Delete Node function in C

[stamatiou]

hey guys,
i know that I have made already many posts but this one is tough! I want to make a function that deletes the Node when it finds a specific value

#include <stdio.h>
#include <stdlib.h>

typedef struct node
{
    struct node *next,*prev;
    int data;
}Node;

typedef struct list{
    Node *head,*tail;
    int len;
}List;

int find_len(List *list)
{
    int counter = 0;
    Node *dummy;
    dummy = (*list).head;
    while(dummy != NULL)
    {
        counter = counter + 1;
        dummy = dummy->next;
    }
    list->len = counter;
    return 0;
}

int del1st(List *list,int value)
{
    Node *dummy;
    dummy  = (*list).head;
    while(dummy != NULL)
    {
        if(dummy->data == value)
        {
            dummy->prev->next = dummy->next;
            free(dummy);
        }
        dummy = dummy -> next;
    }
    return 0;
}

int main()
{
    Node *new1,*new2,*new3;
    List list;
    
    new1 = malloc(sizeof(Node));
    new2 = malloc(sizeof(Node));
    new3 = malloc(sizeof(Node));
    
    new1->data = 26;
    new2->data = 12;
    new3->data = 45;
    
    list.head = new1;
    list.tail = new3;
    
    list.head->next = new2;
    new2->next = new3;
    new3->next = NULL;
    
    new3->prev = new2;
    new2->prev = new1;
    
    find_len(&list);
    printf("LENGTH = %d\n",list.len);
    
    del1st(&list,12);
    
    printf("LENGTH = %d\n",list.len);
    
    return 0;
}

I put in the find_len function to be easier to count the nodes.

[Bachstelze]

        if(dummy->data == value)
        {
            dummy->prev->next = dummy->next;
            free(dummy);
        }
        dummy = dummy -> next;

You cannot access dummy anymore after you free() it.

[stamatiou]

        if(dummy->data == value)
        {
            dummy->prev->next = dummy->next;
            free(dummy);
        }
        dummy = dummy -> next;

You cannot access dummy anymore after you free() it.

So if I make a pointer to dummy->next and then assign dummy to the the pointer?

[Bachstelze]

So if I make a pointer to dummy->next and then assign dummy to the the pointer?

No, you cannot "make a pointer to dummy->next", because dummy does not point to anything anymore. What's that line for anyway? It doesn't seem to serve any purpose.

[stamatiou]

No, you cannot make a pointer to dummy->next since dummy does not point to anything anymore. What's that line for anyway? It doesn't seem to serve any purpose.

I mean before I free dummy, something like this:

#include <stdio.h>
#include <stdlib.h>

typedef struct node
{
    struct node *next,*prev;
    int data;
}Node;

typedef struct list{
    Node *head,*tail;
    int len;
}List;

int find_len(List *list)
{
    int counter = 0;
    Node *dummy;
    dummy = (*list).head;
    while(dummy != NULL)
    {
        counter = counter + 1;
        dummy = dummy->next;
    }
    list->len = counter;
    return 0;
}

int del1st(List *list,int value)
{
    Node *dummy;
    Node *temp;
    dummy  = (*list).head;
    while(dummy != NULL)
    {
        if(dummy->data == value)
        {
            temp = dummy->next;
            free(dummy);
            dummy = temp;
        }
        dummy = dummy -> next;
    }
    return 0;
}

int main()
{
    Node *new1,*new2,*new3;
    List list;
    
    new1 = malloc(sizeof(Node));
    new2 = malloc(sizeof(Node));
    new3 = malloc(sizeof(Node));
    
    new1->data = 26;
    new2->data = 12;
    new3->data = 45;
    
    list.head = new1;
    list.tail = new3;
    
    list.head->next = new2;
    new2->next = new3;
    new3->next = NULL;
    
    new3->prev = new2;
    new2->prev = new1;
    
    find_len(&list);
    printf("LENGTH = %d\n",list.len);
    
    del1st(&list,12);
    
    printf("LENGTH = %d\n",list.len);
    
    return 0;
}

[Bachstelze]

You're making it more complicated than it needs to be. Just make the function return after it has deleted the node:

int del1st(List *list,int value)
{
    Node *dummy;
    dummy = list->head;
    while(dummy != NULL)
    {
        if(dummy->data == value)
        {
            if (dummy->prev != NULL) {
                dummy->prev->next = dummy->next;
            }
            if (dummy->next != NULL) {
                dummy->next->prev = dummy->prev;
            }
            free(dummy);
            return 0;
        }
        dummy = dummy->next;
    }
    return 1; /* Return 1 if object not found */
 }

And if you want to "delete all nodes with value n", just do:

while (del1st(list, n) == 0)
    ;