Doubly-linked list/Definition

Doubly-linked list/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 complete Doubly Linked List.

• The structure should support adding elements to the head, tail and middle of the list.
• The structure should not allow circular loops

See also

• Array
• Associative array: Creation, Iteration
• Collections
• Compound data type
• Doubly-linked list: Definition, Element definition, Element insertion, List Traversal, Element Removal
• Linked list
• Queue: Definition, Usage
• Set
• Singly-linked list: Element definition, Element insertion, List Traversal, Element Removal
• Stack

Action!

The user must type in the monitor the following command after compilation and before running the program!

SET EndProg=*

Library: Action! Tool Kit

<lang Action!>CARD EndProg ;required for ALLOCATE.ACT

INCLUDE "D2:ALLOCATE.ACT" ;from the Action! Tool Kit. You must type 'SET EndProg=*' from the monitor after compiling, but before running this program!

DEFINE PTR="CARD" DEFINE NODE_SIZE="5" TYPE ListNode=[BYTE data PTR prv,nxt]

ListNode POINTER listBegin,listEnd

PROC AddBegin(BYTE v)

 ListNode POINTER n

 n=Alloc(NODE_SIZE)
 n.data=v
 n.prv=0
 n.nxt=listBegin
 IF listBegin THEN
   listBegin.prv=n
 ELSE
   listEnd=n
 FI
 listBegin=n

RETURN

PROC AddEnd(BYTE v)

 ListNode POINTER n

 n=Alloc(NODE_SIZE)
 n.data=v
 n.prv=listEnd
 n.nxt=0
 IF listEnd THEN
   listEnd.nxt=n
 ELSE
   listBegin=n
 FI
 listEnd=n

RETURN

PROC AddBefore(BYTE v ListNode POINTER node)

 ListNode POINTER n,tmp

 IF node=0 THEN
   PrintE("The node is null!") Break()
 ELSEIF node=listBegin THEN
   AddBegin(v)
 ELSE
   n=Alloc(NODE_SIZE)
   n.data=v
   n.prv=node.prv
   n.nxt=node
   tmp=node.prv
   tmp.nxt=n
   node.prv=n
 FI

RETURN

PROC AddAfter(BYTE v ListNode POINTER node)

 ListNode POINTER n,tmp

 IF node=0 THEN
   PrintE("The node is null!") Break()
 ELSEIF node=listEnd THEN
   AddEnd(v)
 ELSE
   n=Alloc(NODE_SIZE)
   n.data=v
   n.nxt=node.nxt
   n.prv=node
   tmp=node.nxt
   tmp.prv=n
   node.nxt=n
 FI

RETURN

PROC Clear()

 ListNode POINTER n,next

 n=listBegin
 WHILE n
 DO
   next=n.nxt
   Free(n,NODE_SIZE)
   n=next
 OD
 listBegin=0
 listEnd=0

RETURN

PROC PrintList()

 ListNode POINTER n

 n=listBegin
 Print("(")
 WHILE n
 DO
   PrintB(n.data)
   IF n.nxt THEN
     Print(", ")
   FI
   n=n.nxt
 OD
 PrintE(")")

RETURN

PROC TestAddBegin(BYTE v)

 AddBegin(v)
 PrintF("Add %B at the begin:%E",v)
 PrintList()

RETURN

PROC TestAddEnd(BYTE v)

 AddEnd(v)
 PrintF("Add %B at the end:%E",v)
 PrintList()

RETURN

PROC TestAddBefore(BYTE v ListNode POINTER node)

 AddBefore(v,node)
 PrintF("Add %B before %B:%E",v,node.data)
 PrintList()

RETURN

PROC TestAddAfter(BYTE v ListNode POINTER node)

 AddAfter(v,node)
 PrintF("Add %B after %B:%E",v,node.data)
 PrintList()

RETURN

PROC TestClear()

 Clear()
 PrintE("Clear the list:")
 PrintList()

RETURN

PROC Main()

 Put(125) PutE() ;clear screen
 
 AllocInit(0)
 listBegin=0
 listEnd=0

 PrintList()
 TestAddBegin(1)
 TestAddBegin(2)
 TestAddEnd(3)
 TestAddBefore(4,listBegin.nxt)
 TestAddBefore(5,listBegin)
 TestAddAfter(6,listEnd.prv)
 TestAddAfter(7,listEnd)
 TestClear()

RETURN</lang>

Output:

Screenshot from Atari 8-bit computer

()
Add 1 at the begin:
(1)
Add 2 at the begin:
(2, 1)
Add 3 at the end:
(2, 1, 3)
Add 4 before 1:
(2, 4, 1, 3)
Add 5 before 2:
(5, 2, 4, 1, 3)
Add 6 after 1:
(5, 2, 4, 1, 6, 3)
Add 7 after 3:
(5, 2, 4, 1, 6, 3, 7)
Clear the list:
()

Ada

Works with: Ada 2005

Examples already in other doubly-linked list tasks: see Doubly-linked list/Element insertion#Ada and Doubly-linked list/Traversal#Ada.

Ada 2005 defines doubly-linked lists in A.18.3 The Package Containers.Doubly_Linked_Lists.

ALGOL 68

Translation of: C

Works with: ALGOL 68 version Revision 1 - one extension to language used - PRAGMA READ - a non standard feature similar to C's #include directive.

Works with: ALGOL 68G version Any - tested with release algol68g-2.7

Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - tested with release 1.8.8d.fc9.i386

File: prelude/Doubly-linked_list_Link.a68<lang algol68># -*- coding: utf-8 -*- # COMMENT REQUIRES:

 MODE VALUE = ~;

1. For example: #

 MODE VALUE = UNION(INT, REAL, COMPL)

END COMMENT

MODE LINKNEW = STRUCT (

 LINK next, prev,
 VALUE value

);

MODE LINK = REF LINKNEW;

SKIP</lang>File: prelude/Doubly-linked_list_Operator.a68<lang algol68># -*- coding: utf-8 -*- # MODE LISTNEW = LINKNEW; MODE LIST = REF LISTNEW;

OP LISTINIT = (LIST self)LIST: (

 self := (self, self, ~);
 self

);

OP ISEMPTY = (LIST self)BOOL:

 (LIST(prev OF self) :=: LIST(self)) AND (LIST(self) :=: LIST(next OF self));

OP HEAD = (LIST self)LINK: next OF self;

OP TAIL = (LIST self)LINK: prev OF self;

1. insert after #

OP +:= = (LINK cursor, LINK link)LINK: (

 next OF link := next OF cursor;
 prev OF link := cursor;
 next OF cursor := link;
 prev OF next OF link := link;
 link

);

1. insert before #

OP +=: = (LINK link, LINK cursor)LINK: prev OF cursor +:= link;

1. delete current and step forward #

OP -:= = (LIST ignore, LINK link)LINK: (

 next OF prev OF link := next OF link;
 prev OF next OF link := prev OF link;
 next OF link := prev OF link := NIL; # garbage collection hint #
 link

);

1. delete current and step backward #

PRIO -=: = 1; OP -=: = (LIST link, LIST ignore)LINK: (

 ignore -:= link; prev OF link

);

PRIO ISIN = 1; # low priority #

OP ISIN = (LINK link, LIST self)BOOL:

 link ISNT LINK(self);

SKIP</lang>File: test/Doubly-linked_list_Operator_Usage.a68<lang algol68>#!/usr/bin/a68g --script #

1. -*- coding: utf-8 -*- #

MODE VALUE = STRING; # user defined data type # PR READ "prelude/Doubly-linked_list_Link.a68" PR; PR READ "prelude/Doubly-linked_list_Operator.a68" PR;

main: (

   []VALUE sample = ("Was", "it", "a", "cat", "I", "saw");
   LIST example list := LISTINIT HEAP LISTNEW;
   LINK this;

1. Add some data to a list #

   FOR i TO UPB sample DO
       this := HEAP LINKNEW;
       value OF this := sample[i];
       TAIL example list +:= this
   OD;

1. Iterate throught the list forward #

   this := HEAD example list;
   print("Iterate forward: ");
   WHILE this ISIN example list DO
       print((value OF this, " "));
       this := next OF this
   OD;
   print(new line);

1. Iterate throught the list backward #

   this := TAIL example list;
   print("Iterate backward: ");
   WHILE this ISIN example list DO
       print((value OF this, " "));
       this := prev OF this
   OD;
   print(new line);

1. Finally empty the list #

   print("Empty from tail: ");
   WHILE NOT ISEMPTY example list DO
         this := (example list -:= TAIL example list);
         print((value OF this, " "))
   OD;
   print(new line)

)</lang>

Output:

Iterate forward: Was it a cat I saw 
Iterate backward: saw I cat a it Was 
Empty from tail: saw I cat a it Was 

ARM Assembly

Works with: as version Raspberry Pi

<lang ARM Assembly> /* ARM assembly Raspberry PI */ /* program defDblList.s */

/* Constantes */ .equ STDOUT, 1 @ Linux output console .equ EXIT, 1 @ Linux syscall .equ READ, 3 .equ WRITE, 4

/*******************************************/ /* Structures */ /********************************************/ /* structure Doublylinkedlist*/

   .struct  0

dllist_head: @ head node

   .struct  dllist_head + 4

dllist_tail: @ tail node

   .struct  dllist_tail  + 4

dllist_fin: /* structure Node Doublylinked List*/

   .struct  0

NDlist_next: @ next element

   .struct  NDlist_next + 4 

NDlist_prev: @ previous element

   .struct  NDlist_prev + 4 

NDlist_value: @ element value or key

   .struct  NDlist_value + 4 

NDlist_fin: /* Initialized data */ .data szMessInitListe: .asciz "List initialized.\n" szCarriageReturn: .asciz "\n" szMessErreur: .asciz "Error detected.\n"

/* UnInitialized data */ .bss dllist1: .skip dllist_fin @ list memory place

/* code section */ .text .global main main:

   ldr r0,iAdrdllist1
   bl newDList                      @ create new list
   ldr r0,iAdrszMessInitListe
   bl affichageMess
   ldr r0,iAdrdllist1               @ list address
   mov r1,#10                       @ value
   bl insertHead                    @ insertion at head
   cmp r0,#-1
   beq 99f
   ldr r0,iAdrdllist1
   mov r1,#20
   bl insertTail                    @ insertion at tail
   cmp r0,#-1
   beq 99f
   ldr r0,iAdrdllist1               @ list address
   mov r1,#10                       @ value to after insered
   mov r2,#15                       @ new value
   bl insertAfter
   cmp r0,#-1
   beq 99f

   b 100f

99:

   ldr r0,iAdrszMessErreur
   bl affichageMess

100: @ standard end of the program

   mov r7, #EXIT                       @ request to exit program
   svc 0                               @ perform system call

iAdrszMessInitListe: .int szMessInitListe iAdrszMessErreur: .int szMessErreur iAdrszCarriageReturn: .int szCarriageReturn iAdrdllist1: .int dllist1 /******************************************************************/ /* create new list */ /******************************************************************/ /* r0 contains the address of the list structure */ newDList:

   push {r1,lr}                         @ save  registers 
   mov r1,#0
   str r1,[r0,#dllist_tail]
   str r1,[r0,#dllist_head]
   pop {r1,lr}                          @ restaur registers
   bx lr                                @ return

/******************************************************************/ /* list is empty ? */ /******************************************************************/ /* r0 contains the address of the list structure */ /* r0 return 0 if empty else return 1 */ isEmpty:

   //push {r1,lr}                         @ save  registers 
   ldr r0,[r0,#dllist_head]
   cmp r0,#0
   movne r0,#1
   //pop {r1,lr}                          @ restaur registers
   bx lr                                @ return

/******************************************************************/ /* insert value at list head */ /******************************************************************/ /* r0 contains the address of the list structure */ /* r1 contains value */ insertHead:

   push {r1-r4,lr}                         @ save  registers 
   mov r4,r0                            @ save address
   mov r0,r1                            @ value
   bl createNode
   cmp r0,#-1                           @ allocation error ?
   beq 100f
   ldr r2,[r4,#dllist_head]             @ load address first node
   str r2,[r0,#NDlist_next]             @ store in next pointer on new node
   mov r1,#0
   str r1,[r0,#NDlist_prev]             @ store zero in previous pointer on new node
   str r0,[r4,#dllist_head]             @ store address new node in address head list 
   cmp r2,#0                            @ address first node is null ?
   strne r0,[r2,#NDlist_prev]           @ no store adresse new node in previous pointer
   streq r0,[r4,#dllist_tail]           @ else store new node in tail address

100:

   pop {r1-r4,lr}                       @ restaur registers
   bx lr                                @ return

/******************************************************************/ /* insert value at list tail */ /******************************************************************/ /* r0 contains the address of the list structure */ /* r1 contains value */ insertTail:

   push {r1-r4,lr}                         @ save  registers 
   mov r4,r0                            @ save list address
   mov r0,r1                            @ value
   bl createNode                        @ create new node
   cmp r0,#-1
   beq 100f                             @ allocation error
   ldr r2,[r4,#dllist_tail]             @ load address last node
   str r2,[r0,#NDlist_prev]             @ store in previous pointer on new node
   mov r1,#0                            @ store null un next pointer
   str r1,[r0,#NDlist_next]
   str r0,[r4,#dllist_tail]             @ store address new node on list tail
   cmp r2,#0                            @ address last node is null ?
   strne r0,[r2,#NDlist_next]           @ no store address new node in next pointer
   streq r0,[r4,#dllist_head]           @ else store in head list

