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{{task|Data Structures}}
 
Define the data structure for a [[Linked List|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.
;Task:
Define the data structure for a [[Linked_List#Doubly-Linked_List|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.
 
 
{{Template:See also lists}}
<br><br>
 
=={{header|Action!}}==
<syntaxhighlight lang="action!">DEFINE PTR="CARD"
 
TYPE ListNode=[
BYTE data
PTR prv,nxt]</syntaxhighlight>
{{out}}
[https://gitlab.com/amarok8bit/action-rosetta-code/-/raw/master/images/Doubly-linked_list_element_definition.png Screenshot from Atari 8-bit computer]
 
=={{header|Ada}}==
<syntaxhighlight lang="ada">type Link;
<lang ada>
type Link;
type Link_Access is access Link;
type Link is record
Line 10 ⟶ 28:
Prev : Link_Access := null;
Data : Integer;
end record;</syntaxhighlight>
</lang>
Using generics, the specification might look like this:
<syntaxhighlight lang="ada">generic
<lang ada>
generic
type Element_Type is private;
package Linked_List is
Line 36 ⟶ 52:
Traversing : Boolean := False; -- True when in a traversal.
end record;
end Linked_List;</syntaxhighlight>
</lang>
In Ada 2005 this example can be written without declaration of an access type:
<syntaxhighlight lang="ada">type Link is limited record
<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;</syntaxhighlight>
</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.
 
Line 51 ⟶ 64:
 
=={{header|ALGOL 68}}==
{{works with|ALGOL 68|Revision 1.}}
<pre>MODE LINK = STRUCT (
{{works with|ALGOL 68G|Any - tested with release [http://sourceforge.net/projects/algol68/files/algol68g/algol68g-2.7 algol68g-2.7].}}
REF LINK prev,
{{works with|ELLA ALGOL 68|Any (with appropriate job cards) - tested with release [http://sourceforge.net/projects/algol68/files/algol68toc/algol68toc-1.8.8d/algol68toc-1.8-8d.fc9.i386.rpm/download 1.8-8d]}}
REF LINK next,
'''File: prelude/link.a68'''<syntaxhighlight lang="algol68"># -*- coding: utf-8 -*- #
DATA value
CO REQUIRES:
MODE OBJVALUE = ~ # Mode/type of actual obj to be queued #
END CO
 
MODE OBJLINK = STRUCT(
REF OBJLINK next,
REF OBJLINK prev,
OBJVALUE value # ... etc. required #
);
 
PROC obj link new = REF OBJLINK: HEAP OBJLINK;
MODE DATA = STRUCT(INT year elected, STRING name);
 
PROC obj link free = (REF OBJLINK free)VOID:
LINK previous, incumbent, elect;
prev OF free := next OF free := obj queue empty # give the garbage collector a big hint #</syntaxhighlight>'''See also:''' [[Queue/Usage#ALGOL_68|Queue/Usage]]
previous := (NIL, incumbent, DATA(1993, "Clinton"));
incumbent:= (previous, elect,DATA(2001, "Bush" ));
elect := (incumbent, NIL, DATA(2008, "Obama" ));
 
=={{header|ALGOL W}}==
REF LINK node := previous;
<syntaxhighlight lang="algolw"> % record type to hold an element of a doubly linked list of integers %
WHILE REF LINK(node) ISNT NIL DO
record DListIElement ( reference(DListIElement) prev
printf(($dddd": "g"; "$,value OF node));
; integer iValue
node := next OF node
; reference(DListIElement) next
OD;
);
print((newline));
% additional record types would be required for other element types %</syntaxhighlight>
 
=={{header|ARM Assembly}}==
node := elect;
{{works with|as|Raspberry Pi}}
WHILE REF LINK(node) ISNT NIL DO
<syntaxhighlight lang="arm assembly">
printf(($dddd": "g"; "$,value OF node));
 
node := prev OF node
/* ARM assembly Raspberry PI */
OD;
 
print((newline))</pre>
/* structure Node Doublylinked List*/
Output:<pre>
.struct 0
1993: Clinton; 2001: Bush; 2008: Obama;
NDlist_next: @ next element
2008: Obama; 2001: Bush; 1993: Clinton; </pre>
.struct NDlist_next + 4
NDlist_prev: @ previous element
.struct NDlist_prev + 4
NDlist_value: @ element value or key
.struct NDlist_value + 4
NDlist_fin:
</syntaxhighlight>
 
