;
    head=(struct node *)malloc(sizeof(struct node));
    head->data = A[0];
    head->next=NULL;
    last = head;
    
    for(int i=1;i<n;i  ){
        t=(struct node *)malloc(sizeof(struct node));
        t->data = A[i];
        last->next=t;
        t->next = NULL;
        last= t;
    }
    
}
void display(struct node *p){
    while(p!=NULL){
        printf("%d ",p->data);
        p=p->next;
    }
}

void midofLL(){
    struct node *q,*p=head;
    while(q!=NULL){
        q=q->next;
        if(q) q=q->next;
        if(q) p=p->next;
    }
   printf("%d",p->data);
    
}

int main()
{
  int A[]={2,3,6,7,8};
  create(A,5);
  display(head);
  printf("\n");
  midofLL();
}

I am finding the middle node in a singly liked list using two pointers p and q .q incremented by 2 nodes,p incremented by 1 node. What is the error in this ? Only the linked list is getting printed.I need the data stored in middle element. How to correct my code?

CodePudding user response：

As stated in comments: in midofLL():

• Pointer q is declared but not assigned a value. Try:

  struct node *q = head, *p = head;
  

• The first advance of q should also be an advance of p... p "misses out" on the possible second advance of q... You'll be off a bit with your current logic... Gotta mind those 'P's & 'Q's...

CodePudding user response：

your problem is in this line struct node *q,*p=head; as you initialized pointer called p but you didn't initialize the pointer q , so you made pointer q as a wild pointer which can make undefined behavior , the line you wrote is equivalent to writing :

struct node *q;
struct node *p = head;

so in order to not get confused , it's the best practice to separate the declaration of the variables , so do this :

struct node *q = head;
struct node *p = head;

instead of this struct node *q,*p=head;

and here is the fill edited code :

    #include <stdio.h>
#include <stdlib.h>

struct node{
    int data;
    struct node * next;
}*head=NULL;

void create(int A[],int n){
    struct node* last,*t;
    head=(struct node *)malloc(sizeof(struct node));
    head->data = A[0];
    head->next=NULL;
    last = head;

    for(int i=1;i<n;i  ){
        t=(struct node *)malloc(sizeof(struct node));
        t->data = A[i];
        last->next=t;
        t->next = NULL;
        last= t;
    }

}
void display(struct node *p){
    while(p!=NULL){
        printf("%d ",p->data);
        p=p->next;
    }
}

void midofLL(){
    struct node *q = head;
    struct node *p =head;
    while(q!=NULL){
        q=q->next;
        if(q) q=q->next;
        if(q) p=p->next;
    }
   printf("%d",p->data);

}

int main()
{
  int A[]={2,3,6,7,8};
  create(A,5);
  display(head);
  printf("\n");
  midofLL();
}