Problem with LinkList in C++ Programming:

[bhattaroshan]

When i write this it works fine

#include<iostream>
#include<conio.h>
#include<stdio.h>

using namespace std;

typedef struct stack
{
        int num;
        stack *lpnext;
};

stack *head=0;

void push_me_back(int num)
{
     stack *tmp,*tmp2;
     
      tmp=new stack;
      tmp->num=num;
      tmp->lpnext=0;
       
     if(head==0)
      {
       head=tmp;
      }
     else
      {
       tmp2=head;
       
       while(tmp2->lpnext)
        tmp2=tmp2->lpnext;

       tmp2->lpnext=tmp;
      }
}

int main()
{
    push_me_back(10);
    push_me_back(20);
    push_me_back(30);
    
    cout<<head->lpnext->num;
    
    getch();
    return 0;
}

But when i make slight change in code as follows code is not working what is the difference. Is there anything difference in the code:

#include<iostream>
#include<conio.h>
#include<stdio.h>

using namespace std;

typedef struct stack
{
        int num;
        stack *lpnext;
};

stack *head=0;

void push_me_back(int num)
{
     stack *tmp,*tmp2;
     
      tmp=new stack;
      tmp->num=num;
      tmp->lpnext=0;
       
     if(head==0)
      {
       head=tmp;
      }
     else
      {
       tmp2=head->lpnext; //made changes here
       
       while(tmp2)  //made changes here
        tmp2=tmp2->lpnext;

       tmp2=tmp; //made changes here
      }
}

int main()
{
    push_me_back(10);
    push_me_back(20);
    push_me_back(30);
    
    cout<<head->lpnext->num;
    
    getch();
    return 0;
}

 

[jeffrey samuel]

Code:
#include<iostream>
#include<conio.h>
#include<stdio.h>

using namespace std;

typedef struct stack
{
        int num;
        stack *lpnext;
};

stack *head=0;

void push_me_back(int num)
{
     stack *tmp,*tmp2;
     
      tmp=new stack;
      tmp->num=num;
      tmp->lpnext=0;
       
     if(head==0)
      {
       head=tmp;
      }
     else
      {
       tmp2=head->lpnext; //made changes here   [COLOR="#FFD700"]since the pointer has null stored in it it flags an error[/COLOR]
       
       while(tmp2)  //made changes here[COLOR="#FF0000"]there is null in tmp2 and so the segment is omitted [/COLOR]
        tmp2=tmp2->lpnext;

       tmp2=tmp; //made changes here[COLOR="#FF0000"]as the above conditions failed this is also flagged as an error [/COLOR]
      }
}

int main()
{
    push_me_back(10);
    push_me_back(20);
    push_me_back(30);
    
    cout<<head->lpnext->num;
    
    getch();
    return 0;
}

my corrections are in colour hope it helps

 

[bhattaroshan]

thanks for your reply

if so then why does the first code that i posted runs without an error
even tmp2->lpnext is null pointer.......

 

[jeffrey samuel]

there you have defined the pointer

there is no memory allocated to the pointer in your program
that is the reason for the error to be flagged

remember that pointer gets defined only when memory is assigned to it

 

[bhattaroshan]

Let me write another example exactly as above but with pointer to integer.

#include<iostream>
#include<conio.h>
#include<stdio.h>

using namespace std;

int main()
{
    int *b,*a,*c;
    
    c=new int;
    
    b=NULL; //it is not pointed any where
    
    a=b;  //according to you this is wrong
    
    while(a); //this is also wrong according to you
    
    a=c; //this is also wrong 
    
    *a=10; //but all these works well
    
    getch();
    return 0;
}

If what you told is correct why does this code run well?

I might not be understanding linklist? If you have any tutorials or site, or anything please help me out.

Thanks for you reply.

 

[jeffrey samuel]

try out a book by NV publications for data structures and stuff like that

also go for pearson publication's book they are worth trying


the difference in these coding is simple int is a default data type

whereas you are using a abstract datatype in list that makes a huge difference in the applications of them in the programs