=={{header|AutoHotkey}}==
see [[Doubly-linked list/AutoHotkey]]
 
=={{header|Axe}}==
<syntaxhighlight lang="axe">Lbl LINK
r₂→{r₁}ʳ
0→{r₁+2}ʳ
0→{r₁+4}ʳ
r₁
Return
 
Lbl NEXT
{r₁+2}ʳ
Return
 
Lbl PREV
{r₁+4}ʳ
Return
 
Lbl VALUE
{r₁}ʳ
Return</syntaxhighlight>
 
=={{header|BASIC}}==
==={{header|BBC BASIC}}===
{{works with|BBC BASIC for Windows}}
<syntaxhighlight lang="bbcbasic"> DIM node{pPrev%, pNext%, iData%}
</syntaxhighlight>
 
=={{header|Bracmat}}==
<syntaxhighlight lang="bracmat">link=(prev=) (next=) (data=)</syntaxhighlight>
 
=={{header|C}}==
It basically doesn't matter if we use the name link, node, Node or some other name. These are matters of taste and aesthetics. However, it is important that the C language is case-sensitive and that the namespace for structures is separate.
struct link {
<syntaxhighlight lang="c">struct linkNode *next;
{
struct link *prev;
struct intNode data*next;
struct Node *prev;
};
void *data;
};</syntaxhighlight>
An alternative technique is to define a pointer type by typedef as shown below. The advantage here is that you do not have to write struct everywhere - assuming that you will most often need a pointer to a struct Node, not the structure itself.
<syntaxhighlight lang="c">
struct Node;
typedef struct Node* Node;
struct Node
{
Node next;
Node prev;
void* data;
};
</syntaxhighlight>
 
=={{header|C sharp|C#}}==
<langsyntaxhighlight lang="csharp">class Link
{
public int itemItem { get; set; }
public Link nextPrev { get; set; }
public Link prevNext { get; set; }
 
}</lang>
//A constructor is not neccessary, but could be useful
public Link(int item, Link prev = null, Link next = null) {
Item = item;
Prev = prev;
Next = next;
}
}</syntaxhighlight>
 
=={{header|C++}}==
C++ has doubly linked list class template in standard library. However actual list noded are treated as implementation detail and encapsulated inside list. If we were to reimplement list, then node could look like that:
<syntaxhighlight lang="cpp">template <typename T>
struct Node
{
Node* next;
Node* prev;
T data;
};</syntaxhighlight>
 
=={{header|Clojure}}==
 
This sort of mutable structure is not idiomatic in Clojure. [[../Definition#Clojure]] or a finger tree implementation would be better.
 
<syntaxhighlight lang="clojure">(defrecord Node [prev next data])
 
(defn new-node [prev next data]
(Node. (ref prev) (ref next) data))</syntaxhighlight>
 
=={{header|Common Lisp}}==
 
<syntaxhighlight lang="lisp">(defstruct dlist head tail)
(defstruct dlink content prev next)</syntaxhighlight>
 
See the functions on the [[Doubly-Linked List]] page for the usage of these structures.
 
=={{header|D}}==
A default constructor is implicit:
struct Node(T) {
<syntaxhighlight lang="d">struct Node(T) {
Node* next, prev;
T data;
typeof(this)* prev, next;
}
}
 
void main() {
alias N = Node!int;
N* n = new N(10);
}</syntaxhighlight>
 
=={{header|Delphi}}==
<syntaxhighlight lang="d">struct Node(T) {
 
type
 
pList = ^List ;
 
list = record
data : pointer ;
prev : pList ;
next : pList ;
end;
 
}</syntaxhighlight>
 
=={{header|E}}==
Line 106 ⟶ 232:
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 <code>element.getNext().getPrev() == element</code>. See [[Doubly-Linked List#E]] for an actual implementation (which uses slightly more elaborate nodes than this).
 