100:

   pop {r1-r4,lr}                          @ restaur registers
   bx lr                                @ return

/******************************************************************/ /* insert value after other element */ /******************************************************************/ /* r0 contains the address of the list structure */ /* r1 contains value to search*/ /* r2 contains value to insert */ insertAfter:

   push {r1-r5,lr}                         @ save  registers 
   mov r4,r0                               @ save list address
   bl searchValue                          @ search this value in r1
   cmp r0,#-1
   beq 100f                                @ not found -> error
   mov r5,r0                               @ save address of node find
   mov r0,r2                               @ new value
   bl createNode                           @ create new node
   cmp r0,#-1
   beq 100f                                @ allocation error
   ldr r2,[r5,#NDlist_next]                @ load pointer next of find node
   str r0,[r5,#NDlist_next]                @ store new node in pointer next
   str r5,[r0,#NDlist_prev]                @ store address find node in previous pointer on new node
   str r2,[r0,#NDlist_next]                @ store pointer next of find node on pointer next on new node
   cmp r2,#0                               @ next pointer is null ?
   strne r0,[r2,#NDlist_prev]              @ no store address new node in previous pointer 
   streq r0,[r4,#dllist_tail]              @ else store in list tail

100:

   pop {r1-r5,lr}                          @ restaur registers
   bx lr                                   @ return

/******************************************************************/ /* search value */ /******************************************************************/ /* r0 contains the address of the list structure */ /* r1 contains the value to search */ /* r0 return address of node or -1 if not found */ searchValue:

   push {r2,lr}                         @ save  registers 
   ldr r0,[r0,#dllist_head]             @ load first node

1:

   cmp r0,#0                            @ null -> end search not found
   moveq r0,#-1
   beq 100f
   ldr r2,[r0,#NDlist_value]            @ load node value
   cmp r2,r1                            @ equal ?
   beq 100f
   ldr r0,[r0,#NDlist_next]             @ load addresse next node 
   b 1b                                 @ and loop

100:

   pop {r2,lr}                          @ restaur registers
   bx lr                                @ return

/******************************************************************/ /* Create new node */ /******************************************************************/ /* r0 contains the value */ /* r0 return node address or -1 if allocation error*/ createNode:

   push {r1-r7,lr}                         @ save  registers 
   mov r4,r0                            @ save value
   @ allocation place on the heap
   mov r0,#0                                   @ allocation place heap
   mov r7,#0x2D                                @ call system 'brk'
   svc #0
   mov r5,r0                                   @ save address heap for output string
   add r0,#NDlist_fin                            @ reservation place one element
   mov r7,#0x2D                                @ call system 'brk'
   svc #0
   cmp r0,#-1                                  @ allocation error
   beq 100f
   mov r0,r5
   str r4,[r0,#NDlist_value]                   @ store value
   mov r2,#0
   str r2,[r0,#NDlist_next]                    @ store zero to pointer next
   str r2,[r0,#NDlist_prev]                    @ store zero to pointer previous

100:

   pop {r1-r7,lr}                          @ restaur registers
   bx lr                                @ return

/******************************************************************/ /* display text with size calculation */ /******************************************************************/ /* r0 contains the address of the message */ affichageMess:

   push {r0,r1,r2,r7,lr}                       @ save  registers 
   mov r2,#0                                   @ counter length */

1: @ loop length calculation

   ldrb r1,[r0,r2]                             @ read octet start position + index 
   cmp r1,#0                                   @ if 0 its over
   addne r2,r2,#1                              @ else add 1 in the length
   bne 1b                                      @ and loop 
                                               @ so here r2 contains the length of the message 
   mov r1,r0                                   @ address message in r1 
   mov r0,#STDOUT                              @ code to write to the standard output Linux
   mov r7, #WRITE                              @ code call system "write" 
   svc #0                                      @ call system
   pop {r0,r1,r2,r7,lr}                        @ restaur registers
   bx lr                                       @ return

</lang>

ATS

Translation of: Scheme

The example is broken into an interface ("static") file dllist.sats, an implementation ("dynamic") file dllist.dats, and a demonstration program dllist-demo.dats. I broke up the example this way because the abstraction thus introduced is important; the implementation is a linear type, whereas the user sees it as a nonlinear type. The interface file completely hides the linear typing.

The reason for this complication is that, in ATS, a linear object can be treated as "mutable", without resorting to embedded C code. One cost of this is that an object of a linear type has to be freed explicitly. However, if it is cast to an "ordinary" (nonlinear) type, a garbage collector to handle freeing the object. Such casting is what I have done.

Here is dllist.sats. <lang ATS>(********************************************************************) (* The public interface *)

typedef dllist_t (t : t@ype+, is_root : bool) = ptr typedef dllist_t (t : t@ype+) = [b : bool] dllist_t (t, b)

(* Make a new dllist_t, consisting of a root node. *) fun {t : t@ype} dllist_t_make () : dllist_t (t, true)

(* Is this the root node, with no element stored? *) fun {t : t@ype} dllist_t_is_root

         {is_root : bool}
         (dl : dllist_t (t, is_root)) :
   [b : bool | b == is_root]
   bool b

(* Is this a non-root node, with an element stored? *) fun {t : t@ype} dllist_t_isnot_root

         {is_root : bool}
         (dl : dllist_t (t, is_root)) :
   [b : bool | b == ~is_root]
   bool b

(* Return the root node of the list. *) fun {t : t@ype} dllist_t_root (dl : dllist_t t) : dllist_t (t, true)

(* Return the previous node. *) fun {t : t@ype} dllist_t_previous (dl : dllist_t t) : dllist_t t

(* Return the next node. *) fun {t : t@ype} dllist_t_next (dl : dllist_t t) : dllist_t t

(* Insert an element before the given node. *) fun {t : t@ype} dllist_t_insert_before (dl : dllist_t t, elem : t) : void

(* Insert an element after the given node. *) fun {t : t@ype} dllist_t_insert_after (dl : dllist_t t, elem : t) : void

(* Remove the given node. It is an error to call this on the root

  node. *)

fun {t : t@ype} dllist_t_remove (dl : dllist_t t) : void

(* Return a copy of the stored element. It is an error to call this on

  the root node. *)

fun {t : t@ype} dllist_t_element (dl: dllist_t t) : t

fun {t : t@ype} dllist_t_make_generator (dl: dllist_t t, direction : int) :

   () -<cloref1> Option t

fun {t : t@ype} dllist_t_to_list

         (dl : dllist_t t) : [n : nat] list (t, n)

fun {t : t@ype} list_to_dllist_t

         {n : int}
         (dl : list (t, n)) : dllist_t t

(********************************************************************)</lang>

Here is dllist.dats. <lang ATS>#define ATS_DYNLOADFLAG 0

1. include "share/atspre_staload.hats"

staload "dllist.sats" staload UN = "prelude/SATS/unsafe.sats"

(********************************************************************) (* The implementation in terms of linear types. *)

absprop NODEPTR (t : t@ype+, is_root : bool, p : addr)

vtypedef nodeptr_vt (t : t@ype+, is_root : bool, p : addr) =

 @(NODEPTR (t, is_root, p) | ptr p)

vtypedef nodeptr_vt (t : t@ype+, p : addr) =

 [is_root : bool] nodeptr_vt (t, is_root, p)

vtypedef nodeptr_vt (t : t@ype+, is_root : bool) =

 [p : addr] nodeptr_vt (t, is_root, p)

vtypedef nodeptr_vt (t : t@ype+) =

 [is_root : bool] [p : addr] nodeptr_vt (t, is_root, p)

datavtype node_vt (t : t@ype+, is_root : bool) = | node_vt_object (t, is_root) of

   (bool is_root,              (* Is it the root node? *)
    nodeptr_vt t,              (* previous*)
    nodeptr_vt t,              (* next *)
    t)                         (* element *)

vtypedef node_vt (t : t@ype+) =

 [is_root : bool] node_vt (t, is_root)

extern castfn node2nodeptr_consuming :

 {t : t@ype}
 {is_root : bool}
 node_vt (t, is_root) -<> nodeptr_vt (t, is_root)

extern castfn nodeptr2node_consuming :

 {t : t@ype}
 {is_root : bool}
 nodeptr_vt (t, is_root) -<> node_vt (t, is_root)

extern castfn node2nodeptr_preserving :

 {t : t@ype}
 {is_root : bool}
 (!node_vt (t, is_root) >> _) -<> nodeptr_vt (t, is_root)

extern castfn nodeptr2node_preserving :

 {t : t@ype}
 {is_root : bool}
 (!nodeptr_vt (t, is_root) >> _) -<> node_vt (t, is_root)

fn {t : t@ype} node_refcopy {is_root : bool}

            (node    : !node_vt (t, is_root)) :
   node_vt (t, is_root) =
 nodeptr2node_preserving (node2nodeptr_preserving node)

fn {t : t@ype} make_root () : node_vt (t, true) =

 (* Create a node that is marked as a root, points to itself, and has
    no actual element stored in it. *)
 let
   var fake_elem : t?
   prval _ = $UN.castview2void_at{t} (view@ fake_elem)

   val node = node_vt_object (true,
                              $UN.castvwtp0 the_null_ptr,
                              $UN.castvwtp0 the_null_ptr,
                              fake_elem)

   val prev_val = node2nodeptr_preserving node
   val next_val = node2nodeptr_preserving node

   val+ @ node_vt_object (_, prev, next, _) = node
   val _ = prev := prev_val
   val _ = next := next_val
   prval _ = fold@ node
 in
   node
 end

fn {t : t@ype} is_root {is_root : bool}

       (node : !node_vt (t, is_root)) :
   [b : bool | b == is_root] bool b =
 case+ node of
 | node_vt_object (is_root, _, _, _) => is_root

fn {t : t@ype} isnot_root {is_root : bool}

          (node : !node_vt (t, is_root)) :
   [b : bool | b == ~is_root] bool b =
 ~is_root<t> node

fn {t : t@ype} find_root (node : !node_vt t) : node_vt (t, true) =

 let
   fun
   loop (node : !node_vt t) : node_vt (t, true) =
     case+ node of
     | node_vt_object (true, _, _, _) => node_refcopy node
     | node_vt_object (false, prev, _, _) =>
       let
         val prev_node = nodeptr2node_preserving prev
         val retval = loop prev_node
         val _ = $UN.castvwtp0{void} prev_node
       in
         retval
       end
 in
   loop node
 end

fn {t : t@ype} get_prev (node : !node_vt t) : node_vt t =

 let
   val+ @ node_vt_object (_, prev, _, _) = node
   val prev_node = nodeptr2node_preserving prev
   prval _ = fold@ node
 in
   prev_node
 end

fn {t : t@ype} get_next (node : !node_vt t) : node_vt t =

 let
   val+ @ node_vt_object (_, _, next, _) = node
   val next_node = nodeptr2node_preserving next
   prval _ = fold@ node
 in
   next_node
 end

fn {t : t@ype} set_prev (node : !node_vt t, prev_val : !node_vt t) : void =

 {
   val+ @ node_vt_object (_, prev, _, _) = node
   val _ = prev := node2nodeptr_preserving prev_val
   prval _ = fold@ node
 }

fn {t : t@ype} set_next (node : !node_vt t, next_val : !node_vt t) : void =

 {
   val+ @ node_vt_object (_, _, next, _) = node
   val _ = next := node2nodeptr_preserving next_val
   prval _ = fold@ node
 }

fn {t : t@ype} insert_before (node : !node_vt t, elem : t) : void =

 {
   val prev_node = get_prev<t> node
   val new_node =
     node_vt_object (false, node2nodeptr_preserving prev_node,
                     node2nodeptr_preserving node, elem)
   val _ = set_next<t> (prev_node, new_node)
   val _ = set_prev<t> (node, new_node)
   val _ = $UN.castvwtp0{void} prev_node
   val _ = $UN.castvwtp0{void} new_node
 }

fn {t : t@ype} insert_after (node : !node_vt t, elem : t) : void =

 {
   val next_node = get_next<t> node
   val new_node =
     node_vt_object (false, node2nodeptr_preserving node,
                     node2nodeptr_preserving next_node, elem)
   val _ = set_next<t> (node, new_node)
   val _ = set_prev<t> (next_node, new_node)
   val _ = $UN.castvwtp0{void} next_node
   val _ = $UN.castvwtp0{void} new_node
 }

fn {t : t@ype} remove (node : !node_vt t) : void =

 {
   val _ = assertloc (isnot_root<t> node)
   val prev_node = get_prev<t> node
   val next_node = get_next<t> node
   val _ = set_next<t> (prev_node, next_node)
   val _ = set_prev<t> (next_node, prev_node)
   val _ = $UN.castvwtp0{void} prev_node
   val _ = $UN.castvwtp0{void} next_node
 }

fn {t : t@ype} get_element (node : !node_vt t) : t =

 case+ node of
 | node_vt_object (is_root, _, _, elem) =>
   begin
     assertloc (~is_root);
     elem
   end

(********************************************************************) (* Implementation of the public interface. *)

(* The public interface is "nonlinear"; that is, its types are

  "ordinary", and will have to be managed by a garbage collector, if
  you do not want them to leak freely. The need to free the linear
  types is bypassed by these interface template functions.

