Doubly-linked list/Element definition
You are encouraged to solve this task according to the task description, using any language you may know.
Define the data structure for a doubly-linked list element. The element should include a data member to hold its value and pointers to both the next element in the list and the previous element in the list. The pointers should be mutable.
Ada
<lang ada>type Link; type Link_Access is access Link; type Link is record
Next : Link_Access := null; Prev : Link_Access := null; Data : Integer;
end record;</lang> Using generics, the specification might look like this: <lang ada>generic
type Element_Type is private;
package Linked_List is
type List_Type is limited private;
... private
type List_Element;
type List_Element_Ptr is access list_element;
type List_Element is
record
Prev : List_Element_Ptr; Data : Element_Type; Next : List_Element_Ptr;
end record;
type List_Type is
record
Head : List_Element_Ptr; -- Pointer to first element. Tail : List_Element_Ptr; -- Pointer to last element. Cursor : List_Element_Ptr; -- Pointer to cursor element. Count : Natural := 0; -- Number of items in list. Traversing : Boolean := False; -- True when in a traversal.
end record;
end Linked_List;</lang> In Ada 2005 this example can be written without declaration of an access type: <lang ada>type Link is limited record
Next : not null access Link := Link'Unchecked_Access; Prev : not null access Link := Link'Unchecked_Access; Data : Integer;
end record;</lang> Here the list element is created already pointing to itself, so that no further initialization is required. The type of the element is marked as limited indicating that such elements have referential semantics and cannot be copied.
Ada's standard container library includes a generic doubly linked list. The structure of the link element is private.
ALGOL 68
<lang algol68>MODE LINK = STRUCT (
REF LINK prev, REF LINK next, DATA value
);
MODE DATA = STRUCT(INT year elected, STRING name);
LINK previous, incumbent, elect; previous := (NIL, incumbent, DATA(1993, "Clinton")); incumbent:= (previous, elect,DATA(2001, "Bush" )); elect := (incumbent, NIL, DATA(2008, "Obama" ));
REF LINK node := previous; WHILE REF LINK(node) ISNT NIL DO
printf(($dddd": "g"; "$,value OF node)); node := next OF node
OD; print(new line);
node := elect; WHILE REF LINK(node) ISNT NIL DO
printf(($dddd": "g"; "$,value OF node)); node := prev OF node
OD; print((newline))</lang> Output:
1993: Clinton; 2001: Bush; 2008: Obama; 2008: Obama; 2001: Bush; 1993: Clinton;
AutoHotkey
see Doubly-linked list/AutoHotkey
C
<lang c>struct link {
struct link *next; struct link *prev; int data;
};</lang>
C#
<lang csharp>class Link {
public int item; public Link next; public Link prev;
}</lang>
Common Lisp
<lang lisp>(defstruct dlist head tail) (defstruct dlink content prev next)</lang>
See the functions on the Doubly-Linked List page for the usage of these structures.
D
<lang d>struct Node(T) {
Node* next, prev; T data;
}</lang>
E
This does no type-checking, under the assumption that it is being used by a containing doubly-linked list object which enforces that invariant along with others such as that element.getNext().getPrev() == element. See Doubly-Linked List#E for an actual implementation (which uses slightly more elaborate nodes than this).
<lang e>def makeElement(var value, var next, var prev) {
def element {
to setValue(v) { value := v }
to getValue() { return value }
to setNext(n) { next := n }
to getNext() { return next }
to setPrev(p) { prev := p }
to getPrev() { return prev }
}
return element
}</lang>
Fortran
In ISO Fortran 95 or later: <lang fortran>type node
real :: data type(node), pointer :: next => null(), previous => null()
end type node ! ! . . . . ! type( node ), target :: head</lang>
Haskell
Haskell in general doesn't have mutability so the following 'mutator' functions use lazy evaluation instead.
Note that unlike naive pointer manipulation which could corrupt the doubly-linked list, updateLeft and updateRight will always yield a well-formed data structure.
<lang haskell> data DList a = Leaf | Node (DList a) a (DList a)
updateLeft _ Leaf = Leaf updateLeft Leaf (Node _ v r) = Node Leaf v r updateLeft new@(Node nl _ _) (Node _ v r) = current
where current = Node prev v r
prev = updateLeft nl new
updateRight _ Leaf = Leaf updateRight Leaf (Node l v _) = Node l v Leaf updateRight new@(Node _ _ nr) (Node l v _) = current
where current = Node l v next
next = updateRight nr new
</lang>
J
As discussed in Doubly-linked_list/Definition#J, doubly linked lists are antithetical to J's design. Defining individual elements as independent structures is even worse. Now each element of the list must contain three arrays (everything in J is an array), all so that we can implement a list.