<syntaxhighlight 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
}</syntaxhighlight>
}</lang>
 
=={{header|Erlang}}==
Using the code in [[Doubly-linked_list/Definition]] the element is defined by:
<syntaxhighlight lang="erlang">
new( Data ) -> erlang:spawn( fun() -> loop( Data, noprevious, nonext ) end ).
</syntaxhighlight>
 
=={{header|F_Sharp|F#}}==
<syntaxhighlight lang="fsharp">
type 'a DLElm = {
mutable prev: 'a DLElm option
data: 'a
mutable next: 'a DLElm option
}
</syntaxhighlight>
 
=={{header|Factor}}==
<syntaxhighlight lang="factor">TUPLE: node data next prev ;</syntaxhighlight>
 
=={{header|Fortran}}==
In ISO Fortran 95 or later:
<syntaxhighlight lang="fortran">type node
real :: data
type(node), pointer :: next => null(), previous => null()
end type node
!
! . . . .
!
type( node ), target :: head</syntaxhighlight>
 
=={{header|FreeBASIC}}==
<syntaxhighlight lang="freebasic">type node
nxt as node ptr
prv as node ptr
dat as any ptr 'points to any kind of data; user's responsibility
'to keep track of what's actually in it
end type</syntaxhighlight>
 
=={{header|Go}}==
<syntaxhighlight lang="go">type dlNode struct {
string
next, prev *dlNode
}</syntaxhighlight>
Or, using the [http://golang.org/pkg/container/list/#Element container/list] package:
<syntaxhighlight lang="go">import "container/list"
 
var node list.Element
// and using: node.Next(), node.Prev(), node.Value</syntaxhighlight>
 
=={{header|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.
 
<syntaxhighlight 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
</syntaxhighlight>
 
==Icon and {{header|Unicon}}==
 
Uses Unicon classes.
 
<syntaxhighlight lang="unicon">
class DoubleLink (value, prev_link, next_link)
initially (value, prev_link, next_link)
self.value := value
self.prev_link := prev_link # links are 'null' if not given
self.next_link := next_link
end
</syntaxhighlight>
 
=={{header|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]].
 
<syntaxhighlight lang="j">coclass'DoublyLinkedListElement'
create=:3 :0
this=:coname''
'predecessor successor data'=:y
successor__predecessor=: predecessor__successor=: this
)</syntaxhighlight>
 
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:
 
<syntaxhighlight lang="j">coclass'DoublyLinkedListHead'
create=:3 :0
predecessor=:successor=:this=: coname''
)</syntaxhighlight>
 
=={{header|Java}}==
{{works with|Java|1.5+}}
<syntaxhighlight 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;
}
}</syntaxhighlight>
 
For use with [[Java]] 1.4 and below, delete all "<T>"s and replace T's with "Object".
 
=={{header|JavaScript}}==
Inherits from LinkedList (see [[Singly-Linked_List_(element)#JavaScript]])
<syntaxhighlight 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;
}
public Node<T>(Node<T> n, Node<T> p, T elem){
 
next = n;
var head = createDoublyLinkedListFromArray([10,20,30,40]);</syntaxhighlight>
prev = p;
 
element = elem;
=={{header|Julia}}==
}
{{works with|Julia|0.6}}
 
public void setNext(Node<T> n){
<syntaxhighlight lang="julia">abstract type AbstractNode{T} end
next = n;
 
}
struct EmptyNode{T} <: AbstractNode{T} end
mutable publicstruct Node<{T>} getNext()<: AbstractNode{T}
return next;value::T
pred::AbstractNode{T}
succ::AbstractNode{T}
end</syntaxhighlight>
 
=={{header|Kotlin}}==
<syntaxhighlight lang="scala">// version 1.1.2
 
class Node<T: Number>(var data: T, var prev: Node<T>? = null, var next: Node<T>? = null) {
override fun toString(): String {
val sb = StringBuilder(this.data.toString())
var node = this.next
while (node != null) {
sb.append(" -> ", node.data.toString())
node = node.next
}
return sb.toString()
}
}
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;
}
}
 
fun main(args: Array<String>) {
For use with [[Java]] 1.4 and below, delete all "<T>"s and replace T's with "Object".
val n1 = Node(1)
val n2 = Node(2, n1)
n1.next = n2
val n3 = Node(3, n2)
n2.next = n3
println(n1)
println(n2)
println(n3)
}</syntaxhighlight>
 
{{out}}
<pre>
1 -> 2 -> 3
2 -> 3
3
</pre>
 
=={{header|Lang}}==
<syntaxhighlight lang="lang">
&Node = {
$next
$prev
$data
}
</syntaxhighlight>
 