  This, of course, is the situation with a great many programming
  languages, and with more of them all the time. So it is nothing
  extraordinary.

  The usual garbage collector to use with ATS2 (Postiats) is Boehm
  GC. *)

implement {t} dllist_t_make () =

 $UN.castvwtp0 (make_root<t> ())

implement {t} dllist_t_is_root {is_root} dl =

 let
   val node = $UN.castvwtp0{node_vt (t, is_root)} dl
   val retval = is_root<t> node
   val _ = $UN.castvwtp0{void} node
 in
   retval
 end

implement {t} dllist_t_isnot_root {is_root} dl =

 let
   val node = $UN.castvwtp0{node_vt (t, is_root)} dl
   val retval = isnot_root<t> node
   val _ = $UN.castvwtp0{void} node
 in
   retval
 end

implement {t} dllist_t_root dl =

 let
   val node = $UN.castvwtp0{node_vt t} dl
   val root = find_root<t> node
   val _ = $UN.castvwtp0{void} node
 in
   $UN.castvwtp0 root
 end

implement {t} dllist_t_previous dl =

 let
   val node = $UN.castvwtp0{node_vt t} dl
   val prev = get_prev<t> node
   val _ = $UN.castvwtp0{void} node
 in
   $UN.castvwtp0 prev
 end

implement {t} dllist_t_next dl =

 let
   val node = $UN.castvwtp0{node_vt t} dl
   val next = get_next<t> node
   val _ = $UN.castvwtp0{void} node
 in
   $UN.castvwtp0 next
 end

implement {t} dllist_t_insert_before (dl, elem) =

 let
   val node = $UN.castvwtp0{node_vt t} dl
   val next = insert_before<t> (node, elem)
   val _ = $UN.castvwtp0{void} node
 in
 end

implement {t} dllist_t_insert_after (dl, elem) =

 let
   val node = $UN.castvwtp0{node_vt t} dl
   val next = insert_after<t> (node, elem)
   val _ = $UN.castvwtp0{void} node
 in
 end

implement {t} dllist_t_remove dl =

 let
   val node = $UN.castvwtp0{node_vt t} dl
   val next = remove<t> node
   val _ = $UN.castvwtp0{void} node
 in
 end

implement {t} dllist_t_element dl =

 let
   val node = $UN.castvwtp0{node_vt t} dl
   val elem = get_element<t> node
   val _ = $UN.castvwtp0{void} node
 in
   elem
 end

implement {t} dllist_t_make_generator (dl, direction) =

 if isltz direction then
   let
     val node_ref = ref (dllist_t_previous<t> (dllist_t_root<t> dl))
     val p_node = $UN.castvwtp0{ptr} node_ref
   in
     lam () =>
       let
         val node_ref = $UN.castvwtp0{ref (dllist_t t)} p_node
         val node = !node_ref
       in
         if dllist_t_is_root<t> node then
           None ()
         else
           let
             val elem = dllist_t_element<t> node
           in
             !node_ref := dllist_t_previous<t> node;
             Some elem
           end
       end
   end
 else
   let
     val node_ref = ref (dllist_t_next<t> (dllist_t_root<t> dl))
     val p_node = $UN.castvwtp0{ptr} node_ref
   in
     lam () =>
       let
         val node_ref = $UN.castvwtp0{ref (dllist_t t)} p_node
         val node = !node_ref
       in
         if dllist_t_is_root<t> node then
           None ()
         else
           let
             val elem = dllist_t_element<t> node
           in
             !node_ref := dllist_t_next<t> node;
             Some elem
           end
       end
   end

implement {t} dllist_t_to_list dl =

 let
   var lst : List0 t = list_nil ()
   var xopt : Option t
   val gen = dllist_t_make_generator<t> (dl, ~1)
 in
   for (xopt := gen (); option_is_some xopt; xopt := gen ())
     case+ xopt of
     | Some x => lst := list_cons (x, lst);
   lst
 end

implement {t} list_to_dllist_t lst =

 let
   val root = dllist_t_make<t> ()
   var dl : dllist_t t = root
   var p : List t
 in
   for (p := lst; isneqz p; p := list_tail p)
     begin
       dllist_t_insert_after<t> (dl, list_head p);
       dl := dllist_t_next<t> dl
     end;
   root
 end

(********************************************************************)</lang>

And here is dllist-demo.dats. <lang ATS>#include "share/atspre_staload.hats"

staload "dllist.sats" staload _ = "dllist.dats"

(* Using macdefs as follows, rather than implementing compiled

  functions in terms of the templates, means the templates will be
  expanded each time you call the macro. You may or may not wish
  this. You *might* get big code that optimizes well. *)

typedef dl_t = dllist_t int

macdef dlmake = dllist_t_make<int>

macdef insert_before = dllist_t_insert_before<int> macdef insert_after = dllist_t_insert_after<int> macdef remove = dllist_t_remove<int>

macdef get_root = dllist_t_root<int> macdef get_prev = dllist_t_previous<int> macdef get_next = dllist_t_next<int>

macdef is_root = dllist_t_is_root<int> macdef isnot_root = dllist_t_isnot_root<int>

macdef get_element = dllist_t_element<int>

macdef make_generator = dllist_t_make_generator<int> macdef dl2list = dllist_t_to_list<int> macdef list2dl = list_to_dllist_t<int>

fn print_forwards (dl : dl_t) =

 let
   val gen = make_generator (dl, 1)
   var xopt : Option int
   var separator : string = ""
 in
   for (xopt := gen (); option_is_some xopt; xopt := gen ())
     case+ xopt of
     | Some x =>
       begin
         print! separator;
         print! x;
         separator := " "
       end
 end

fn print_backwards (dl : dl_t) =

 let
   val gen = make_generator (dl, ~1)
   var xopt : Option int
   var separator : string = ""
 in
   for (xopt := gen (); option_is_some xopt; xopt := gen ())
     case+ xopt of
     | Some x =>
       begin
         print! separator;
         print! x;
         separator := " "
       end
 end

implement main0 () =

 {
   val dl = list2dl ($list{int} (10, 20, 30, 40, 50))
   val _ = print! ("doubly linked list: ")
   val _ = print_forwards dl
   val _ = println! ()

   val _ = print! ("conversion to a regular list: ")
   val _ = println! (dl2list dl)

   val _ = print! ("traversal backwards: ")
   val _ = print_backwards dl
   val _ = println! ()

   val _ = print! ("traversal forwards, given a non-root node: ")
   val _ = print_forwards (get_prev (get_prev dl))
   val _ = println! ()

   val _ = print! ("traversal backwards, given a non-root node: ")
   val _ = print_backwards (get_prev (get_prev dl))
   val _ = println! ()

   val _ = print! ("insertion after the root: ")
   val _ = insert_after (dl, 5)
   val _ = print_forwards dl
   val _ = println! ()

   val _ = print! ("insertion before the root: ")
   val _ = insert_before (dl, 55)
   val _ = print_forwards dl
   val _ = println! ()

   val _ = print! ("insertion after the second element: ")
   val _ = insert_after (get_next (get_next dl), 15)
   val _ = print_forwards dl
   val _ = println! ()

   val _ = print! ("insertion before the second from last element: ")
   val _ = insert_before (get_prev (get_prev dl), 45)
   val _ = print_forwards dl
   val _ = println! ()

   val _ = print! ("removal of the element 30: ")
   val _ =
     let
       var p : dl_t
     in
       for (p := get_next dl; get_element p <> 30; p := get_next p)
         ();
       remove p
     end
   val _ = print_forwards dl
   val _ = println! ()
 }</lang>

AutoHotkey

see Doubly-linked list/AutoHotkey

C

<lang c>/* double linked list */

1. include <stdio.h>
2. include <stdlib.h>

struct List {

  struct MNode *head;
  struct MNode *tail;
  struct MNode *tail_pred;

};

struct MNode {

  struct MNode *succ;
  struct MNode *pred;

};

typedef struct MNode *NODE; typedef struct List *LIST;

/*

• • LIST l = newList()
  • create (alloc space for) and initialize a list
• /

LIST newList(void);

/*

• • int isEmpty(LIST l)
  • test if a list is empty
• /

int isEmpty(LIST);

/*

• • NODE n = getTail(LIST l)
  • get the tail node of the list, without removing it
• /

NODE getTail(LIST);

/*

• • NODE n = getHead(LIST l)
  • get the head node of the list, without removing it
• /

NODE getHead(LIST);

/*

• • NODE rn = addTail(LIST l, NODE n)
  • add the node n to the tail of the list l, and return it (rn==n)
• /

NODE addTail(LIST, NODE);

/*

• • NODE rn = addHead(LIST l, NODE n)
  • add the node n to the head of the list l, and return it (rn==n)
• /

NODE addHead(LIST, NODE);

/*

• • NODE n = remHead(LIST l)
  • remove the head node of the list and return it
• /

NODE remHead(LIST);

/*

• • NODE n = remTail(LIST l)
  • remove the tail node of the list and return it
• /

NODE remTail(LIST);

/*

• • NODE rn = insertAfter(LIST l, NODE r, NODE n)
  • insert the node n after the node r, in the list l; return n (rn==n)
• /

NODE insertAfter(LIST, NODE, NODE);

/*

• • NODE rn = removeNode(LIST l, NODE n)
  • remove the node n (that must be in the list l) from the list and return it (rn==n)
• /

NODE removeNode(LIST, NODE);

LIST newList(void) {

   LIST tl = malloc(sizeof(struct List));
   if ( tl != NULL )
   {
      tl->tail_pred = (NODE)&tl->head;
      tl->tail = NULL;
      tl->head = (NODE)&tl->tail;
      return tl;
   }
   return NULL;

}

int isEmpty(LIST l) {

  return (l->head->succ == 0);

}

NODE getHead(LIST l) {

 return l->head;

}

NODE getTail(LIST l) {

 return l->tail_pred;

}

NODE addTail(LIST l, NODE n) {

   n->succ = (NODE)&l->tail;
   n->pred = l->tail_pred;
   l->tail_pred->succ = n;
   l->tail_pred = n;
   return n;

}

NODE addHead(LIST l, NODE n) {

   n->succ = l->head;
   n->pred = (NODE)&l->head;
   l->head->pred = n;
   l->head = n;
   return n;

}

NODE remHead(LIST l) {

  NODE h;
  h = l->head;
  l->head = l->head->succ;
  l->head->pred = (NODE)&l->head;
  return h;

}

NODE remTail(LIST l) {

  NODE t;
  t = l->tail_pred;
  l->tail_pred = l->tail_pred->pred;
  l->tail_pred->succ = (NODE)&l->tail;
  return t;

}

NODE insertAfter(LIST l, NODE r, NODE n) {

  n->pred = r; n->succ = r->succ;
  n->succ->pred = n; r->succ = n;
  return n;

}

NODE removeNode(LIST l, NODE n) {

  n->pred->succ = n->succ;
  n->succ->pred = n->pred;
  return n;

}</lang>

Simple test:

<lang c>/* basic test */

struct IntNode {

 struct MNode node;
 int data;

};

int main() {

   int i;
   LIST lista;
   struct IntNode *m;
   NODE n;
   
   lista = newList();
   if ( lista != NULL )
   {
     for(i=0; i < 5; i++)
     {
         m = malloc(sizeof(struct IntNode));
         if ( m != NULL )
         {
            m->data = rand()%64;
            addTail(lista, (NODE)m);
         }
     }
     while( !isEmpty(lista) )
     {
           m = (struct IntNode *)remTail(lista);
           printf("%d\n", m->data);
           free(m);
     }
     free(lista);
   }

}</lang>

C#

The .NET framework provides the LinkedListNode<T> class, which represents an individual node of a linked list, and the LinkedList<T> class, which provides abstractions to read and modify a list as if it were a single object. The LinkedListNode<T>.Next and LinkedListNode<T>.Previous properties is read-only, ensuring that all lists must be created using LinkedList<T> and that each list can only be mutated using the methods of its LinkedList<T> instance (the appropriate .NET accessibility modifiers are used to hide these implementation details).

One node instance is forbidden to be in multiple lists; this is enforced using the LinkedListNode<T>.List property, which is set accordingly when a node is added to a LinkedList<T> and set to null when it is removed. This also has the effect of preventing cycles.

Though mutating the structure of a list can only be done through the LinkedList<T> class, mutating the values contained by the nodes of a list is done through its individual LinkedListNode<T> instances, as the LinkedListNode<T>.Next.Value property is settable.

