How to move elements in doubly linked list? [closed] - c++

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I have custom list (doubly linked list and not std::list) implemented in my code. My requirement is to move the element by one left or right by updating the references. Is it possible?
class Elem
{
Elem *next;
Elem *prev;
}
.......
void move_element_left(Elem *e)
{
if(e->prev()==NULL)
return; //Left most ... so return
Elem *left = e->prev();
left->next() = e->next();
e->prev() = left->prev();
if (left->next())
left->next()->prev() = left;
if (e->prev())
e->prev()->next() = e;
e->next() = left;
left->prev() = e;
}
.......
int main()
{
ElemList ls;
...
...
move_element_left(e); //e of type Elem *
...
}
Above code works except for the 2nd object in the list which I want to move to left most (or to top most). (i.e. say if list(obj5, obj9, obj11, obj12,..), moving obj9 to the first in the list gives error)

Answer 1

See Bubble-sorting doubly linked list
I assume your Elem class does also contain data, so move the data or - if it's a simple data pointer - swap the pointers: C++ Swapping Pointers.
If that's not possible I would - from a "Don't Repeat Yourself" point of view - reuse those simple linked list functions you most probably already have:
void move_element_left(Elem *e)
{
Elem *left = e->prev();
if(left)
{
remove_element(e);
insert_element_before(e, left);
}
}

Answer 2

Works as designed ?
Following your code in a schema, shows that it works as designed:
void move_element_left(Elem *e)
{
if(e->prev()==NULL)
return; //ok ! Left most ... so return
Elem *left = e->prev(); // ok ! (1)
left->next() = e->next(); // ok ! (2)
e->prev() = left->prev(); // ok ! (3)
if (left->next()) // ok !
left->next()->prev() = left; // ok ! (4)
if (e->prev()) // ok ! e prev is left prev is null
e->prev()->next() = e;
e->next() = left; // ok ! (5)
left->prev() = e; // ok ! (6)
}
Here the schema (sorry for the childish aspect ;-) ):
So the list is in fact fine. The problem is that ElemList certainly contains a pointer to the head of the list . And this pointer still points to the old first and now second element. So the list is then no longer consitent.
How to fix it ?
One way out, would be to make move_element_left() a member function of ElemList. In this case you could take care of the special case where e->left becomes null, in which case you need to update ElemList's pointer to the first element.

Answer 3

Below is the updated code. You need to change the head pointer if the left pointer points to head of double linked list.
//**head is the address of pointer of head of double linked list
void move_element_left(Elem *e,Elem **head)
{
if(e->prev()==NULL)
return; //Left most ... so return
Elem *left = e->prev();
if(left==*head){
*head=e;
}
left->next() = e->next();
e->prev() = left->prev();
if (left->next())
left->next()->prev() = left;
if (e->prev())
e->prev()->next() = e;
e->next() = left;
left->prev() = e;
}