=={{header|Lua}}==
see [[Doubly-linked_list/Definition#Lua]], essentially:
<syntaxhighlight lang="lua">local node = { data=data, prev=nil, next=nil }</syntaxhighlight>
 
=={{header|Mathematica}}/{{header|Wolfram Language}}==
Mathematica and the Wolfram Language have no lower-level way of handling pointers. It does have a built-in, compilable doubly-linked list data structure:
<syntaxhighlight lang="mathematica">CreateDataStructure["DoublyLinkedList"]</syntaxhighlight>
 
=={{header|Modula-2}}==
 
<syntaxhighlight lang="modula2">TYPE
Link = POINTER TO LinkRcd;
LinkRcd = RECORD
Prev, Next: Link;
Data: INTEGER
END;</syntaxhighlight>
 
=={{header|Nim}}==
<syntaxhighlight lang="nim">type
Node[T] = ref TNode[T]
 
TNode[T] = object
next, prev: Node[T]
data: T</syntaxhighlight>
 
=={{header|Oberon-2}}==
<syntaxhighlight lang="oberon2">
MODULE Box;
TYPE
Object* = POINTER TO ObjectDesc;
ObjectDesc* = (* ABSTRACT *) RECORD
END;
 
(* ... *)
END Box.
 
MODULE Collections;
TYPE
Node* = POINTER TO NodeDesc;
NodeDesc* = (* ABSTRACT *) RECORD
prev-,next-: Node;
value-: Box.Object;
END;
 
(* ... *)
END Collections.
</syntaxhighlight>
 
=={{header|Objeck}}==
<syntaxhighlight lang="objeck">class ListNode {
@value : Base;
@next : ListNode;
@previous: ListNode;
 
New(value : Base) {
@value := value;
}
method : public : Set(value : Base) ~ Nil {
@value := value;
}
 
method : public : Get() ~ Base {
return @value;
}
 
method : public : SetNext(next : Collection.ListNode) ~ Nil {
@next := next;
}
 
method : public : GetNext() ~ ListNode {
return @next;
}
 
method : public : SetPrevious(previous : Collection.ListNode) ~ Nil {
@previous := previous;
}
 
method : public : GetPrevious() ~ ListNode {
return @previous;
}
}</syntaxhighlight>
 
=={{header|OCaml}}==
===Imperative===
<langsyntaxhighlight lang="ocaml">type 'a dlink = {
mutable data: 'a;
mutable next: 'a dlink option;
Line 188 ⟶ 574:
 
let dlink_of_list li =
let rec auxf prev_dlink x = function
|let []dlink ->= prev_dlink{
| hd::tl ->data = x;
let dlinkprev = {None;
next = prev_dlink data = hd;}
in
prev = None;
begin next =match prev_dlink }with
| None -> in()
| begin matchSome prev_dlink with->
| Noneprev_dlink.prev <-> ()Some dlink
end;
| Some prev_dlink ->
prev_dlink.prev <- Some dlink
end;
aux (Some dlink) tl
in
auxList.fold_left f None (List.rev li)
;;
 
Line 224 ⟶ 608:
in
aux
;;</langsyntaxhighlight>
 
<langsyntaxhighlight lang="ocaml"># let dl = dlink_of_list [1;2;3;4;5] in
iter_forward_dlink (Printf.printf "%d\n") dl ;;
1
Line 233 ⟶ 617:
4
5
- : unit = ()</langsyntaxhighlight>
 
===Functional===
Line 240 ⟶ 624:
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:
 
<langsyntaxhighlight lang="ocaml">type 'a nav_list = 'a list * 'a * 'a list</langsyntaxhighlight>
 
The middle element is the pointed item, and the two lists are the
previous and the following items.
Here are the associated functions:
<syntaxhighlight lang="ocaml">let nav_list_of_list = function
<lang ocaml>
let nav_list_of_list = function
| hd::tl -> [], hd, tl
| [] -> invalid_arg "empty list"
Line 263 ⟶ 646:
prev_tl, prev, item::next
| _ ->
failwith "begin of nav_list reached"</syntaxhighlight>
<syntaxhighlight lang="ocaml"># let nl = nav_list_of_list [1;2;3;4;5] ;;
</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 ;;
Line 274 ⟶ 655:
 