Yo Dawg, we heard you like lists, so we put lists in your lists so you can list while you list.
Nevertheless, this is doable, though it necessarily departs from the definition specified at Doubly-linked_list/Definition#J.
<lang j>coclass'DoublyLinkedListElement' create=:3 :0
this=:coname 'predecessor successor data'=:y successor__predecessor=: predecessor__successor=: this
)</lang>
Here, when we create a new list element, we need to specify its successor node and its predecessor node and the data to be stored in the node. To start a new list we will need a node that can be the head and the tail of the list -- this will be the successor node for the last element of the list and the predecessor node for the first element of the list:
<lang j>coclass'DoublyLinkedListHead' create=:3 :0
predecessor=:successor=:this=: coname
)</lang>
Java
<lang java>public class Node<T> {
private T element; private Node<T> next, prev;
public Node<T>(){
next = prev = element = null;
}
public Node<T>(Node<T> n, Node<T> p, T elem){
next = n;
prev = p;
element = elem;
}
public void setNext(Node<T> n){
next = n;
}
public Node<T> getNext(){
return next;
}
public void setElem(T elem){
element = elem;
}
public T getElem(){
return element;
}
public void setNext(Node<T> n){
next = n;
}
public Node<T> setPrev(Node<T> p){
prev = p;
}
public getPrev(){
return prev;
}
}</lang>
For use with Java 1.4 and below, delete all "<T>"s and replace T's with "Object".
JavaScript
Inherits from LinkedList (see Singly-Linked_List_(element)#JavaScript) <lang javascript>function DoublyLinkedList(value, next, prev) {
this._value = value; this._next = next; this._prev = prev;
} // from LinkedList, inherit: value(), next(), traverse(), print() DoublyLinkedList.prototype = new LinkedList();
DoublyLinkedList.prototype.prev = function() {
if (arguments.length == 1)
this._prev = arguments[0];
else
return this._prev;
}
function createDoublyLinkedListFromArray(ary) {
var node, prev, head = new DoublyLinkedList(ary[0], null, null);
prev = head;
for (var i = 1; i < ary.length; i++) {
node = new DoublyLinkedList(ary[i], null, prev);
prev.next(node);
prev = node;
}
return head;
}
var head = createDoublyLinkedListFromArray([10,20,30,40]);</lang>
OCaml
Imperative
<lang ocaml>type 'a dlink = {
mutable data: 'a; mutable next: 'a dlink option; mutable prev: 'a dlink option;
}
let dlink_of_list li =
let rec aux prev_dlink = function
| [] -> prev_dlink
| hd::tl ->
let dlink = {
data = hd;
prev = None;
next = prev_dlink }
in
begin match prev_dlink with
| None -> ()
| Some prev_dlink ->
prev_dlink.prev <- Some dlink
end;
aux (Some dlink) tl
in
aux None (List.rev li)
let list_of_dlink =
let rec aux acc = function
| None -> List.rev acc
| Some{ data = d;
prev = _;
next = next } -> aux (d::acc) next
in
aux []
let iter_forward_dlink f =
let rec aux = function
| None -> ()
| Some{ data = d;
prev = _;
next = next } -> f d; aux next
in
aux
- </lang>
<lang ocaml># let dl = dlink_of_list [1;2;3;4;5] in
iter_forward_dlink (Printf.printf "%d\n") dl ;;
1 2 3 4 5 - : unit = ()</lang>
Functional
The previous implementation is the strict equivalent of the other examples of this page and its task, but in regular OCaml these kind of imperative structures can be advantageously replaced by a functional equivalent, that can be use in the same area, which is to have a list of elements and be able to point to one of these. We can use this type:
<lang ocaml>type 'a nav_list = 'a list * 'a * 'a list</lang>
The middle element is the pointed item, and the two lists are the previous and the following items. Here are the associated functions: <lang ocaml>let nav_list_of_list = function
| hd::tl -> [], hd, tl | [] -> invalid_arg "empty list"
let current = function
| _, item, _ -> item
let next = function
| prev, item, next::next_tl ->
item::prev, next, next_tl
| _ ->
failwith "end of nav_list reached"
let prev = function
| prev::prev_tl, item, next ->
prev_tl, prev, item::next
| _ ->
failwith "begin of nav_list reached"</lang>
<lang ocaml># let nl = nav_list_of_list [1;2;3;4;5] ;; val nl : 'a list * int * int list = ([], 1, [2; 3; 4; 5])
- let nl = next nl ;;
val nl : int list * int * int list = ([1], 2, [3; 4; 5])
- let nl = next nl ;;
val nl : int list * int * int list = ([2; 1], 3, [4; 5])
- current nl ;;
- : int = 3</lang>
Oz
We show how to create a new node as a record value. <lang oz>fun {CreateNewNode Value}
node(prev:{NewCell _}
next:{NewCell _} value:Value) end</lang> Note: this is for illustrative purposes only. In a real Oz program, you would use one of the existing data types.
