C double linked list insert based on location

Question

I am working on a generic form of a double linked list, so I have :
list structure :

int size;  
struct elem *head;  
struct elem *current;  
struct elem *tail;

elem structure :

void *data;  
struct elem *next;  
struct elem *prev;

I am having a hard time implementing a function allowing me to push a new element before the current one. Here is what I came up with for the moment :

t_list *my_list_add_before(t_list *list, void *data)  
{
    struct elem *elem = malloc(sizeof(*elem));
    elem->data = data;
    if(list->size == 0)
    {
        list->head = elem;
        list->current = elem;
        list->tail = elem;
        elem->prev = NULL;
        elem->next = NULL;
    }
    else
    {
       elem->next = list->current;
       elem->prev = list->current->prev;
       elem->prev->next = elem;
       list->current->prev = elem;
    }
    list->size = list->size + 1;
    return (list);
}

But my list->head->next seem to point to NULL. What is the issue?

Brave Sir Robin · Accepted Answer · 2014-03-04 00:05:22Z

2

You should modify list->current->prev->next before list->current->prev, currently you effectively do elem->next = elem.

answered Mar 4, 2014 at 0:05

Brave Sir Robin

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2 Comments

Swann Over a year ago

I swapped both line as you suggested, but when I try to display my linked list, I can only display one line. (my list-size = 45)

Brave Sir Robin Over a year ago

You are also not treatingk the case where list->current == list->head correctly. You need to update list->head after inserting before the current head.

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