<lang C sharp>using System.Collections.Generic;

class Program {

   static void Main(string[] args)
   {
       LinkedList<string> list = new LinkedList<string>();
       list.AddFirst(".AddFirst() adds at the head.");
       list.AddLast(".AddLast() adds at the tail.");
       LinkedListNode<string> head = list.Find(".AddFirst() adds at the head.");
       list.AddAfter(head, ".AddAfter() adds after a specified node.");
       LinkedListNode<string> tail = list.Find(".AddLast() adds at the tail.");
       list.AddBefore(tail, "Betcha can't guess what .AddBefore() does.");

       System.Console.WriteLine("Forward:");
       foreach (string nodeValue in list) { System.Console.WriteLine(nodeValue); }

       System.Console.WriteLine("\nBackward:");
       LinkedListNode<string> current = tail;
       while (current != null)
       {
           System.Console.WriteLine(current.Value);
           current = current.Previous;
       }
   }

}</lang>

Output:

Forward:
.AddFirst() adds at the head.
.AddAfter() adds after a specified node.
Betcha can't guess what .AddBefore() does.
.AddLast() adds at the tail.

Backward:
.AddLast() adds at the tail.
Betcha can't guess what .AddBefore() does.
.AddAfter() adds after a specified node.
.AddFirst() adds at the head.

C++

Works with: C++11

<lang cpp>#include <iostream>

1. include <list>

int main () {

   std::list<int> numbers {1, 5, 7, 0, 3, 2};
   numbers.insert(numbers.begin(), 9); //Insert at the beginning
   numbers.insert(numbers.end(), 4); //Insert at the end
   auto it = std::next(numbers.begin(), numbers.size() / 2); //Iterator to the middle of the list
   numbers.insert(it, 6); //Insert in the middle
   for(const auto& i: numbers)
       std::cout << i << ' ';
   std::cout << '\n';

}</lang>

Output:

9 1 5 7 6 0 3 2 4 

Clojure

<lang Clojure>(ns double-list)

(defprotocol PDoubleList

 (get-head [this])
 (add-head [this x])
 (get-tail [this])
 (add-tail [this x])
 (remove-node [this node])
 (add-before [this node x])
 (add-after [this node x])
 (get-nth [this n]))

(defrecord Node [prev next data])

(defn make-node

 "Create an internal or finalized node"
 ([prev next data] (Node. prev next data))
 ([m key] (when-let [node (get m key)]
           (assoc node :m m :key key))))

(defn get-next [node] (make-node (:m node) (:next node))) (defn get-prev [node] (make-node (:m node) (:prev node)))

(defn- seq* [m start next]

 (seq
  (for [x (iterate #(get m (next %)) (get m start))
        :while x]
    (:data x))))

(defmacro when->

 ([x pred form] `(let [x# ~x] (if ~pred (-> x# ~form) x#)))
 ([x pred form & more] `(when-> (when-> ~x ~pred ~form) ~@more)))

(declare get-nth-key)

(deftype DoubleList [m head tail]

 Object
   (equals [this x]
     (and (instance? DoubleList x)
          (= m (.m ^DoubleList x))))
   (hashCode [this] (hash (or this ())))
 clojure.lang.Sequential
 clojure.lang.Counted
   (count [_] (count m))
 clojure.lang.Seqable
   (seq [_] (seq* m head :next))
 clojure.lang.Reversible
   (rseq [_] (seq* m tail :prev))
 clojure.lang.IPersistentCollection
   (empty [_] (DoubleList. (empty m) nil nil))
   (equiv [this x]
     (and (sequential? x)
          (= (seq x) (seq this))))
   (cons [this x] (.add-tail this x))
 PDoubleList
   (get-head [_] (make-node m head))
   (add-head [this x]
     (let [new-key (Object.)
           m (when-> (assoc m new-key (make-node nil head x))
               head (assoc-in [head :prev] new-key))
           tail (if tail tail new-key)]
       (DoubleList. m new-key tail)))
   (get-tail [_] (make-node m tail))
   (add-tail [this x]
     (if-let [tail (.get-tail this)]
       (.add-after this tail x)
       (.add-head this x)))
   (remove-node [this node]
     (if (get m (:key node))
       (let [{:keys [prev next key]} node
             head (if prev head next)
             tail (if next tail prev)
             m (when-> (dissoc m key)
                 prev (assoc-in [prev :next] next)
                 next (assoc-in [next :prev] prev))]
         (DoubleList. m head tail))
       this))
   (add-after [this node x]
     (if (get m (:key node))
       (let [{:keys [prev next key]} node
             new-key (Object.)
             m (when-> (-> (assoc m new-key  (make-node key next x))
                           (assoc-in , [key :next] new-key))
                 next (assoc-in [next :prev] new-key))
             tail (if next tail new-key)]
         (DoubleList. m head tail))
       this))
   (add-before [this node x]
     (if (:prev node)
       (.add-after this (get-prev node) x)
       (.add-head this x)))
   (get-nth [this n] (make-node m (get-nth-key this n))))

(defn get-nth-key [^DoubleList this n]

 (if (< -1 n (.count this))
   (let [[start next n] (if (< n (/ (.count this) 2))
                          [(.head this) :next n]
                          [(.tail this) :prev (- (.count this) n 1)])]
     (nth (iterate #(get-in (.m this) [% next]) start) n))
   (throw (IndexOutOfBoundsException.))))

(defn double-list

 ([] (DoubleList. nil nil nil))
 ([coll] (into (double-list) coll)))

(defmethod print-method DoubleList [dl w]

 (print-method (interpose '<-> (seq dl)) w))

(defmethod print-method Node [n w]

 (print-method (symbol "#:double_list.Node") w)
 (print-method (into {} (dissoc n :m)) w))</lang>

Usage: <lang Clojure>(use 'double-list)

=> nil

(def dl (double-list (range 10)))

=> #'user/dl

dl

=> (0 <-> 1 <-> 2 <-> 3 <-> 4 <-> 5 <-> 6 <-> 7 <-> 8 <-> 9)

(remove-node dl (get-tail dl))

=> (0 <-> 1 <-> 2 <-> 3 <-> 4 <-> 5 <-> 6 <-> 7 <-> 8)

dl

=> (0 <-> 1 <-> 2 <-> 3 <-> 4 <-> 5 <-> 6 <-> 7 <-> 8 <-> 9)

((juxt seq rseq) dl)

=> [(0 1 2 3 4 5 6 7 8 9) (9 8 7 6 5 4 3 2 1 0)]

(remove-node dl (get-nth dl 5))

=> (0 <-> 1 <-> 2 <-> 3 <-> 4 <-> 6 <-> 7 <-> 8 <-> 9)

(add-after *1 (get-nth *1 4) 10)

=> (0 <-> 1 <-> 2 <-> 3 <-> 4 <-> 10 <-> 6 <-> 7 <-> 8 <-> 9)

(get-head *1)

=> #

double_list.Node{:prev nil, :next #<Object ...>, :data 0, :key <Object ...>}

(get-next *1)

=> #

double_list.Node{:prev #<Object ...>, :next #<Object ...>, :data 1, :key #<Object ...>}

(get-prev *1)

=> #

double_list.Node{:prev #<Object ...>, :next #<Object ...>, :data 1, :key #<Object ...>}</lang>

Common Lisp

<lang lisp>(defstruct dlist head tail) (defstruct dlink content prev next)

(defun insert-between (dlist before after data)

 "Insert a fresh link containing DATA after existing link BEFORE if not nil and before existing link AFTER if not nil"
 (let ((new-link (make-dlink :content data :prev before :next after)))
   (if (null before)
       (setf (dlist-head dlist) new-link)
       (setf (dlink-next before) new-link))
   (if (null after)
       (setf (dlist-tail dlist) new-link)
       (setf (dlink-prev after) new-link))
   new-link))

(defun insert-before (dlist dlink data)

 "Insert a fresh link containing DATA before existing link DLINK"
 (insert-between dlist (dlink-prev dlink) dlink data))

(defun insert-after (dlist dlink data)

 "Insert a fresh link containing DATA after existing link DLINK"
 (insert-between dlist dlink (dlink-next dlink) data))

(defun insert-head (dlist data)

 "Insert a fresh link containing DATA at the head of DLIST"
 (insert-between dlist nil (dlist-head dlist) data))

(defun insert-tail (dlist data)

 "Insert a fresh link containing DATA at the tail of DLIST"
 (insert-between dlist (dlist-tail dlist) nil data))

(defun remove-link (dlist dlink)

 "Remove link DLINK from DLIST and return its content"
 (let ((before (dlink-prev dlink))
       (after (dlink-next dlink)))
   (if (null before)
       (setf (dlist-head dlist) after)
       (setf (dlink-next before) after))
   (if (null after)
       (setf (dlist-tail dlist) before)
       (setf (dlink-prev after) before))))

(defun dlist-elements (dlist)

 "Returns the elements of DLIST as a list"
 (labels ((extract-values (dlink acc)
            (if (null dlink)
                acc
                (extract-values (dlink-next dlink) (cons (dlink-content dlink) acc)))))
   (reverse (extract-values (dlist-head dlist) nil))))</lang>

The following produces (1 2 3 4).

<lang lisp>(let ((dlist (make-dlist)))

 (insert-head dlist 1)
 (insert-tail dlist 4)
 (insert-after dlist (dlist-head dlist) 2)
 (let* ((next-to-last (insert-before dlist (dlist-tail dlist) 3))
        (bad-link (insert-before dlist next-to-last 42)))
   (remove-link dlist bad-link))
 (print (dlist-elements dlist)))</lang>

D

<lang d>import std.stdio;

class LinkedList(T) {

Node!(T) head, tail;

/** Iterate in the forward direction. */
int opApply (int delegate(uint, Node!(T)) dg)
{
 uint i = 0;
 auto link = head;
 int result = 0;
 while (link)
 {
  result = dg (i, link);
  if (result) return result;
  i++;
  link = link.next;
 }
 return result;
}

static LinkedList!(T) fromArray (T[] array)
{
 Node!(T) link = null;
 auto head = link;
 auto self = new LinkedList!(T);
 foreach (elem; array)
 {
  link = new Node!(T)(null, link, elem, self);
  if (!head)
   head = link;
 }
 return self;
}

}

class Node(T) {

Node!(T) next;
Node!(T) previous;
LinkedList!(T) parent;
T value;

this (Node!(T) next, Node!(T) previous, T value, LinkedList!(T) parent)
in
{
 assert (parent !is null);
}
body
{
 this.next = next;
 if (next)
  next.previous = this;
 if (previous)
  previous.next = this;
 this.previous = previous;
 this.value = value;
 this.parent = parent;

 if (parent.head == next)
  parent.head = this;
 if (parent.tail == previous)
  parent.tail = this;
}

/** Insert an element after this one. */
void insertAfter (T value)
{
 new Node!(T)(next, this, value, parent);
}

/** Insert an element before this one. */
void insertBefore (T value)
{
 new Node!(T)(this, previous, value, parent);
}

/** Remove the current node from the list. */
void remove ()
{
 if (next)
  next.previous = previous;
 if (previous)
  previous.next = next;
 if (parent.tail == this)
  parent.tail = previous;
 if (parent.head == this)
  parent.head = next;
}

}

void main () {

string[] sample = ["was", "it", "a", "cat", "I", "saw"];
auto list = LinkedList!string.fromArray (sample);
for (auto elem = list.head; elem; elem = elem.next)
{
 writef ("%s ", elem.value);
 if (elem.value == "it") elem.insertAfter("really");
}
writeln;
for (auto elem = list.tail; elem; elem = elem.previous)
{
 writef ("%s ", elem.value);
}
writeln;

}</lang>

Output:

Iterate forward: Was it really a cat I saw 
Iterate backward: saw I cat a really it Was 
Empty from tail: saw I cat a really it Was 

Delphi

Library: System.SysUtils

Library: boost.LinkedList

[[1]]

Translation of: C#

<lang Delphi> program Doubly_linked;

{$APPTYPE CONSOLE}

uses

 System.SysUtils,
 boost.LinkedList;

var

 List: TLinkedList<string>;
 Head, Tail,Current: TLinkedListNode<string>;
 Value:string;

begin

 List := TLinkedList<string>.Create;

 List.AddFirst('.AddFirst() adds at the head.');
 List.AddLast('.AddLast() adds at the tail.');
 Head := List.Find('.AddFirst() adds at the head.');
 List.AddAfter(Head, '.AddAfter() adds after a specified node.');
 Tail := List.Find('.AddLast() adds at the tail.');
 List.AddBefore(Tail, 'Betcha cant guess what .AddBefore() does.');

 Writeln('Forward:');
 for value in List do
   Writeln(value);

 Writeln(#10'Backward:');

 Current:= Tail;
 while Assigned(Current) do
 begin
   Writeln(Current.Value);
   Current:= Current.Prev;
 end;