Pascal
<lang pascal>type link_ptr = ^link;
data_ptr = ^data; (* presumes that type 'data' is defined above *)
link = record
prev: link_ptr;
next: link_ptr;
data: data_ptr;
end;</lang>
Perl
<lang perl>my %node = (
data => 'say what',
next => \%foo_node,
prev => \%bar_node,
); $node{next} = \%quux_node; # mutable</lang>
PL/I
<lang PL/I> define structure
1 Node,
2 value fixed decimal,
2 back_pointer handle(Node),
2 fwd_pointer handle(Node);
P = NEW(: Node :); /* Creates a node, and lets P point at it. */ get (P => value); /* Reads in a value to the node we just created. */
/* Assuming that back_pointer and fwd_pointer point at other nodes, */ /* we can say ... */ P = P => fwd_pointer; /* P now points at the next node. */ ... P = P => back_pointer; /* P now points at the previous node. */ </lang>
PicoLisp
We use (in addition to the header structure described in Doubly-linked list/Definition#PicoLisp) two cells per doubly-linked list element:
+-----+-----+ +-----+-----+
| Val | ---+---> | | | ---+---> next
+-----+-----+ +--+--+-----+
|
prev <---+
With that, 'cddr' can be used to access the next, and 'cadr' to access the previous element. <lang PicoLisp># 'cons' an element to a doubly-linked list (de 2cons (X DLst)
(let L (car DLst) # Get current data list
(set DLst (cons X NIL L)) # Prepend two new cons pairs
(if L # Unless DLst was empty
(set (cdr L) (car DLst)) # set new 'prev' link
(con DLst (car DLst)) ) ) ) # otherwise set 'end' link
- We prepend 'not' to the list in the previous example
(2cons 'not *DLst)</lang> For output of the example data, see Doubly-linked list/Traversal#PicoLisp.
Pop11
<lang pop11>uses objectclass; define :class Link;
slot next = []; slot prev = []; slot data = [];
enddefine;</lang>
PureBasic
<lang PureBasic>Structure node
*prev.node *next.node value.i
EndStructure</lang>
Python
<lang python>class Node(object):
def __init__(self, data = None, prev = None, next = None):
self.prev = prev
self.next = next
self.data = data
def __str__(self):
return str(self.data)
def __repr__(self):
return repr(self.data)
def iter_forward(self):
c = self
while c != None:
yield c
c = c.next
def iter_backward(self):
c = self
while c != None:
yield c
c = c.prev</lang>
Ruby
Extending Singly-Linked List (element)#Ruby <lang ruby>class DListNode < ListNode
attr_accessor :prev # accessors :succ and :value are inherited
def initialize(value, prev=nil, succ=nil) @value = value @prev = prev @prev.succ = self if prev @succ = succ @succ.prev = self if succ end
def self.from_values(*ary)
ary << (f = ary.pop)
ary.map! {|i| new i }
ary.inject(f) {|p, c| p.succ = c; c.prev = p; c }
end
end
list = DListNode.from_values 1,2,3,4</lang>
Tcl
or
<lang tcl>oo::class create List {
variable content next prev
constructor {value {list ""}} {
set content $value
set next $list
set prev ""
if {$next ne ""} {
$next previous [self]
}
}
method value args {
set content {*}$args
}
method next args {
set next {*}$args
}
method previous args {
set prev {*}$args
}
}</lang>
Visual Basic .NET
<lang vbnet>Public Class Node(Of T)
Public Value As T Public [Next] As Node(Of T) Public Previous As Node(Of T)
End Class</lang>