# current nl ;;
- : int = 3</syntaxhighlight>
 
</lang>
=={{header|Oforth}}==
 
Complete definition is here : [[../Definition#Oforth]]
 
<syntaxhighlight lang="oforth">Object Class new: DNode(value, mutable prev, mutable next)</syntaxhighlight>
 
=={{header|Oz}}==
We show how to create a new node as a record value.
<syntaxhighlight lang="oz">fun {CreateNewNode Value}
node(prev:{NewCell _}
next:{NewCell _}
value:Value)
end</syntaxhighlight>
Note: this is for illustrative purposes only. In a real Oz program, you would use one of the existing data types.
 
=={{header|Pascal}}==
 
<syntaxhighlight 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;</syntaxhighlight>
 
=={{header|Perl}}==
Just use an array. You can traverse and splice it any way. Linked lists are way too low level.
 
However, if all you have is an algorithm in another language, you can use references to accomplish the translation.
 
<langsyntaxhighlight lang="perl">my %node = (
data => 'say what',
next => \%foo_node,
prev => \%bar_node,
);
$node{next} = \%quux_node; # mutable</langsyntaxhighlight>
 
=={{header|Phix}}==
In Phix, types are used for validation and debugging rather than specification purposes. For extensive run-time checking you could use
<!--<syntaxhighlight lang="phix">-->
<span style="color: #008080;">enum</span> <span style="color: #000000;">NEXT</span><span style="color: #0000FF;">,</span><span style="color: #000000;">PREV</span><span style="color: #0000FF;">,</span><span style="color: #000000;">DATA</span>
<span style="color: #008080;">type</span> <span style="color: #000000;">slnode</span><span style="color: #0000FF;">(</span><span style="color: #004080;">object</span> <span style="color: #000000;">x</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">return</span> <span style="color: #0000FF;">(</span><span style="color: #004080;">sequence</span><span style="color: #0000FF;">(</span><span style="color: #000000;">x</span><span style="color: #0000FF;">)</span> <span style="color: #008080;">and</span> <span style="color: #7060A8;">length</span><span style="color: #0000FF;">(</span><span style="color: #000000;">x</span><span style="color: #0000FF;">)=</span><span style="color: #000000;">DATA</span> <span style="color: #008080;">and</span> <span style="color: #0000FF;"><</span><span style="color: #000000;">i</span><span style="color: #0000FF;">></span><span style="color: #000000;">udt</span><span style="color: #0000FF;"></</span><span style="color: #000000;">i</span><span style="color: #0000FF;">>(</span><span style="color: #000000;">x</span><span style="color: #0000FF;">[</span><span style="color: #000000;">DATA</span><span style="color: #0000FF;">])</span> <span style="color: #008080;">and</span> <span style="color: #004080;">integer</span><span style="color: #0000FF;">(</span><span style="color: #000000;">x</span><span style="color: #0000FF;">[</span><span style="color: #000000;">NEXT</span><span style="color: #0000FF;">]</span> <span style="color: #008080;">and</span> <span style="color: #004080;">integer</span><span style="color: #0000FF;">(</span><span style="color: #000000;">x</span><span style="color: #0000FF;">[</span><span style="color: #000000;">PREV</span><span style="color: #0000FF;">]))</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">type</span>
<!--</syntaxhighlight>-->
But more often you would just use the builtin sequences. See also [[Singly-linked_list/Element_definition#Phix|Singly-linked_list/Element_definition]].
 
Memory is automatically reclaimed the moment items are no longer needed.
 
Note that automatic typechecking does not occur under pwa/p2js, that is desktop/Phix only (for the debugging stage) but you can invoke a type such as the above explicitly.
 
=={{header|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.
<syntaxhighlight lang="picolisp">(de 2tail (X DLst)
(let L (cdr DLst)
(con DLst (cons X L NIL))
(if L
(con (cdr L) (cdr DLst))
(set DLst (cdr DLst)) ) ) )
 
(de 2head (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
(2head 'not *DLst)</syntaxhighlight>
For output of the example data, see [[Doubly-linked list/Traversal#PicoLisp]].
 