 List.Free;
 Readln;

end.</lang>

E

<lang e>def makeDLList() {

   def firstINode
   def lastINode
   
   def makeNode(var value, var prevI, var nextI) {
       # To meet the requirement that the client cannot create a loop, the 
       # inter-node refs are protected: clients only get the external facet 
       # with invariant-preserving operations.
       def iNode
       
       def node { # external facet
           
           to get() { return value }
           to put(new) { value := new }
           
           /** Return the value of the element of the list at the specified offset
               from this element. */
           to get(index :int) {
               if (index > 0 && node.hasNext()) {
                   return nextI.node().get(index - 1)
               } else if (index < 0 && node.hasPrev()) {
                   return prevI.node().get(index + 1)
               } else if (index <=> 0) {
                   return value
               } else {
                   throw("index out of range in dlList")
               }
           }
           to hasPrev() {
               return prevI != firstINode && prevI != null
           }
           to prev() {
               if (!node.hasPrev()) {
                   throw("there is no previous node")
               }
               return prevI.node()
           }
           to hasNext() {
               return nextI != lastINode && nextI != null
           }
           to next() {
               if (!node.hasNext()) {
                   throw("there is no next node")
               }
               return nextI.node()
           }
           to remove() {
               if (prevI == null || nextI == null) { return }
               prevI.setNextI(nextI)
               nextI.setPrevI(prevI)
               prevI := null
               nextI := null
           }
           to insertAfter(newValue) {
               def newI := makeNode(newValue, iNode, nextI)
               nextI.setPrevI(newI)
               nextI := newI
           }
           to insertBefore(newValue) {
               prevI.node().insertAfter(newValue)
           }
       }
       
       bind iNode { # internal facet
           to node() { return node }
           to nextI() { return nextI }
           to prevI() { return prevI }
           to setNextI(new) { nextI := new }
           to setPrevI(new) { prevI := new }
       }
       
       return iNode
   } # end makeNode

   bind firstINode := makeNode(null, Ref.broken("no first prev"), lastINode)
   bind lastINode := makeNode(null, firstINode, Ref.broken("no last next"))

   def dlList {
       to __printOn(out) {
           out.print("<")
           var sep := ""
           for x in dlList {
               out.print(sep)
               out.quote(x)
               sep := ", "
           }
           out.print(">")
       }
       to iterate(f) {
           var n := firstINode
           while (n.node().hasNext()) {
               n := n.nextI()
               f(n.node(), n.node()[])
           }
       }
       to atFirst() { return firstINode.nextI().node() }
       to atLast() { return lastINode.prevI().node() }
       to insertFirst(new) { return firstINode.node().insertAfter(new) }
       to push(new) { return lastINode.node().insertBefore(new) }
       
       /** Return the node which has the specified value */
       to nodeOf(value) {
           for node => v ? (v == value) in dlList { return node }
       }
   }
   return dlList

}</lang>

<lang e>? def list := makeDLList()

1. value: <>

? list.push(1) ? list

1. value: <1>

? list.push(10) ? list.push(100) ? list

1. value: <1, 10, 100>

? list.atFirst().insertAfter(5) ? list

1. value: <1, 5, 10, 100>

? list.insertFirst(0) ? list

1. value: <0, 1, 5, 10, 100>

? list.atLast().prev().remove() ? list

1. value: <0, 1, 5, 100>

? list.atLast()[] := 10 ? list

1. value: <0, 1, 5, 10>

? for x in 11..20 { list.push(x) } ? list

1. value: <0, 1, 5, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20></lang>

Erlang

As with Singly-linked_list/Element_insertion a process is used to get mutability in Erlang's single assignment world. <lang Erlang> -module( doubly_linked_list ).

-export( [append/2, foreach_next/2, foreach_previous/2, free/1, insert/3, new/1, task/0] ).

append( New, Start ) -> Start ! {append, New}.

foreach_next( Fun, Start ) -> Start ! {foreach_next, Fun}.

foreach_previous( Fun, Start ) -> Start ! {foreach_previous, Fun}.

free( Element ) -> Element ! {free}.

insert( New, After, Start ) -> Start ! {insert, New, After}.

new( Data ) -> erlang:spawn( fun() -> loop( Data, noprevious, nonext ) end ).

task() ->

   A = new( a ),
   B = new( b ),
   append( B, A ),
   C = new( c ),
   insert( C, A, A ),
   foreach_next( fun(Data) -> io:fwrite("foreach_next ~p~n", [Data]) end, A ),
   timer:sleep( 100 ),
   foreach_previous( fun(Data) -> io:fwrite("foreach_previous ~p~n", [Data]) end, B ).

loop( Data, Previous, Next ) ->

     My_pid = erlang:self(),
     receive
     {append, New} ->
            New_next = loop_append( New, Next, My_pid ),
            loop( Data, Previous, New_next );
     {foreach_next, Fun} ->
               catch Fun( Data ),
               loop_foreach_next( Fun, Next ),
               loop( Data, Previous, Next );
     {foreach_previous, Fun} ->
               catch Fun( Data ),
               loop_foreach_previous( Fun, Previous ),
               loop( Data, Previous, Next );
     {free} ->
            ok;
     {insert, New, My_pid} ->

New ! {previous, My_pid},

            loop_append( Next, New, My_pid ),
            loop( Data, Previous, New );
     {insert, New, After} ->
            Next ! {insert, New, After},
            loop( Data, Previous, Next );
     {previous, New_previous} ->
            loop( Data, New_previous, Next )
       end.

loop_append( New, nonext, My_pid ) ->

       New ! {previous, My_pid},
       New;

loop_append( New, Next, _My_pid ) ->

       Next ! {append, New},
       Next.

loop_foreach_next( _Fun, nonext ) -> ok; loop_foreach_next( Fun, Next ) -> Next ! {foreach_next, Fun}.

loop_foreach_previous( _Fun, noprevious ) -> ok; loop_foreach_previous( Fun, Next ) -> Next ! {foreach_previous, Fun}. </lang>

F#

<lang fsharp>type DListAux<'T> = {mutable prev: DListAux<'T> option; data: 'T; mutable next: DListAux<'T> option} type DList<'T> = {mutable front: DListAux<'T> option; mutable back: DListAux<'T> option} //'

let empty() = {front=None; back=None}

let addFront dlist elt =

 match dlist.front with
 | None ->
     let e = Some {prev=None; data=elt; next=None}
     dlist.front <- e
     dlist.back <- e
 | Some e2 ->
     let e1 = Some {prev=None; data=elt; next=Some e2}
     e2.prev <- e1
     dlist.front <- e1

let addBack dlist elt =

 match dlist.back with
 | None -> addFront dlist elt
 | Some e2 ->
     let e1 = Some {prev=Some e2; data=elt; next=None}
     e2.next <- e1
     dlist.back <- e1

let addAfter dlist link elt =

 if link.next = dlist.back then addBack dlist elt else
   let e = Some {prev=Some link; data=elt; next=link.next}
   link.next <- e</lang>

Fortran

Tested with g95 and gfortran v. 4.6. <lang fortran> module dlist

 implicit none
 type node
    type(node), pointer :: next => null()
    type(node), pointer :: prev => null()
    integer :: data
 end type node

 type dll
    type(node), pointer :: head => null()
    type(node), pointer :: tail => null()
    integer :: num_nodes = 0
 end type dll

 public  :: node, dll, append, prepend, insert, dump, reverse_dump, tidy
 private :: init

contains

 ! Create a new doubly-linked list
 elemental type(dll) function new_dll()
   new_dll = dll(null(),null(),0)
   return
 end function new_dll

 ! Append an element to the end of the list
 elemental subroutine append(dl2, value)
   type(dll), intent(inout) :: dl2
   integer, intent(in)      :: value

   type(node), pointer :: np

   ! If the list is empty
   if (dl2%num_nodes == 0) then
      call init(dl2, value)
      return
   end if

   ! Add new element to the end
   dl2%num_nodes = dl2%num_nodes + 1
   np => dl2%tail
   allocate(dl2%tail)
   dl2%tail%data = value
   dl2%tail%prev => np
   dl2%tail%prev%next => dl2%tail
 end subroutine append

 ! Prepend an element to the beginning of the list
 elemental subroutine prepend(dl2, value)
   type(dll), intent(inout) :: dl2
   integer, intent(in)      :: value

   type(node), pointer :: np

   if (dl2%num_nodes == 0) then
      call init(dl2, value)
      return
   end if

   dl2%num_nodes = dl2%num_nodes + 1
   np => dl2%head
   allocate(dl2%head)
   dl2%head%data = value
   dl2%head%next => np
   dl2%head%next%prev => dl2%head
 end subroutine prepend

 ! Insert immediately before the given index
 elemental subroutine insert(dl2, index, value)
   type(dll), intent(inout) :: dl2
   integer, intent(in)      :: index
   integer, intent(in)      :: value

   type(node), pointer :: element
   type(node), pointer :: np1, np2
   integer             :: i

   if (dl2%num_nodes == 0) then
      call init(dl2, value)
      return
   end if

   ! If index is beyond the end then append
   if (index > dl2%num_nodes) then
      call append(dl2, value)
      return
   end if

   ! If index is less than 1 then prepend
   if (index <= 1) then
      call prepend(dl2, value)
      return
   end if

   ! Find the node at position 'index' counting from 1
   np1 => dl2%head
   do i=1, index-2
      np1 => np1%next
   end do
   np2 => np1%next

   ! Create the new node
   allocate(element)
   element%data = value

   ! Connect it up
   element%prev => np1
   element%next => np2
   np1%next => element
   np2%prev => element
   dl2%num_nodes = dl2%num_nodes + 1
 end subroutine insert

 subroutine dump(dl2)
   type(dll), intent(in) :: dl2
   type(node), pointer :: current
   integer :: i

   write(*,fmt='(a,i0,a)',advance='no') 'Doubly-linked list has ',dl2%num_nodes,' element - fwd = '
   current => dl2%head
   i = 1
   write(*,fmt='(i0,a)',advance='no') current%data,', '
   do
      current => current%next
      if (.not. associated(current)) then
         exit
      end if
      i = i + 1
      if (i == dl2%num_nodes) then
         write(*,'(i0)') current%data
      else
         write(*,fmt='(i0,a)',advance='no') current%data,', '
      end if
   end do
 end subroutine dump

 subroutine reverse_dump(dl2)
   type(dll), intent(in) :: dl2
   type(node), pointer :: current
   integer :: i

   write(*,fmt='(a,i0,a)',advance='no') 'Doubly-linked list has ',dl2%num_nodes,' element - bwd = '
   current => dl2%tail
   write(*,fmt='(i0,a)',advance='no') current%data,', '
   i = 1
   do
      current => current%prev
      if (.not. associated(current)) then
         exit
      end if
      i = i + 1
      if (i == dl2%num_nodes) then
         write(*,'(i0)') current%data
      else
         write(*,fmt='(i0,a)',advance='no') current%data,', '
      end if
   end do
 end subroutine reverse_dump

 ! Deallocate all allocated memory
 elemental subroutine tidy(dl2)
   type(dll), intent(inout) :: dl2
   type(node), pointer :: current, last

   current => dl2%head
   do
      last => current
      current => current%next
      if (associated(last)) then
         deallocate(last)
      end if
      if (associated(current, dl2%tail)) then
         deallocate(current)
         exit
      end if
   end do
 end subroutine tidy

 elemental subroutine init(dl2, value)
   type(dll), intent(inout) :: dl2
   integer, intent(in)      :: value
   allocate(dl2%head)
   dl2%tail => dl2%head
   dl2%tail%data = value
   dl2%num_nodes = 1
   return
 end subroutine init

end module dlist

program dl

 use dlist
 implicit none

 type(dll) :: mydll

 mydll = new_dll()
 call append(mydll, 5)
 call append(mydll, 7)
 call prepend(mydll, 3)
 call prepend(mydll, 1)
 call insert(mydll, 3, 4)
 call dump(mydll)

 call reverse_dump(mydll)

 call tidy(mydll)

end program dl </lang>

Output:

Doubly-linked list has 5 element - fwd = 1, 3, 4, 5, 7
Doubly-linked list has 5 element - bwd = 7, 5, 4, 3, 1

Go

Go has nothing like an enforced invariant. Responsibility for preventing circular loops must be shared by all code that modifies the list. Given that, the following declaration enables code to do that efficiently. <lang go>type dlNode struct {

   int
   next, prev *dlNode

}

// Field 'members' allows loops to be prevented. All nodes // inserted should be added to members. Code that operates // on the list can check any pointer against members to // find out if the pointer is already in the list. type dlList struct {

   members map[*dlNode]int
   head, tail **dlNode

}</lang> Or, just use the container/list package: <lang go>package main

import "fmt" import "container/list"

func main() {

       // Create a new list and put some values in it.
       l := list.New()
       e4 := l.PushBack(4)
       e1 := l.PushFront(1)
       l.InsertBefore(3, e4)
       l.InsertAfter("two", e1)
       
       // Iterate through list and print its contents.
       for e := l.Front(); e != nil; e = e.Next() {
           fmt.Println(e.Value)
       }

}</lang>

Haskell

For an efficient implementation, see the Data.FDList module provided by liboleg. But before using doubly linked lists at all, see this discussion on Stack Overflow.

