infinite loop while trying to insert node before a node in singly linked list

Question

I tried to insert a node before a given node by specifying the position of the node before which I want to insert the newnode. I got the data present inside that node's position and using a while loop, compared this data with each node's data till I reached the point where I was supposed to insert the node.

But when I tried displaying the elements using a while statement my program went into an infinte loop.I checked where the head node was pointing to and its to pointing to the first node of the singly list only. Could someone please help me out?

#include <iostream>
#include<stdlib.h>
using namespace std;
void display(struct node *);
struct node{
    int data;
    struct node *next;
};
struct node *ptr,*head=NULL;
void insertt(struct node *head){   //insert function to insert node before a node
    struct node *ptr1=head;
    int pos,poss,value;
    cin>>poss;   //getting position and value
    cin>>value;
    pos=poss-1; 
    while(pos--){   //moving to that position
        ptr1=ptr1->next;
    }
    struct node *ptr2=ptr1;
    struct node *newnode = (struct node*)malloc(sizeof(struct node)); //creating new node for insertion
    newnode->next=NULL;
    newnode->data=value;
    struct node *preptr;
    preptr=ptr1;
    int c=ptr2->data;  //getting value present in that particular position(node) of the list
    while(ptr1->data!=c){  //inserting before node
        preptr=ptr1;
        ptr1=ptr1->next;  
    }
    preptr->next=newnode;
    newnode->next=ptr1;
    
    display(head);
    
    
}
void display(struct node *head){  //displaying linked list
    struct node *ptr2=head;
    while(ptr2!=NULL){
        cout<<ptr2->data;
        ptr2=ptr2->next;
    }
}
int main()
{
    int n,val,i;
    cin>>n;   //number of nodes
    for(i=0;i<n;i++){        //node creation
        cin>>val;
        struct node *newnode = (struct node*)malloc(sizeof(struct node));
        newnode->data=val;
        newnode->next=NULL;
        if(head==NULL){
            ptr=head;
            head=newnode;
        }
        else{
            ptr=head;
            while(ptr->next!=NULL){
                ptr=ptr->next;
            }
            ptr->next=newnode;
        }
    }
        insertt(head); //insertion

    return 0;
}

Answer 1

• Once you find ptr1, you set preptr and ptr2 to ptr1.
• Then you get c = ptr2->data, which happens to be the same as ptr1->data.
• And you go on and check while (ptr1->data != c), which is always false. So that bottom while loop does nothing
• And you get to the last lines with preptr and ptr1 pointing to the same node n.
• Now you make n->next (preptr->next) point to newnode, and newnode->next point to ptr1 (n): an infinite loop.

               n           newnode
            --------      ---------
preptr ---> |  c   |      | value |
ptr1   ---> --------      ---------
            | next | ---> |       |
            |      | <--- | next  |
            --------      ---------

void insertt(struct node *head) {
    struct node *ptr1 = head;
    int pos, poss, value;
    cin >> poss;
    cin >> value;
    pos = poss - 1;
    while (pos--) {
        ptr1 = ptr1->next;
    }
    struct node *ptr2 = ptr1;
    struct node *newnode = (struct node *) malloc(sizeof(struct node));
    newnode->next = NULL;
    newnode->data = value;
    struct node *preptr;
    preptr = ptr1;  // ptr1, ptr2, and preptr point to the same node n
    int c = ptr2->data;  // c = ptr1->data too
    while (ptr1->data != c) {  // always false
        preptr = ptr1;
        ptr1 = ptr1->next;
    }
    preptr->next = newnode;  // n->next = newnode
    newnode->next = ptr1;  // newnode-next = n

    display(head);
}