=={{header|PL/I}}==
<syntaxhighlight 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. */
</syntaxhighlight>
 
=={{header|Plain English}}==
When you define a <code>thing</code>, you are defining a record as a doubly-linked list element. <code>next</code> and <code>previous</code> fields are implicitly added to the record that can be used to build and traverse a list.
<syntaxhighlight lang="plainenglish">An element is a thing with a number.</syntaxhighlight>
 
=={{header|Pop11}}==
 
<syntaxhighlight lang="pop11">uses objectclass;
define :class Link;
slot next = [];
slot prev = [];
slot data = [];
enddefine;</syntaxhighlight>
 
=={{header|PureBasic}}==
<syntaxhighlight lang="purebasic">Structure node
*prev.node
*next.node
value.i
EndStructure</syntaxhighlight>
 
=={{header|Python}}==
 
<langsyntaxhighlight lang="python">class Node(object):
def __init__(self, data = None, prev = None, next = None):
self.prev = prev
Line 328 ⟶ 794:
while c != None:
yield c
c = c.prev</langsyntaxhighlight>
 
=={{header|Racket}}==
 
<syntaxhighlight lang="racket">
(define-struct dlist (head tail) #:mutable)
(define-struct dlink (content prev next) #:mutable)
</syntaxhighlight>
 
See the functions on the [[Doubly-Linked List]] page for the usage of these structures.
 
=={{header|Raku}}==
(formerly Perl 6)
 
<syntaxhighlight lang="raku" line>role DLElem[::T] {
has DLElem[T] $.prev is rw;
has DLElem[T] $.next is rw;
has T $.payload = T;
 
method pre-insert(T $payload) {
die "Can't insert before beginning" unless $!prev;
my $elem = ::?CLASS.new(:$payload);
$!prev.next = $elem;
$elem.prev = $!prev;
$elem.next = self;
$!prev = $elem;
$elem;
}
 
method post-insert(T $payload) {
die "Can't insert after end" unless $!next;
my $elem = ::?CLASS.new(:$payload);
$!next.prev = $elem;
$elem.next = $!next;
$elem.prev = self;
$!next = $elem;
$elem;
}
 
method delete {
die "Can't delete a sentinel" unless $!prev and $!next;
$!next.prev = $!prev;
$!prev.next = $!next; # conveniently returns next element
}
}</syntaxhighlight>
 
=={{header|REXX}}==
REXX doesn't have linked lists, as there are no pointers (or handles).
<br>However, linked lists can be simulated with lists in REXX.
 
