Doubly-linked list/Element insertion
You are encouraged to solve this task according to the task description, using any language you may know.
Use the link structure defined in Doubly-Linked List (element) to define a procedure for inserting a link into a doubly-linked list. Call this procedure to insert element C into a list {A,B}, between elements A and B.
This is much like inserting into a Singly-Linked List, but with added assignments so that the backwards-pointing links remain correct.
Ada
Define the procedure:
procedure Insert (Anchor : Link_Access; New_Link : Link_Access) is begin if Anchor /= Null and New_Link /= Null then New_Link.Next := Anchor.Next; New_Link.Prev := Anchor; if New_Link.Next /= Null then New_Link.Next.Prev := New_Link; end if; Anchor.Next := New_Link; end if; end Insert;
Create links and form the list.
procedure Make_List is Link_Access : A, B, C; begin A := new Link; B := new Link; C := new Link; A.Data := 1; B.Data := 2; C.Data := 2; Insert(Anchor => A, New_Link => B); -- The list is (A, B) Insert(Anchor => B, New_Link => C); -- The list is (A, B, C) end Make_List;
Element insertion using the generic doubly linked list defined in the standard Ada containers library.
with Ada.Containers.Doubly_Linked_Lists; with Ada.Text_Io; use Ada.Text_Io; with Ada.Strings.Unbounded; use Ada.Strings.Unbounded; procedure List_Insertion is package String_List is new Ada.Containers.Doubly_Linked_Lists(Unbounded_String); use String_List; procedure Print(Position : Cursor) is begin Put_Line(To_String(Element(Position))); end Print; The_List : List; begin The_List.Append(To_Unbounded_String("A")); The_List.Append(To_Unbounded_String("B")); The_List.Insert(Before => The_List.Find(To_Unbounded_String("B")), New_Item => To_Unbounded_String("C")); The_List.Iterate(Print'access); end List_Insertion;
C
Define the function:
void insert(link* anchor, link* newlink) { newlink->next = anchor->next; newlink->prev = anchor; (newlink->next)->prev = newlink; anchor->next = newlink; }
Production code should also include checks that the passed links are valid (e.g. not null pointers). There should also be code to handle special cases, such as when *anchor is the end of the existing list (i.e. anchor->next is a null pointer).
To call the function:
Create links, and form the list:
link a, b, c; a.next = &b; a.prev = null; a.data = 1; b.next = null; b.prev = &a; b.data = 3; c.data = 2;
This list is now {a,b}, and c is separate.
Now call the function:
insert(&a, &c);
This function call changes the list from {a,b} to {a,b,c}.
Pascal
procedure insert_link( a, b, c: link_ptr ); begin a^.next := c; if b <> nil then b^.prev := c; c^.next := b; c^.prev := a; end;
Actually, a more likely real world scenario is 'insert after A'. So...
procedure realistic_insert_link( a, c: link_ptr ); begin if a^.next <> nil then a^.next^.prev := c; (* 'a^.next^' is another way of saying 'b', if b exists *) c^.next := a^.next; a^.next := c; c^.prev := a; end;
Pop11
define insert_double(list, element); lvars tmp; if list == [] then ;;; Insertion into empty list, return element element else next(list) -> tmp; list -> prev(element); tmp -> next(element); element -> next(list); if tmp /= [] then element -> prev(tmp) endif; ;;; return original list list endif; enddefine; lvars A = newLink(), B = newLink(), C = newLink(); ;;; Build the list of A and B insert_double(A, B) -> _; ;;; insert C between A and b insert_double(A, C) -> _;
Python
def insert(anchor, new): new.next = anchor.next new.prev = anchor anchor.next.prev = new anchor.next = new
Ruby
class ListNode def insertAt(ind,newEl) # where ind > 0 if ind==1 ListNode.new(newEl,self,nxt) elsif ind > 1 && nxt nxt.insertAt(ind-1,newEl) else fail "cannot insert at index #{ind}" end end end
a=ListNode.new(:a) b=ListNode.new(:b,a)
a.insertAt(1,:c)