Thread: remove entry from doubly linked list

Hello, I am trying to delete an entry from a doubly linked list, where the function takes a pointer to the entry to be removed as its argument. I have tried various different things, and I can get the insertEntry function to work, but I am at a loss for what to put for the remove Entry.

I can get it if I put that it will receive the entry before the entry to be removed as the argument but not when it receives the entry to be removed as the argument. Can you guys help me? This is not homework, I am learning on my own. Here is the code I have so far.

Code:

/* 6. Develop insertEntry and removeEntry functions for a doubly linked list that are
similar in function to those developed in previous exercises for a singly linked list.
Why can your removeEntry function now take as its argument a direct pointer to
the entry to be removed from the list?


 */


#include <stdio.h>

    struct entry
    {
        int value;
        struct entry *next;
        struct entry *previous;
    };
    
    struct entry n2, n3, n4, insert;

int main (void)
{
    
    void insertEntry (struct entry *here, struct entry *insert);
    void removeEntry (struct entry *remove);
    
    struct entry *list_pointer = &n2;

    n2.value = 200;
    n2.next = &n3;
    n2.previous = (struct entry *) 0;
    
    n3.value = 300;
    n3.next = &n4;
    n3.previous = &n2;
    
    n4.value = 400;
    n4.next = (struct entry *) 0;
    n4.previous = &n3;
    
    insert.value = 250;
    
    
    
    while ( list_pointer != (struct entry *) 0 ) 
        {
        printf ("%i\n", list_pointer->value);
        list_pointer = list_pointer->next;
        }
 
    
    list_pointer = &n2;
    
    insertEntry(&n2, &insert);
    removeEntry(&n4);
    
    while ( list_pointer != (struct entry *) 0 ) 
        {
        printf ("%i\n", list_pointer->value);
        list_pointer = list_pointer->next;
        }

}

void insertEntry (struct entry *here, struct entry *insert)
{
    insert->next = here->next;
    here->next = insert; 
}

void removeEntry (struct entry *remove)
{
    remove->previous = remove->next;
}

Your list isn't doubly-linked yet. You haven't set any of the previous members to anything useful. What you just added would simply crash due to writing through a NULL pointer.

This is also a somewhat longer alternative to just writing NULL: (struct entry *) 0

Originally Posted by sdbuilt

Huh? My program doesn't crash.. It removes the desired entry from the list as long as it isn't the first entry. I am not worried about that corner case I am just trying to understand the concepts. I went ahead and updated the function to update the previous pointer of the next entry of the list by using

Code:

remove->next->previous = remove->previous;

Also I have no idea what you mean when you say to just write "NULL: (struct entry *) 0". Write that where? What do you mean?

Oh, you've set up the pointers in some of the static instances of the struct you have made. This needs to happen from your insertEntry function, otherwise it is broken in terms of every real-world scenario.

I mean, you stop using "(struct entry *) 0". Just use "NULL" instead like everyone else. How much clearer can I be?

Oh I see what you mean. You mean the insertFunction should insert the first member of the list and then the second etc instead of having those things already set up in main. Got ya. Yeah that makes sense for a real world scenario.

Yeah I was not understanding the way you worded that last part. The colon symbol through me off.. But when you worded it differently I understood. Replace the extra code with NULL like everybody else does gosh dang it!

Ah nice. I hope it works well for you once you start using dynamic memory allocation.

Just remember, it never hurts to use pen an paper for to draw diagrams if you run into trouble. Speaking from experience, doubly-linked lists are one of the things where it is most beneficial. Many things you can do with them are much harder than anyone ever realises at first.