╔═════════════════════════════════════════════════════════════════════════╗
║ ☼☼☼☼☼☼☼☼☼☼☼ Functions of the List Manager ☼☼☼☼☼☼☼☼☼☼☼ ║
║ @init ─── initializes the List. ║
║ ║
║ @size ─── returns the size of the List [could be a 0 (zero)]. ║
║ ║
║ @show ─── shows (displays) the complete List. ║
║ @show k,1 ─── shows (displays) the Kth item. ║
║ @show k,m ─── shows (displays) M items, starting with Kth item. ║
║ @show ,,─1 ─── shows (displays) the complete List backwards. ║
║ ║
║ @get k ─── returns the Kth item. ║
║ @get k,m ─── returns the M items starting with the Kth item. ║
║ ║
║ @put x ─── adds the X items to the end (tail) of the List. ║
║ @put x,0 ─── adds the X items to the start (head) of the List. ║
║ @put x,k ─── adds the X items to before of the Kth item. ║
║ ║
║ @del k ─── deletes the item K. ║
║ @del k,m ─── deletes the M items starting with item K. ║
╚═════════════════════════════════════════════════════════════════════════╝
<syntaxhighlight lang="rexx">/*REXX program implements various List Manager functions (see the documentation above).*/
call sy 'initializing the list.' ; call @init
call sy 'building list: Was it a cat I saw' ; call @put "Was it a cat I saw"
call sy 'displaying list size.' ; say "list size="@size()
call sy 'forward list' ; call @show
call sy 'backward list' ; call @show ,,-1
call sy 'showing 4th item' ; call @show 4,1
call sy 'showing 5th & 6th items' ; call @show 5,2
call sy 'adding item before item 4: black' ; call @put "black",4
call sy 'showing list' ; call @show
call sy 'adding to tail: there, in the ...' ; call @put "there, in the shadows, stalking its prey (and next meal)."
call sy 'showing list' ; call @show
call sy 'adding to head: Oy!' ; call @put "Oy!",0
call sy 'showing list' ; call @show
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
p: return word(arg(1), 1) /*pick the first word out of many items*/
sy: say; say left('', 30) "───" arg(1) '───'; return
@init: $.@=; @adjust: $.@=space($.@); $.#=words($.@); return
@hasopt: arg o; return pos(o, opt)\==0
@size: return $.#
/*──────────────────────────────────────────────────────────────────────────────────────*/
@del: procedure expose $.; arg k,m; call @parms 'km'
_=subword($.@, k, k-1) subword($.@, k+m)
$.@=_; call @adjust; return
/*──────────────────────────────────────────────────────────────────────────────────────*/
@get: procedure expose $.; arg k,m,dir,_
call @parms 'kmd'
do j=k for m by dir while j>0 & j<=$.#
_=_ subword($.@, j, 1)
end /*j*/
return strip(_)
/*──────────────────────────────────────────────────────────────────────────────────────*/
@parms: arg opt /*define a variable based on an option.*/
if @hasopt('k') then k=min($.#+1, max(1, p(k 1)))
if @hasopt('m') then m=p(m 1)
if @hasopt('d') then dir=p(dir 1); return
/*──────────────────────────────────────────────────────────────────────────────────────*/
@put: procedure expose $.; parse arg x,k; k=p(k $.#+1); call @parms 'k'
$.@=subword($.@, 1, max(0, k-1)) x subword($.@, k); call @adjust
return
/*──────────────────────────────────────────────────────────────────────────────────────*/
@show: procedure expose $.; parse arg k,m,dir; if dir==-1 & k=='' then k=$.#
m=p(m $.#); call @parms 'kmd'; say @get(k,m, dir); return</syntaxhighlight>
'''output'''
<pre>
─── initializing the list. ───
 
─── building list: Was it a cat I saw ───
 
─── displaying list size. ───
list size=6
 
─── forward list ───
Was it a cat I saw
 
─── backward list ───
saw I cat a it Was
 
─── showing 4th item ───
cat
 
─── showing 6th & 6th items ───
I saw
 
─── adding item before item 4: black ───
 
─── showing list ───
Was it a black cat I saw
 
─── adding to tail: there, in the ... ───
 
─── showing list ───
Was it a black cat I saw there, in the shadows, stalking its prey (and next meal).
 
─── adding to head: Oy! ───
 
─── showing list ───
Oy! Was it a black cat I saw there, in the shadows, stalking its prey (and next meal).
</pre>
 
=={{header|Ruby}}==
Extending [[Singly-Linked List (element)#Ruby]]
<lang ruby>class ListNode
<syntaxhighlight lang="ruby">class DListNode < ListNode
attr_accessor :val, :nxt, :prv
attr_accessor :prev
def initialize(mval,mprv=nil,mnxt=nil)
# accessors :succ and :value are inherited
self.val=mval
 
self.prv=mprv
def initialize(value, prev=nil, succ=nil)
prv.nxt=self if prv
@value self.nxt=mnxt value
@prev nxt.prv=self if nxtprev
@prev.succ = self if prev
end
def each(&b)@succ = succ
@succ.prev = yieldself valif succ
end
nxt.each(&b) if nxt
 
self
def self.from_values(*ary)
end
ary << (f = ary.pop)
include Enumerable
ary.map! {|i| new i }
end</lang>
ary.inject(f) {|p, c| p.succ = c; c.prev = p; c }
end
end
 
list = DListNode.from_values 1,2,3,4</syntaxhighlight>
 
=={{header|Rust}}==
 
 
=== Simply using the standard library ===
<syntaxhighlight lang="rust">use std::collections::LinkedList;
fn main() {
// Doubly linked list containing 32-bit integers
let list = LinkedList::<i32>::new();
}</syntaxhighlight>
 
=== The behind-the-scenes implementation ===
Doubly linked lists present a problem in Rust due to its ownership model. There cannot be two mutable references to the same object, so what are we to do? Below are the relevant lines (with added comments) from the <code>std</code> implementation ([https://doc.rust-lang.org/std/collections/struct.LinkedList.html Documentation] [https://github.com/rust-lang/rust/blob/master/src/libcollections/linked_list.rs Source]).
 
