delete node in Linked List

This is a discussion on delete node in Linked List within the C Programming forums, part of the General Programming Boards category; This function is deleting a node from linked list. It is OK with nodes which are not the first node. ...

  1. #1
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    delete node in Linked List

    This function is deleting a node from linked list. It is OK with nodes which are not the first node. When I send anchor to function, I lose the link to the rest of the list. Why is it so?


    Code:
    void delete(visit_ptr_node_type this_is_the_node)
    {
      visit_ptr_node_type one_back;
      if(anchor == NULL)
        printf("\n The list is empty");
      else
        {
        if(this_is_the_node==anchor)
          anchor=anchor->next_ptr;
        else
          {
          one_back=anchor;
          while(one_back->next_ptr != this_is_the_node)
    	one_back=one_back->next_ptr;
          one_back->next_ptr = (this_is_the_node) ->next_ptr;
          }
        free(this_is_the_node);
        }
    }
    TIA,
    Ronen

  2. #2
    Gawking at stupidity
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    Are you sure it's not in the way you're calling delete()? The function you showed looks ok to me. I'm assuming anchor is a global variable.
    If you understand what you're doing, you're not learning anything.

  3. #3
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    This is my full code:
    Code:
    #include <stdio.h>
    #include <malloc.h>
    
    typedef struct visit_node
      {
        char v_name;
        struct visit_node *next_ptr;
      }
    visit_node_type, *visit_ptr_node_type;
    
    visit_ptr_node_type anchor;
    
    
    void insert_after(visit_ptr_node_type after_this_node, visit_ptr_node_type new_node)
    {
      if(anchor==NULL)
        {
        new_node->next_ptr=NULL;
        anchor=new_node;
        }
      else
        {
        new_node->next_ptr=after_this_node->next_ptr;
        after_this_node->next_ptr=new_node;
        }
    }
    
    void delete(visit_ptr_node_type this_is_the_node)
    {
      visit_ptr_node_type one_back;
      if(anchor == NULL)
        printf("\n The list is empty");
      else
        {
        if(this_is_the_node==anchor)
          anchor=anchor->next_ptr;
        else
          {
          one_back=anchor;
          while(one_back->next_ptr != this_is_the_node)
    	one_back=one_back->next_ptr;
          one_back->next_ptr = (this_is_the_node) ->next_ptr;
          }
        free(this_is_the_node);
        }
    }
    
    main()
    { 
      visit_ptr_node_type p1,p2;
      visit_ptr_node_type hadash;
    
      anchor=(visit_ptr_node_type)malloc(sizeof(visit_node_type));
      p1=(visit_ptr_node_type)malloc(sizeof(visit_node_type));
      p2=(visit_ptr_node_type)malloc(sizeof(visit_node_type));
    
      anchor->v_name ='A';
      p1->v_name='I';
      p2->v_name='Y';
    
      anchor->next_ptr=p1;
      p1->next_ptr=p2;
      p2->next_ptr=NULL;
    
    //  clrscr();
      printf("\n FIRST LIST");
      printf("\n The characters in the list are :");
      printf("\n first= %c second= %c third= %c\n",anchor->v_name,
    	anchor->next_ptr->v_name,anchor->next_ptr->next_ptr->v_name);
      hadash=malloc(sizeof(visit_node_type));
      hadash->v_name='N';
      printf("\n Insert the char N after the char I");
      insert_after(p1,hadash);
      printf("\n The characters in the list are :");
      printf("\n first= %c second= %c third= %c fourth= %c\n",anchor->v_name,
    	 anchor->next_ptr->v_name,anchor->next_ptr->next_ptr->v_name,
    	 anchor->next_ptr->next_ptr->next_ptr->v_name);
      printf("\n Delete the char I from the list");
      delete(p1);
      printf("\n The characters in the list are :");
      printf("\n first= %c second= %c third= %c\n",anchor->v_name,
    	 anchor->next_ptr->v_name,anchor->next_ptr->next_ptr->v_name);
      printf("\n Delete the first char A {anchor} from the list");
      delete(anchor);
      printf("\n The characters in the list are :");
      printf("\n first= %c second= %c third= %c\n",anchor->v_name,
    	 anchor->next_ptr->v_name,anchor->next_ptr->next_ptr->v_name);
      printf("\n THE SECOND LIST IS EMPTY");
      anchor=NULL;
      printf("\n Delete one char from the empty list");
      delete(anchor);
      printf("\n \n Insert the char N into the empty list:");
      insert_after(anchor,hadash);
      printf("\n The characters in the list are : first= %c",anchor->v_name);
    }
    If I send anchor to function as a pointer (&anchor), then also the nodes in the middle don't get deleted (p1).


