how do i reverse a circular double linked list

[kobra_swe]

Hi everyone!
I have a circular double linked list see
http://www.openasthra.com/c-tidbits/...-or-tail-node/
for a description of circular double linked list.
I want to reverse the list with a function called
void reverse(list_t** head ); or maybe void reverse(list_t** head, list_t* last);
where head is the first element and last is the last element.
My first implementation was to start from last element until i reach the first element and put those element in a new double linked list until. (every element is inserted last in the list)
However i changed the requirement for me and i do not want to have a new list to move element on it.
I want to use same list but i always succeed to loose reference to some element.
Therefore i hope that some of you can help me with that and i appreciate if somebody can help me with this ( it is enough with a pseudo code description)
To make it shorter i want to use same list and reverse it without using a temporary linked list
Thanks for all help

[Thantos]

Just move backwards in the list.

Forward: ele = ele->next;
Backwards: ele = ele->previous;

[tabstop]

I'm with Thantos: if it's a circular doubly-linked list, how could you tell it was reversed?

[brewbuck]

I'm with Thantos: if it's a circular doubly-linked list, how could you tell it was reversed?

Iteration in the "forward" direction would traverse the elements in the opposite order as before.

Anyway, to reverse a circular list, just step through every element and swap the "prev" and "next" pointers.

[Salem]

If it's circular, and doubly linked, then how different will it really look after you've reversed it?

[brewbuck]

If it's circular, and doubly linked, then how different will it really look after you've reversed it?

I don't see why it's such a weird request. Maybe some piece of code is set up to always traverse in the "forward" direction by following the "next" pointers, and you want to traverse the opposite direction, and you don't want to change the other piece of code.

[Thantos]

I don't think it is weird and there are some very practical applications for reversing directions (I.E. changing motor direction). The problem becomes that going through each element and swapping the pointers is rather expensive compared to changing the direction you transverse.

[Salem]

That's my whole point. A circular double linked list means it's already set up to start at any point and traverse in either direction.

Even if it is a student exercise, it's a pretty dumb one since it's not something you'd ever do in real life.

[brewbuck]

That's my whole point. A circular double linked list means it's already set up to start at any point and traverse in either direction.

Even if it is a student exercise, it's a pretty dumb one since it's not something you'd ever do in real life.

How easy it is to make a sweeping generalization. Have you never worked with code that is out of your control or cannot be changed for some reason?


As far as student exercises comparing with real life. Real life wins hands down, in terms of absolute stupidity I've dealt with. School was not even close. School was nice.

[matsp]

How easy it is to make a sweeping generalization. Have you never worked with code that is out of your control or cannot be changed for some reason?


As far as student exercises comparing with real life. Real life wins hands down, in terms of absolute stupidity I've dealt with. School was not even close. School was nice.

Yeah, almost everything you do at school is easily solvable in a few hundred or thousand lines of code. Not so in real life. And in School, you are almost always in control of ALL the code in the application. In real life that's rarely the case - often the majority of the code is either written by someone else, or belongs to say the OS that you can't modify at all.

--
Mats

[kobra_swe]

The reason why i asked this question is that i wanted to refresh my knowledge in c. I know the difference beetween the school and industri since i have both studied before and worked since 2,5 years ago. However i have been working with java and ui-application until now and from may i will change profile to hardware/device driver and before i starting the new job i only want to repeat and refresh my not so advanced knowledge in c.
However i want to thanks people who helps and i also want to give an advice to people who does not have a meaningful life/job and need to give a comment on every thread. Get a life

[brewbuck]

To actually answer the original question, you might do something like this:

void reverse_list(node *head)
{
    node *curr, *next, *tmp;
    curr = head;
    do
    {
        next = curr->next;
        tmp = curr->prev;
        curr->prev = curr->next;
        curr->next = tmp;
        curr = next;
    } while(curr != head);
}

[Thantos]

of course if you do

void reverse_list(node *head)
{
    node *curr, *tmp;
    curr = head;
    do
    {
        tmp = curr->next;
        curr->next = curr->prev;
        curr->prev = tmp;
        curr = tmp;
    } while(curr != head);
}

You save yourself an assignment. If the list is large it might be worth it

I would still suggest finding a better way if at all possible.

[brewbuck]

You save yourself an assignment. If the list is large it might be worth it

The compiler's data flow analysis will probably figure that out anyway.