The standard library uses the (currently) unstable `Shared<T>` type which indicates that the ownership of its contained type has shared ownership. It is guaranteed not to be null, is variant over <code>T</code> (meaning that an <code>&Shared<&'static T></code> may be used where a <code>&Shared<&'a T></code> is expected, indicates to the compiler that it may own a <code>T</code>) and may be dereferenced to a mutable pointer (<code>*mut T</code>). All of the above may be accomplished in standard stable Rust, except for the non-null guarantee which allows the compiler to make a few extra optimizations.
 
<syntaxhighlight lang="rust">pub struct LinkedList<T> {
head: Option<Shared<Node<T>>>,
tail: Option<Shared<Node<T>>>,
len: usize,
marker: PhantomData<Box<Node<T>>>, // Indicates that we logically own a boxed (owned pointer) Node<T>>
}
 
struct Node<T> {
next: Option<Shared<Node<T>>>,
prev: Option<Shared<Node<T>>>,
element: T,
}</syntaxhighlight>
 
=={{header|Sidef}}==
<syntaxhighlight lang="ruby">var node = Hash.new(
data => 'say what',
next => foo_node,
prev => bar_node,
);
 
node{:next} = quux_node; # mutable</syntaxhighlight>
 
=={{header|Swift}}==
 
<syntaxhighlight lang="swift">typealias NodePtr<T> = UnsafeMutablePointer<Node<T>>
 
class Node<T> {
var value: T
fileprivate var prev: NodePtr<T>?
fileprivate var next: NodePtr<T>?
 
init(value: T, prev: NodePtr<T>? = nil, next: NodePtr<T>? = nil) {
self.value = value
self.prev = prev
self.next = next
}
}
</syntaxhighlight>
 
=={{header|Tcl}}==
{{eff note|Tcl|list}}
While it is highly unusual to implement linked lists in Tcl, since the language has a built-in list type (that internally uses arrays of references), it is possible to simulate it with objects.
{{works with|Tcl|8.6}} or {{libheader|TclOO}}
<br>
<syntaxhighlight lang="tcl">oo::class create List {
{{works with|Tcl|8.6}}
<lang tcl>oo::class create List {
variable content next prev
constructor {value {list ""}} {
Line 371 ⟶ 1,045:
set prev {*}$args
}
}</langsyntaxhighlight>
 
=={{header|Visual Basic .NET}}==
 
<syntaxhighlight 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</syntaxhighlight>
 
=={{header|Wren}}==
{{libheader|Wren-llist}}
The DNode class in the above module is the element type for the DLinkedList class which is a generic doubly-linked list. The latter is implemented in such a way that the user does not need to deal directly with DNode though for the purposes of the task we show below how instances of it can be created and manipulated.
<syntaxhighlight lang="wren">import "./llist" for DNode
 
var dn1 = DNode.new(1)
var dn2 = DNode.new(2)
dn1.next = dn2
dn1.prev = null
dn2.prev = dn1
dn2.next = null
System.print(["node 1", "data = %(dn1.data)", "prev = %(dn1.prev)", "next = %(dn1.next)"])
System.print(["node 2", "data = %(dn2.data)", "prev = %(dn2.prev)", "next = %(dn2.next)"])</syntaxhighlight>
 
{{out}}
<pre>
[node 1, data = 1, prev = null, next = 2]
[node 2, data = 2, prev = 1, next = null]
</pre>
 
=={{header|XPL0}}==
<syntaxhighlight lang "XPL0">
def \Node\ Prev, Data, Next; \Element (Node) definition
</syntaxhighlight>
 
=={{header|zkl}}==
<syntaxhighlight lang="zkl">class Node{
fcn init(_value,_prev=Void,_next=Void)
{ var value=_value, prev=_prev, next=_next; }
fcn toString{ value.toString() }
}</syntaxhighlight>
<syntaxhighlight lang="zkl">a,b:=Node(1),Node("three");
a.next=b; b.prev=a;
println(a.next," ",b.prev);</syntaxhighlight>
{{out}}
<pre>
three 1
</pre>
 
{{omit from|ACL2}}
{{omit from|TI-83 BASIC}} {{omit from|TI-89 BASIC}} <!-- Does not have user-defined data structures or objects. -->
PureFox
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