c++ - Why is my output not showing the node containing 14 when I print?

Question

Why is my output not showing the node containing 14 when I print it?

I guess I am missing some crucial principles in implementing a linked list:

#include<iostream>
using namespace std;
struct ListNode
{
    int value;
    ListNode *next;
    ListNode(int d,ListNode* p=NULL) //constructor
    {
        value=d;
        next=p;
    }
};

int main()
{
    ListNode* header=NULL;
    header=new ListNode(5);
    ListNode* ptr=header; //pointer to find the correct position

    ListNode* sptr=new ListNode(13);
    header->next=sptr;
    ListNode* tptr=new ListNode(19);
    sptr->next=tptr;
    ListNode* t=new ListNode(14);
    while((ptr->value) < (t->value))
    {
        ptr=ptr->next;
    }

    ListNode* g=ptr;
    ptr=t;
    t->next=g;

    while(header!=NULL)
    {
        cout<<header->value<<" ";
        header=header->next;
    }

    return 0;
}

question from:https://stackoverflow.com/questions/66046335/why-is-my-output-not-showing-the-node-containing-14-when-i-print

Answer 1

You did not add the node with the value 14 to the list. That is neither node in the list points to the node created in this declaration

 ListNode* t=new ListNode(14);

The only what you did is set the data member next of the node to the address of the node with the value 19.

ListNode* g=ptr;
ptr=t;
t->next=g;

Here the statement

ptr=t;

does not make an effect.

If after this loop

while(header!=NULL)
{
    cout<<header->value<<" ";
    header=header->next;
}

you will place this code snippet in your program

cout << '
';

while ( ptr != NULL )
{
    cout<<ptr->value<<" ";
    ptr=ptr->next;
}

you will get the program output as

5 13 19 
14 19 

To make it clear then pay attention to that after this loop

  while((ptr->value) < (t->value))
  {
    ptr=ptr->next;
  }

the pointer ptr points to the node with the value 19. And you can not use the pointer to insert the new node with the value 14 before this node because you have a singly-linked list and the node with the value 19 has only one reference to the next node.

So you can insert the new node with the value 14 only after the node with the value 19 using the pointer ptr obtained after the while loop.

If you want to insert the new node with the value 14 before the node with the value 19 then instead of this code snippet

  while((ptr->value) < (t->value))
  {
    ptr=ptr->next;
  }
  
ListNode* g=ptr;
ptr=t;
t->next=g;

you should write something like the following

  ListNode *prev = nullptr;   
  while( ptr != nullptr && ptr->value < t->value )
  {
    prev = ptr;
    ptr=ptr->next;
  }
  
  t->next = ptr;
  prev == nullptr ? header = t : prev->next = t;