    Ronen

  4. #4
    ATH0 quzah's Avatar
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    And now you know why double linked lists are so much better. Want to delete a node? With only that node?
    Code:
    struct Node *list;
    ...
    delete( thisnode );
    ...
    void delete( struct Node *n )
    {
        if( n->prev )
            n->prev->next = n->next;
        if( n->next )
            n->next->prev = n->prev;
        if( n == list )
            list = n->next;
        free( n );
    }
    Other than that, I hate hidden pointers with a passion, and refuse to read code which contains them.

    Quzah.
    Hope is the first step on the road to disappointment.

  5. #5
    Gawking at stupidity
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    Aside from other things, I see that you have you 3 items in the list after calling delete(p1)

    Then you call delete(anchor) and try printing 3 items again. This should give you a segfault since you'll end up dereferencing a NULL pointer.
    If you understand what you're doing, you're not learning anything.

  6. #6
    Gawking at stupidity
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    Here you go Quzah:
    Code:
    #include <stdio.h>
    #include <malloc.h>
    
    typedef struct visit_node
    {
      char v_name;
      struct visit_node *next;
    } NODE;
    
    NODE *anchor;
    
    void insert_after(NODE *after, NODE *new)
    {
      if(!anchor)
      {
        new->next = NULL;
        anchor = new;
      }
      else
      {
        new->next = after->next;
        after->next = new;
      }
    }
    
    void delete(NODE *node)
    {
      NODE *one_back;
    
      if(!anchor)
        printf("\n The list is empty");
      else
      {
        if(node == anchor)
          anchor = anchor->next;
        else
        {
          one_back = anchor;
          while(one_back->next != node)
            one_back = one_back->next;
          one_back->next = node->next;
        }
    
        free(node);
      }
    }
    
    int main(void)
    {
      NODE *p1, *p2;
      NODE *hadash;
    
      anchor = malloc(sizeof(NODE));
      p1 = malloc(sizeof(NODE));
      p2 = malloc(sizeof(NODE));
    
      anchor->v_name = 'A';
      p1->v_name = 'I';
      p2->v_name = 'Y';
    
      anchor->next = p1;
      p1->next = p2;
      p2->next = NULL;
    
      printf("\n FIRST LIST");
      printf("\n The characters in the list are :");
      printf("\n first= %c second= %c third= %c\n", anchor->v_name,
        anchor->next->v_name, anchor->next->next->v_name);
    
      hadash = malloc(sizeof(NODE));
      hadash->v_name = 'N';
      printf("\n Insert the char N after the char I");
      insert_after(p1, hadash);
      printf("\n The characters in the list are :");
      printf("\n first= %c second= %c third= %c fourth= %c\n",
        anchor->v_name, anchor->next->v_name, anchor->next->next->v_name,
        anchor->next->next->next->v_name);
    
      printf("\n Delete the char I from the list");
      delete(p1);
      printf("\n The characters in the list are :");
      printf("\n first= %c second= %c third= %c\n", anchor->v_name,
        anchor->next->v_name, anchor->next->next->v_name);
    
      printf("\n Delete the first char A {anchor} from the list");
      delete(anchor);
      printf("\n The characters in the list are :");
      printf("\n first= %c second= %c third= %c\n", anchor->v_name,
        anchor->next->v_name, anchor->next->next->v_name);
    
      printf("\n THE SECOND LIST IS EMPTY");
      anchor = NULL;
      printf("\n Delete one char from the empty list");
      delete(anchor);
    
      printf("\n \n Insert the char N into the empty list:");
      insert_after(anchor, hadash);
      printf("\n The characters in the list are : first= %c",
        anchor->v_name);
    
      return 0;
    }
    I had to clean it up so I could read it (and it's still pretty bad)
    If you understand what you're doing, you're not learning anything.

  7. #7
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    Other then changing variables names I don't see any significant change in the code above. The output is left the same; I still seem to lose the link to the list.

    Other than that, I hate hidden pointers with a passion, and refuse to read code which contains them.
    Quzah, what are hidden pointers with a passion? I honestly don't understand this phrase.

  8. #8
    Gawking at stupidity
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    You have a visit_ptr_node_type which is really just a visit_node_type *

    The visit_ptr_node_type is what he's talking about. You'll notice that in my restructured code I don't hide pointers behind a typedef.

    And you're right about changing some variable names but not doing anything significant. My goal was to make your code more readable without changing its functionality. If I changed the way it works then I couldn't be much help to you in solving your problem.

    I didn't fix the problem in the code I posted. I just posted a "more readable" version of your code (to me anyway). I explained what your problem is though. You shouldn't be losing your list, you should be getting a segfault. Did you try only printing 2 items instead of 3 after calling delete(anchor) like I suggested?
    If you understand what you're doing, you're not learning anything.

  9. #9
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    I didn't fix the problem in the code I posted. I just posted a "more readable" version of your code (to me anyway).
    Ok, now I see your point.

    Did you try only printing 2 items instead of 3 after calling delete(anchor) like I suggested?
    I did. this solved the problem.
    Thanx!

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