<lang haskell>import qualified Data.Map as M

type NodeID = Maybe Rational data Node a = Node

  {vNode :: a,
   pNode, nNode :: NodeID}

type DLList a = M.Map Rational (Node a)

empty = M.empty

singleton a = M.singleton 0 $ Node a Nothing Nothing

fcons :: a -> DLList a -> DLList a fcons a list | M.null list = singleton a

            | otherwise   = M.insert newid new $
                            M.insert firstid changed list
 where (firstid, Node firstval _ secondid) = M.findMin list
       newid = firstid - 1
       new     = Node a        Nothing      (Just firstid)
       changed = Node firstval (Just newid) secondid

rcons :: a -> DLList a -> DLList a rcons a list | M.null list = singleton a

            | otherwise   = M.insert lastid changed $
                            M.insert newid new list
 where (lastid, Node lastval penultimateid _) = M.findMax list
       newid = lastid + 1
       changed = Node lastval penultimateid (Just newid)
       new     = Node a       (Just lastid) Nothing

mcons :: a -> Node a -> Node a -> DLList a -> DLList a mcons a n1 n2 = M.insert n1id left .

   M.insert midid mid . M.insert n2id right
 where Node n1val farleftid   (Just n2id) = n1
       Node n2val (Just n1id) farrightid  = n2
       midid = (n1id + n2id) / 2   -- Hence the use of Rationals.
       mid = Node a (Just n1id) (Just n2id)
       left  = Node n1val farleftid    (Just midid)
       right = Node n2val (Just midid) farrightid

firstNode :: DLList a -> Node a firstNode = snd . M.findMin

lastNode :: DLList a -> Node a lastNode = snd . M.findMax

nextNode :: DLList a -> Node a -> Maybe (Node a) nextNode l n = nNode n >>= flip M.lookup l

prevNode :: DLList a -> Node a -> Maybe (Node a) prevNode l n = pNode n >>= flip M.lookup l

fromList = foldr fcons empty

toList = map vNode . M.elems</lang>

An example of use:

<lang haskell>main = putStrLn $ toList l

 where l = mcons 'M' n1 n2 x
       x = rcons 'Z' $ fcons 'a' $ fcons 'q' $ singleton 'w'
       n1 = firstNode x
       Just n2 = nextNode x n1</lang>

Icon and Unicon

Uses Unicon's classes.

The DoubleList is made from elements of DoubleLink. Doubly-Linked List (element)#Icon_and_Unicon, Doubly-Linked List (element insertion)#Icon_and_Unicon and Doubly-Linked List (traversal)#Icon_and_Unicon

<lang Unicon> class DoubleList (item)

 method head ()
   node := item
   every (node := node.traverse_backwards ()) # move to start of list
   return node
 end

 method tail ()
   node := item
   every (node := node.traverse_forwards ()) # move to end of list
   return node
 end
 
 method insert_at_head (value)
   head().insert_before (DoubleLink(value))
 end

 method insert_at_tail (value)
   tail().insert_after (DoubleLink (value))
 end  

 # insert a node for new_value after that for target_value, 
 # i.e. in the middle of the list
 method insert_after (target_value, new_value)
   node := head ()
   every node := head().traverse_forwards () do 
     if (node.value = target_value) 
       then { 
         node.insert_after (DoubleLink (new_value))
        break 
       }
 end

 # constructor initiates a list making a node from given value
 initially (value)
   self.item := DoubleLink (value)

end </lang>

An insert_before method was added to the DoubleLink class:

<lang Unicon>

 # insert given node before this one, losing its existing connections
 method insert_before (node)
   if (\prev_link) then prev_link.next_link := node
   node.prev_link := prev_link
   node.next_link := self
   self.prev_link := node
 end

</lang>

To test the double-linked list:

<lang Unicon> procedure main ()

 dlist := DoubleList (5)
 every i := 4 to 1 by -1 do 
   dlist.insert_at_head (i)
 every i := 6 to 10 do
   dlist.insert_at_tail (i)

 dlist.insert_after (3, 11)

 every node := dlist.head().traverse_forwards () do
   write (node.value)

end </lang>

Output:

1
2
3
11
4
5
6
7
8
9
10

J

Doubly linked lists are antithetical to J.

First, J already has a built in list data type which is heavily optimized, and micromanaging issues like list traversal bypasses all of that design and architecture.

Second, an implementation of "doubly linked" conflicts with the "once and only once" character of many good implementations. In a doubly linked list order must be specified redundantly and that redundancy creates maintenance costs which are justified only in rare cases.

So, first, here is a native J list:

  list=: 2 3 5 7 11

To implement a doubly linked list, one could create a list of successor indices and another list of predecessor indices.

First, let us define a different order for our list element, so we can easily show that our doubly linked list is logically distinct from the built in list. If we use "alphabeted order by names of numbers" we would have the list 11 5 7 3 2

  data=:11 5 7 3 2

3 is followed by 2
5 is followed by 7
7 is followed by 3
11 is followed by 5

and

2 is preceded by 3
3 is preceded by 7
5 is preceded by 11
7 is preceded by 5

To represent this in J, we can define additional lists with the successor index and predecessor index for each node:

  successors=:   _ 0 3 1 2
  predecessors=: 1 3 4 2 __

Note that the successor for the end of the list is _ and the successor for the beginning of the list is __

To check for loops, look for repeated indices in either of these ordering lists. To add an element to the doubly linked list, you would add an element to the data list, and then update the successor and predecessor list by appending to the end the index of the item designated as the successor/predecessor of the new item and replacing the previous holder of that value with the newly valid index.

Finally, note that we can remove elements from the doubly linked list without removing them from the data list. We might wish to chain removed elements together to facilitate re-use of their positions. If we want to do this, we will need a place to start:

  garbage=: __

When we delete an item we place the old garbage value as its successor index and we define the garbage variable to be the index we just deleted. And when adding to the list we first check if garbage has a valid index and if so we take over that position in the structure and update garbage with the previous value of the successor.

Needless to say, this approach is expensive and inefficient. (But, granted, there will be cases where the cost is worth the expense.)

That said, note also that while the native J lists do not support cycles or loops, this high-cost substitute is general enough to support them.

Java

The LinkedList<T> class is the Doubly-linked list implementation in Java. There are a large number of methods supporting the list. An example is shown below.

<lang java> import java.util.LinkedList;

public class DoublyLinkedList {

   public static void main(String[] args) {
       LinkedList<String> list = new LinkedList<String>();
       list.addFirst("Add First");
       list.addLast("Add Last 1");
       list.addLast("Add Last 2");
       list.addLast("Add Last 1");
       traverseList(list);
       
       list.removeFirstOccurrence("Add Last 1");
       traverseList(list);
   }
   
   private static void traverseList(LinkedList<String> list) {
       System.out.println("Traverse List:");
       for ( int i = 0 ; i < list.size() ; i++ ) {
           System.out.printf("Element number %d - Element value = '%s'%n", i, list.get(i));
       }
       System.out.println();
   }
   

} </lang>

Output:

Traverse List:
Element number 0 - Element value = 'Add First'
Element number 1 - Element value = 'Add Last 1'
Element number 2 - Element value = 'Add Last 2'
Element number 3 - Element value = 'Add Last 1'

Traverse List:
Element number 0 - Element value = 'Add First'
Element number 1 - Element value = 'Add Last 2'
Element number 2 - Element value = 'Add Last 1'

JavaScript

See Doubly-Linked List (element)#JavaScript, Doubly-Linked List (element insertion)#JavaScript and Doubly-Linked List (traversal)#JavaScript

Julia

Regarding the avoidance or circular loops part of this task, a call to <lang julia>show(DLNode)</lang> reveals that Julia considers all of the nodes of doubly linked lists of this kind to contain circular references to their adjacent nodes. <lang julia>mutable struct DLNode{T}

   value::T
   pred::Union{DLNode{T}, Nothing}
   succ::Union{DLNode{T}, Nothing}
   DLNode(v) = new{typeof(v)}(v, nothing, nothing)

end

function insertpost(prevnode, node)

   succ = prevnode.succ
   prevnode.succ = node
   node.pred = prevnode
   node.succ = succ
   if succ != nothing
       succ.pred =  node
   end
   node

end

function insertpre(postnode, node)

   pred = postnode.pred
   postnode.pred = node
   node.succ = postnode
   node.pred = pred
   if pred != nothing
       pred.succ = node
   end
   node

end

function delete(nd)

   if nd != nothing
       pred = nd.pred
       succ = nd.succ
       if pred != nothing pred.succ = succ end
       if succ != nothing succ.pred = pred end
   end
   nothing

end

first(nd) = (while nd.pred != nothing nd = nd.prev end; nd) last(nd) = (while nd.succ != nothing nd = nd.succ end; nd)

function printconnected(nd; fromtail = false)

   if fromtail
       nd = last(nd)
       print(nd.value)
       while nd.pred != nothing 
           nd = nd.pred
           print(" -> $(nd.value)")
       end
   else
       nd = first(nd)
       print(nd.value)
       while nd.succ != nothing
           nd = nd.succ
           print(" -> $(nd.value)")
       end
   end
   println()

end

node1 = DLNode(1) node2 = DLNode(2) node3 = DLNode(3) node4 = DLNode(4) insertpost(node1, node2) insertpre(node2, node4) insertpost(node2, node3) println("First value is ", first(node1).value, " and last value is ", last(node1).value) print("From beginning to end: "); printconnected(node1) delete(node4) print("From end to beginning post deletion: "); printconnected(node1, fromtail = true)

</lang>

Output:

First value is 1 and last value is 3
From beginning to end: 1 -> 4 -> 2 -> 3
From end to beginning post deletion: 3 -> 2 -> 1

Kotlin

Rather than use the java.util.LinkedList<E> class, we will write our own simple LinkedList<E> class for this task: <lang scala>// version 1.1.2

class LinkedList<E> {

   class Node<E>(var data: E, var prev: Node<E>? = null, var next: Node<E>? = 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()
       }
   }

   var first: Node<E>? = null
   var last:  Node<E>? = null

   fun addFirst(value: E) {
       if (first == null) {
           first = Node(value)
           last =  first
       }
       else {
           val node = first!!
           first = Node(value, null, node)
           node.prev = first
       }
   }

   fun addLast(value: E) {
       if (last == null) {
           last = Node(value)
           first = last
       }
       else {
           val node = last!!
           last = Node(value, node, null)
           node.next = last
       }
   }

   fun insert(after: Node<E>?, value: E) {
       if (after == null)
           addFirst(value)
       else if (after == last)
           addLast(value)
       else {
           val next = after.next
           val new = Node(value, after, next)
           after.next = new
           if (next != null) next.prev = new
       }
   }

   override fun toString() = first.toString()

}

fun main(args: Array<String>) {

   val ll = LinkedList<Int>()
   ll.addFirst(1)
   ll.addLast(4)
   ll.insert(ll.first, 2)
   ll.insert(ll.last!!.prev, 3)
   println(ll)

}</lang>

Output:

1 -> 2 -> 3 -> 4

Lua

<lang lua>-- Doubly Linked List in Lua 6/15/2020 db

---

-- IMPLEMENTATION:

---

local function Node(data)

 return { data=data } --implied: return { data=data, prev=nil, next=nil }

end

local List = {

 head = nil,
 tail = nil,
 insertHead = function(self, data)
   local node = Node(data)
   if (self.head) then
     self.head.prev = node
     node.next = self.head
     self.head = node
   else
     self.head = node
     self.tail = node
   end
   return node
 end,
 insertTail = function(self, data)
   local node = Node(data)
   if self.tail then
     self.tail.next = node
     node.prev = self.tail
     self.tail = node
   else
     self.head = node
     self.tail = node
   end
   return node
 end,
 insertBefore = function(self,mark,data)
   if (mark) then
     local node = Node(data)
     if (mark == self.head) then
       self.head.next = node
       node.next = self.head
       self.head = node
     else
       mark.prev.next = node
       node.prev = mark.prev
       mark.prev = node
       node.next = mark
     end
     return node
   else
     -- if no mark given, then insertBefore()==insertHead()
     return self:insertHead(data)
   end
 end,
 insertAfter = function(self,mark,data)
   if (mark) then
     local node = Node(data)
     if (mark == self.tail) then
       self.tail.next = node
       node.prev = self.tail
       self.tail = node
     else
       mark.next.prev = node
       node.next = mark.next
       mark.next = node
       node.prev = mark
     end
     return node
   else
     -- if no mark given, then insertAfter()==insertTail()
     return self:insertTail(data)
   end
 end,
 values = function(self)
   local result, node = {}, self.head
   while (node) do result[#result+1], node = node.data, node.next end
   return result
 end,

} List.__index = List setmetatable(List, {__call=function(self) return setmetatable({},self) end })

---

-- TEST:

---

local function validate(list, expected)

 local values = list:values()
 local actual = table.concat(values, ",")
 print(actual==expected, actual)

end local list = List() validate(list, "") local n1 = list:insertTail(1) validate(list, "1") local n2 = list:insertTail(2) validate(list, "1,2") local n3 = list:insertTail(3) validate(list, "1,2,3") local n4 = list:insertTail(4) validate(list, "1,2,3,4") local n33 = list:insertAfter(n3, 33) validate(list, "1,2,3,33,4") local n22 = list:insertAfter(n2, 22) validate(list, "1,2,22,3,33,4") local n11 = list:insertAfter(n1, 11) validate(list, "1,11,2,22,3,33,4") local n44 = list:insertAfter(n4, 44) validate(list, "1,11,2,22,3,33,4,44") local n5 = list:insertTail(5) validate(list, "1,11,2,22,3,33,4,44,5") local n444 = list:insertBefore(n5, 444) validate(list, "1,11,2,22,3,33,4,44,444,5") local n55 = list:insertAfter(nil, 55) validate(list, "1,11,2,22,3,33,4,44,444,5,55") local n0 = list:insertHead(0) validate(list, "0,1,11,2,22,3,33,4,44,444,5,55") local nm1 = list:insertBefore(nil, -1) validate(list, "-1,0,1,11,2,22,3,33,4,44,444,5,55") local n111 = list:insertBefore(n2, 111) validate(list, "-1,0,1,11,111,2,22,3,33,4,44,444,5,55") local n222 = list:insertAfter(n22, 222) validate(list, "-1,0,1,11,111,2,22,222,3,33,4,44,444,5,55") local n333 = list:insertBefore(n4, 333) validate(list, "-1,0,1,11,111,2,22,222,3,33,333,4,44,444,5,55") local n555 = list:insertAfter(n55, 555) validate(list, "-1,0,1,11,111,2,22,222,3,33,333,4,44,444,5,55,555")</lang>

Output:

true
true    1
true    1,2
true    1,2,3
true    1,2,3,4
true    1,2,3,33,4
true    1,2,22,3,33,4
true    1,11,2,22,3,33,4
true    1,11,2,22,3,33,4,44
true    1,11,2,22,3,33,4,44,5
true    1,11,2,22,3,33,4,44,444,5
true    1,11,2,22,3,33,4,44,444,5,55
true    0,1,11,2,22,3,33,4,44,444,5,55
true    -1,0,1,11,2,22,3,33,4,44,444,5,55
true    -1,0,1,11,111,2,22,3,33,4,44,444,5,55
true    -1,0,1,11,111,2,22,222,3,33,4,44,444,5,55
true    -1,0,1,11,111,2,22,222,3,33,333,4,44,444,5,55
true    -1,0,1,11,111,2,22,222,3,33,333,4,44,444,5,55,555

M2000 Interpreter

M2000 from 9th version has pointers for groups. Also there is IS operator to check if two pointers are the same. There is no Null pointer except ->0 for groups which decrement object reference counter. So here we use a Null class to define an empty group and then we make a global pointer Null to hold that and compare it with Is operator later.

To check that all works we have to use deconstructor ( Remove {}) which call with Clear statement if only one reference exist. So when we remove from list nodes we check it if destroyed, visually.

We can use ? as print.

There is no garbage collector for pointers for groups, except the reference counter. We can use groups without pointers and we place them in containers, and use indexes or keys for those containers as pointers. Using groups with no pointers make each group unique, so there is no circular dependency between two groups. Containers have garbage collector.

Using pointers inside groups make harder to program.

Groups are two kinds in M2000. The named group (or static in a way) and the float group (unnamed). Pointers for groups are also two types, internal, but we handle it as one type. If we get a pointer to a named group, actually we get a weak reference. If this named grouped erased then weak pointer can't work, we get error. If we get a pointer from an expression we get a real pointer (with counting reference). To get a real pointer from a named group is not possible. We can use A->(namedGroup) to get a pointer of a copy of namedGroup. Then we can use namedGroup=Group(A) to merge a copy of A to namedGroup (same members get new values, new members added, unique members of namedGroup stay as is).

<lang M2000 Interpreter> Module Checkit {

     Form 80, 50
     Class Null {}
     Global Null->Null()
     Class Node {
           group pred, succ
           dat=0
           Remove {
                 Print "destroyed", .dat
           }
           class: 
           module Node {
                 .pred->Null
                 .succ->Null
                 if match("N") Then Read .dat
           }
     }
     Class LList {
           Group Head, Tail
           Module PushTail(k as pointer) {
                 if .Tail is Null then {
                       .Head<=k
                       .Tail<=k
                 } else {
                       n=.Tail
                       .Tail<=k
                       k=>pred=n=>pred
                       n=>pred=k
                       k=>succ=n
                 }
           }
           Function RemoveTail {
                 n=.Tail
                 if n is .Head then {
                       .Head->Null
                       .Tail->Null
                 } Else { 
                       .Tail<=n=>succ
                       .Tail=>pred=n=>pred
                       n=>pred->Null
                 }
                 for n {
                       .succ->Null
                       .pred->Null
                 }
                 =n
           }
           Module PushHead(k as pointer) {
                 if .head is Null then {
                       .Head<=k
                       .Tail<=k
                 } else {
                       n=.head
                       .head<=k
                       k=>succ=n=>succ
                       n=>succ=k
                       k=>pred=n
                 }
           }
           Function RemoveHead {
                 n=.Head
                 if n is .Tail then {
                       .Head->Null
                       .Tail->Null
                 } Else { 
                     .Head<=n=>pred
                     .Head=>succ=n=>succ
                      n=>succ->Null    
                  }
                 for n {
                       .succ->Null
                       .pred->Null
                 }
                 =n
           }
           Module RemoveNode(k as pointer) {
                 pred=k=>pred
                 succ=k=>succ
                 if pred is succ then {
                       if .head is k else Error "Can't remove this node"
                       k=.RemoveHead()
                       clear k
                 } else {
                      pred=>succ=succ
                      succ=>pred=pred 
                 }
           }
           Module InsertAfter(k as pointer, n as pointer) {
                 pred=k=>pred
                 n=>pred=pred
                 n=>succ=k
                 pred=>succ=n
                 k=>pred=n
           }
           Function IsEmpty {
                 = .Head is null or .tail is null
           }
     class:
           Module LList {
                 .Head->Null
                 .Tail->Null
           }
     }
     m->Node(100)
     
     L=LList()
     L.PushTail m
     If not L.Head is Null then Print L.Head=>dat=100
     for i=101 to 103 {
           m->Node(i)
           L.PushTail m
           Print "ok....", i
     }
     for i=104 to 106 {
           m->Node(i)
           L.PushHead m
           Print "ok....", i
     }
     
     Print "Use Head to display from last to first"
     m=L.Head
     do {
           Print m=>dat
           m=m=>pred
     } Until m is null
     Print "ok, now find 3rd and remove it"
     m1=L.Head
     i=1 
     Index=3
     While i<Index {
           if m1 is null then exit
           m1=m1=>pred
           i++
     }
     If i<>Index then {
           Print "List has less than "; Index;" Items"
     } Else {
           Print "First add one new node"
                 newNode->Node(1000)
                 L.InsertAfter m1, newNode
                 L.RemoveNode m1
                 clear m1  ' last time m1 used here
                 newNode=Null
           Print "ok.............."
     }
     Print "Use Tail to display from first to last"
     m=L.Tail
     do {
           Print m=>dat
           m=m=>succ
     } Until m is null
     
     
     useother=True
     While not L.IsEmpty(){
           For This {
                 \\ we have to use a temporary variable name, here A
                        A=If(useother->L.RemoveTail(),L.RemoveHead())
                        ? A=>dat
                       useother~
                       \\ now we can try to perform removing
                       clear A
            }
     }
     Print "list is empty:"; L.IsEmpty()    

} Checkit </lang>

Mathematica / Wolfram Language

<lang Mathematica>ds = CreateDataStructure["DoublyLinkedList"]; ds["Append", #] & /@ {1, 5, 7, 0, 3, 2}; ds["Prepend", 9]; ds["Append", 4]; (* This is adding at the end and then swap to insert in to the middle: *) ds["Append", 14]; ds["SwapPart", Round[ds["Length"]/2], ds["Length"]]; ds["Elements"]</lang>

Output:

{9, 1, 5, 14, 0, 3, 2, 4, 7}

Nim

Nim has a doubly linked list already in the lists module of the standard library. <lang nim>type

 List[T] = object
   head, tail: Node[T]

 Node[T] = ref TNode[T]

 TNode[T] = object
   next, prev: Node[T]
   data: T

proc initList[T](): List[T] = discard

proc newNode[T](data: T): Node[T] =

 new(result)
 result.data = data

proc prepend[T](l: var List[T], n: Node[T]) =

 n.next = l.head
 if l.head != nil: l.head.prev = n
 l.head = n
 if l.tail == nil: l.tail = n

proc append[T](l: var List[T], n: Node[T]) =

 n.next = nil
 n.prev = l.tail
 if l.tail != nil:
   l.tail.next = n
 l.tail = n
 if l.head == nil:
   l.head = n

proc insertAfter[T](l: var List[T], r, n: Node[T]) =

 n.prev = r
 n.next = r.next
 n.next.prev = n
 r.next = n
 if r == l.tail: l.tail = n

proc remove[T](l: var List[T], n: Node[T]) =

 if n == l.tail: l.tail = n.prev
 if n == l.head: l.head = n.next
 if n.next != nil: n.next.prev = n.prev
 if n.prev != nil: n.prev.next = n.next

proc `$`[T](l: var List[T]): string =

 result = ""
 var n = l.head
 while n != nil:
   if result.len > 0: result.add(" -> ")
   result.add($n.data)
   n = n.next

var l = initList[int]() var n = newNode(12) var m = newNode(13) var i = newNode(14) var j = newNode(15) l.append(n) l.prepend(m) l.insertAfter(m, i) l.prepend(j) l.remove(m) echo l

var l2 = initList[string]() l2.prepend newNode("hello") l2.append newNode("world") echo l2</lang>

Output:

15 -> 14 -> 12
hello -> world

Oberon-2

<lang oberon2> IMPORT Basic; TYPE Node* = POINTER TO NodeDesc; NodeDesc* = (* ABSTRACT *) RECORD prev-,next-: Node; END;

DLList* = POINTER TO DLListDesc; DLListDesc* = RECORD first-,last-: Node; size-: INTEGER; END; </lang>

Objeck

<lang objeck> use Collection;

class Program {

 function : Main(args : String[]) ~ Nil {
   list := List->New();
   list->AddFront("first");
   list->AddBack("last");
   list->Insert("middle");

   list->Forward();
   do {
     list->Get()->As(String)->PrintLine();
     list->Previous();
   }
   while(list->Get() <> Nil);
 }

}</lang>

Oforth

<lang oforth>Object Class new: DNode(value, mutable prev, mutable next)

DNode method: initialize  := next := prev := value ; DNode method: value @value ; DNode method: prev @prev ; DNode method: next @next ; DNode method: setPrev := prev ; DNode method: setNext  := next ; DNode method: << @value << ;

DNode method: insertAfter(node)

  node setPrev(self)
  node setNext(@next)
  @next ifNotNull: [ @next setPrev(node) ]
  node := next ;

// Double linked list definition Collection Class new: DList(mutable head, mutable tail) DList method: head @head ; DList method: tail @tail ;

DList method: insertFront(v) | p |

  @head ->p
  DNode new(v, null, p) := head
  p ifNotNull: [ p setPrev(@head) ]
  @tail ifNull: [ @head := tail ] ;

DList method: insertBack(v) | n |

  @tail ->n
  DNode new(v, n, null) := tail 
  n ifNotNull: [ n setNext(@tail) ]
  @head ifNull: [ @tail := head ] ;

DList method: forEachNext

  dup ifNull: [ drop @head ifNull: [ false ] else: [ @head @head true] return ]
  next dup ifNull: [ drop false ] else: [ dup true ] ;

DList method: forEachPrev

  dup ifNull: [ drop @tail ifNull: [ false ] else: [ @tail @tail true] return ]
  prev dup ifNull: [ drop false ] else: [ dup true ] ;

test // ( -- aDList )

| dl dn |

  DList new ->dl
  dl insertFront("A") 
  dl insertBack("B")
  dl head insertAfter(DNode new("C", null , null))
  dl ;</lang>

Output:

>test .s
[1] (DList) [A, C, B]

Phix

See Doubly-linked_list/Traversal for a complete example.

PicoLisp

For the list of double-cell structures described in Doubly-linked list/Element definition#PicoLisp, we define a header structure, containing one pointer to the start and one to the end of the list.

           +------------> start
           |
        +--+--+-----+
        |  |  |  ---+---> end
        +-----+-----+

<lang PicoLisp># Build a doubly-linked list (de 2list @

  (let Prev NIL
     (let L
        (make
           (while (args)
              (setq Prev (chain (list (next) Prev))) ) )
        (cons L Prev) ) ) )

(setq *DLst (2list 'was 'it 'a 'cat 'I 'saw))</lang> For output of the example data, see Doubly-linked list/Traversal#PicoLisp.

