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Delete unsucessful.

renzokuzen (11)
Urm....i have a problem with the delete link part and i really do not know what's wrong with my codes...i am showing such a lengthy code just in case you guys need to really know what i am doing. If else, please scroll down to the bold part of the code for my problem (the codes cant really be bold tho =.="). I hope you guys can help me cause i am really in dead end and do not know what's wrong with my code.

My problem:
The code works fine but when i try to delete a link which doesnt exist, there will be a fatal error which prompts microsoft window's "error message" or rather, a pop up window which writes, "listfnb(name for this codes) has stopped working". What's wrong with my code actually?



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#include <iostream>
#include <conio.h>
using namespace std;

class link
{  
   public:
      link *previous;
      int data;
      link *next;
   
      link(): previous(NULL), next (NULL), data (0)
      {}
};

class list
{
   private:
           link *first;
           link *last;
           int count;
   
   public:
          list():first(NULL), last(NULL), count(0)
          {}
          
          void InsertFront(int d)
          {
               link *newlink = new link;
               newlink->data = d;
               newlink->next = first;
               newlink->previous = NULL;
               if(count>0)
                  first-> previous = newlink;
               else
                  last = newlink;
               
               first = newlink;
               count++;
          }
          
          void InsertBack(int d)
          {
               link *newlink = new link;
               newlink->data = d;
               newlink->next = NULL;
               newlink->previous = last;
               if(count>0)
                  last-> next = newlink;
               else
                  first = newlink;
               
               last =newlink;
               count++;
          }
          
          void countItem()
          {
               cout << "The total number of items are: " << count <<endl;
          }
          
          void DisplayFront()
          {
               link *current = first;
               cout << "First --> ";
               while(current!=NULL)
               {
               cout <<  current -> data << " --> ";
               current = current->next;
               }
               cout << "End\n";
          }
          
          void DisplayBack()
          {
               link *current = last;
               cout << "Last --> ";
               while(current!=NULL)
               {
               cout <<  current -> data << " --> ";
               current = current->previous;
               }
               cout << "End\n";
          }
          
          link *SearchValue(int n)
          {
               link *current = first;
               //link *current2 = last;
               while (current !=NULL)
               {
                     if ( n == current->data)
                     {
                     return current;
                     }
               current = current->next;
               }
               cout << "No value";
          }
          
          void DeleteValue (int n)
          {
               link *current = SearchValue(n);
               
               if (current == NULL)
               {
                  cout << "\nNo such value; delete unsuccessful";
               }
               
                else if (current->previous == NULL && current->next == NULL)
               {
                  delete current;
                  first = NULL;
                  last = NULL;
                  cout << "\nNo data left; link deleted";         
               }
               
               else if (current->previous == NULL)
               {
                  (current -> previous) -> next = current -> next;
               }
               
               else if (current->next == NULL)
               {
                  (current -> next) -> previous = current -> previous;
               }
               else
               
                  (current -> previous) -> next = current -> next;
                  delete current;
                  cout << "\n\nDelete Successful";
          }
          
};



int main()
{
    //list4.InsertFront(3);
    list4.DeleteValue(4);
    getch();
}

Last edited on
helios (17574)
1. SearchValue() doesn't return a value if n is not found.

2.
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else if (current->previous == NULL)
{
  //This line...
  (current -> previous) -> next = current -> next;
}

else if (current->next == NULL)
{
  //...and this line.
  (current -> next) -> previous = current -> previous;
}

This code invariably produces a segmentation fault, because you inverted those two lines. In fact, there's not even need to do all that derefencing:
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else if (current->previous == NULL)
{
  current->next->previous=NULL;
}

else if (current->next == NULL)
{
  //...and this line.
  current->previous->next=NULL;
}

3. Your last case in DeleteValue() (when the link is in the middle of the list) is only updating current->previous->next, but not current->next->previous.
4. DeleteValue() is not decrementing count.
renzokuzen (11)
Oh! Thanks a lot! You rock, helios :D You're the SUN!!!
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