PL/I

<lang PL/I> define structure

  1 Node,
     2 value        fixed decimal,
     2 back_pointer handle(Node),
     2 fwd_pointer  handle(Node);

</lang>

PowerShell

Create and populate the list: <lang PowerShell> $list = New-Object -TypeName 'Collections.Generic.LinkedList[PSCustomObject]'

for($i=1; $i -lt 10; $i++) {

  $list.AddLast([PSCustomObject]@{ID=$i; X=100+$i;Y=200+$i}) | Out-Null

}

$list </lang>

Output:

ID   X   Y
--   -   -
 1 101 201
 2 102 202
 3 103 203
 4 104 204
 5 105 205
 6 106 206
 7 107 207
 8 108 208
 9 109 209

Insert a value at the head: <lang PowerShell> $list.AddFirst([PSCustomObject]@{ID=123; X=123;Y=123}) | Out-Null

$list </lang>

Output:

 ID   X   Y
 --   -   -
123 123 123
  1 101 201
  2 102 202
  3 103 203
  4 104 204
  5 105 205
  6 106 206
  7 107 207
  8 108 208
  9 109 209

Insert a value in the middle: <lang PowerShell> $current = $list.First

while(-not ($current -eq $null)) {

  If($current.Value.X -eq 105)
  {
      $list.AddAfter($current, [PSCustomObject]@{ID=345;X=345;Y=345}) | Out-Null
      break
  }

  $current = $current.Next

}

$list </lang>

Output:

 ID   X   Y
 --   -   -
123 123 123
  1 101 201
  2 102 202
  3 103 203
  4 104 204
  5 105 205
345 345 345
  6 106 206
  7 107 207
  8 108 208
  9 109 209

Insert a value at the end: <lang PowerShell> $list.AddLast([PSCustomObject]@{ID=789; X=789;Y=789}) | Out-Null

$list </lang>

Output:

 ID   X   Y
 --   -   -
123 123 123
  1 101 201
  2 102 202
  3 103 203
  4 104 204
  5 105 205
345 345 345
  6 106 206
  7 107 207
  8 108 208
  9 109 209
789 789 789

PureBasic

<lang PureBasic>DataSection

 ;the list of words that will be added to the list
 words:
 Data.s "One", "Two", "Three", "Four", "Five", "Six", "EndOfData"

EndDataSection

Procedure displayList(List x.s(), title$)

 ;display all elements from list of strings
 Print(title$)  
 ForEach x()
   Print(x() + " ")
 Next
 PrintN("")

EndProcedure

OpenConsole()

NewList a.s() ;create a new list of strings

add words to the head of list

Restore words Repeat

 Read.s a$
 If a$ <> "EndOfData"
   ResetList(a()) ;Move to head of list
   AddElement(a()) 
   a() = a$
 EndIf

Until a$ = "EndOfData" displayList(a(),"Insertion at Head: ")

ClearList(a())

add words to the tail of list

Restore words LastElement(a()) ;Move to the tail of the list Repeat

 Read.s a$
 If a$ <> "EndOfData"
   AddElement(a()) ;after insertion the new position is still at the tail 
   a() = a$
 EndIf

Until a$ = "EndOfData" displayList(a(),"Insertion at Tail: ")

ClearList(a())

add words to the middle of list

Restore words ResetList(a()) ;Move to the tail of the list Repeat

 Read.s a$
 If a$ <> "EndOfData"
   c = CountList(a())
   If c > 1
     SelectElement(a(),Random(c - 2)) ;insert after a random element but before tail
   Else
     FirstElement(a())
   EndIf 
   AddElement(a())
   a() = a$
 EndIf

Until a$ = "EndOfData" displayList(a(),"Insertion in Middle: ")

Repeat: Until Inkey() <> ""</lang>

Output:

Insertion at Head: Six Five Four Three Two One
Insertion at Tail: One Two Three Four Five Six
Insertion at Middle: One Five Six Three Four Two

Python

In the high level language Python, its list native datatype might be able to be used used. It automatically preserves the integrity of the list w.r.t. loops and allows insertion at any point using list.insert() via an integer index into the list rather than a machine-code level pointer to a list element.But if you need a DLL then you need it!

collections.deque

If, as you would expect from a doubly-linked list, you need to efficiently append items to the start of an ordered collection, use Python’s built-in collections.deque datatype. See TimeComplexity.

<lang python> from collections import deque

some_list = deque(["a", "b", "c"]) print(some_list)

some_list.appendleft("Z") print(some_list)

for value in reversed(some_list):

   print(value)

</lang>

Output:

$ python deque_example.py 
deque(['a', 'b', 'c'])
deque(['Z', 'a', 'b', 'c'])
c
b
a
Z

Alternative

Although a deque is preferable in most cases, this implementation gives you the option to access nodes and slice "views" from a DoublyLinkedList. Things that collections.deque does not do.

Retrieving an item by it’s index returns the node’s value. Similarly, iterating a DoublyLinkedList yields a sequence of values, not a sequence of nodes. The return value of a slice is a DoublyLinkedListView, which shares common nodes with the original list.

Note that, without a concrete use case, the decision as to which of the special methods should return a node’s value or a DoublyLinkedListView is somewhat arbitrary. One might also choose to do away with DoublyLinkedListView and return a new DoublyLinkedList, accepting that update operations on a derived list could break links between nodes in other lists.

<lang python> """A doubly-linked list. Requires Python >=3.7""" from __future__ import annotations

import math

from abc import ABC

from collections.abc import MutableSequence from collections.abc import Sequence from dataclasses import dataclass from dataclasses import field

from typing import Any from typing import Iterable from typing import Iterator from typing import Optional from typing import Union

@dataclass class Node:

   value: Any
   prev: Optional[Node] = field(default=None)
   next: Optional[Node] = field(default=None)

class BaseDoublyLinkedList(ABC):

   def __len__(self):
       return self.size

   def __iter__(self) -> Iterator[Any]:
       node = self.head
       cnt = 1
       while node is not None and cnt <= self.size:
           yield node.value
           node = node.next
           cnt += 1

   def __reversed__(self) -> Iterator[Any]:
       node = self.tail
       while node is not None:
           yield node.value
           node = node.prev

   def __getitem__(self, key: Union[int, slice]) -> Optional[Any]:
       if isinstance(key, int):
           node = self._find_node(key)
           return node.value

       if key.step not in (None, 1):
           raise IndexError("can't step more than 1")

       start = key.start or 0
       node = self._find_node(start)
       return DoublyLinkedListView(node, key.stop - start - 1)

   def _find_node(self, index) -> Node:
       if index > self.size - 1 or index < -self.size:
           raise IndexError("list index out of range")

       if index >= 0:
           node = self.head
           for _ in range(index):
               node = node.next
       else:
           node = self.tail
           for _ in range(self.size - 1, self.size - abs(index), -1):
               node = node.prev

       return node

class DoublyLinkedListView(BaseDoublyLinkedList, Sequence):

   def __init__(self, node, stop=math.inf):
       self.head = node

       # Find the tail
       cnt = 1
       while node is not None and cnt <= stop:
           node = node.next
           cnt += 1

       self.tail = node
       self.size = cnt

   def __repr__(self):
       return f"DoublyLinkedListView([{', '.join(repr(v) for v in self)}])"

   def __str__(self):
       return repr(self)

class DoublyLinkedList(BaseDoublyLinkedList, MutableSequence):

   def __init__(self, iterable: Iterable):
       it = iter(iterable)

       # Possibly empty iterable
       try:
           self.head = Node(value=next(it))
           self.tail = self.head
           self.size = 1
       except StopIteration:
           self.head = None
           self.tail = None
           self.size = 0

       node = self.head

       for val in it:
           self.tail = Node(value=val, prev=node)
           node.next = self.tail
           node = self.tail
           self.size += 1

   def __repr__(self):
       return f"DoublyLinkedList([{', '.join(repr(v) for v in self)}])"

   def __str__(self):
       return repr(self)

   def __setitem__(self, key, value):
       node = self._find_node(key)
       node.value = value

   def __delitem__(self, key) -> None:
       node = self._find_node(key)
       node.prev.next = node.next
       node.next.prev = node.prev
       self.size -= 1

   def insert(self, index: int, value: Any) -> None:
       node = self._find_node(index)
       new_node = Node(value=value, prev=node.prev, next=node)

       node.prev.next = new_node
       node.prev = node

       self.size += 1

   def append(self, value: Any) -> None:
       tail = self.tail
       node = Node(value=value, prev=tail)
       tail.next = node

       self.tail = node
       self.size += 1

   def appendleft(self, value: Any) -> None:
       head = self.head
       node = Node(value=value, next=head)
       head.prev = node

       self.head = node
       self.size += 1

   def pop(self) -> Any:
       value = self.tail.value
       self.tail = self.tail.prev
       self.tail.next = None
       self.size -= 1
       return value

   def popleft(self) -> Any:
       value = self.head.value
       self.head = self.head.next
       self.head.prev = None
       self.size -= 1
       return value

</lang>

Racket

The following is a port of the Common Lisp solution. The ouput is '(1 2 3 4).

<lang racket>

1. lang racket

(define-struct dlist (head tail) #:mutable #:transparent) (define-struct dlink (content prev next) #:mutable #:transparent)

(define (insert-between dlist before after data)

 ; Insert a fresh link containing DATA after existing link 
 ; BEFORE if not nil and before existing link AFTER if not nil
 (define new-link (make-dlink data before after))
 (if before
     (set-dlink-next! before new-link)
     (set-dlist-head! dlist new-link))
 (if after
     (set-dlink-prev! after new-link)
     (set-dlist-tail! dlist new-link))
   new-link)

(define (insert-before dlist dlink data)

 ; Insert a fresh link containing DATA before existing link DLINK
 (insert-between dlist (dlink-prev dlink) dlink data))

(define (insert-after dlist dlink data)

 ; Insert a fresh link containing DATA after existing link DLINK
 (insert-between dlist dlink (dlink-next dlink) data))

(define (insert-head dlist data)

 ; Insert a fresh link containing DATA at the head of DLIST
 (insert-between dlist #f (dlist-head dlist) data))

(define (insert-tail dlist data)

 ; Insert a fresh link containing DATA at the tail of DLIST
 (insert-between dlist (dlist-tail dlist) #f data))

(define (remove-link dlist dlink)

 ; Remove link DLINK from DLIST and return its content
 (let ((before (dlink-prev dlink))
       (after (dlink-next dlink)))
   (if before
       (set-dlink-next! before after)
       (set-dlist-head! dlist after))
   (if after
       (set-dlink-prev! after before)
       (set-dlist-tail! dlist before))))

(define (dlist-elements dlist)

 ; Returns the elements of DLIST as a list
 (define (extract-values dlink acc)
   (if dlink
       (extract-values (dlink-next dlink) (cons (dlink-content dlink) acc))
       acc))
 (reverse (extract-values (dlist-head dlist) '())))

(let ((dlist (make-dlist #f #f)))

 (insert-head dlist 1)
 (insert-tail dlist 4)
 (insert-after dlist (dlist-head dlist) 2)
 (let* ((next-to-last (insert-before dlist (dlist-tail dlist) 3))
        (bad-link (insert-before dlist next-to-last 42)))
   (remove-link dlist bad-link))
 (dlist-elements dlist))

</lang>

Raku

(formerly Perl 6) This shows a complete example. (Other entries in the section focus on aspects of this solution.) <lang perl6>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

   }

}

role DLList[::DLE] {

   has DLE $.first;
   has DLE $.last;

   submethod BUILD {

$!first = DLE.new; $!last = DLE.new; $!first.next = $!last; $!last.prev = $!first;

   }

   method list { ($!first.next, *.next ...^ !*.next).map: *.payload }
   method reverse { ($!last.prev, *.prev ...^ !*.prev).map: *.payload }

}

class DLElem_Int does DLElem[Int] {} class DLList_Int does DLList[DLElem_Int] {}

my $dll = DLList_Int.new;

$dll.first.post-insert(1).post-insert(2).post-insert(3); $dll.first.post-insert(0);

$dll.last.pre-insert(41).pre-insert(40).prev.delete; # (deletes 3) $dll.last.pre-insert(42);

say $dll.list; # 0 1 2 40 41 42 say $dll.reverse; # 42 41 40 2 1 0</lang>

Output:

0 1 2 40 41 42
42 41 40 2 1 0

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.         ║
        ╚═════════════════════════════════════════════════════════════════════════╝

REXX doesn't have linked lists, as there are no pointers (or handles).
However, linked lists can be simulated with lists in REXX. <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</lang>

output

                               ─── 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 5th & 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). 

Ring

<lang ring>

1. Project : Doubly-linked list/Definition

test = [1, 5, 7, 0, 3, 2] insert(test, 0, 9) insert(test, len(test), 4) item = len(test)/2 insert(test, item, 6) showarray(test)

func showarray(vect)

       svect = ""
       for n = 1 to len(vect)
             svect = svect + vect[n] + " "
       next
       svect = left(svect, len(svect) - 1)
       see svect

</lang> Output:

9 1 5 7 6 0 3 2 4

Ruby

See Doubly-Linked List (element)#Ruby, Doubly-Linked List (element insertion)#Ruby and Doubly-Linked List (traversal)#Ruby

Scheme

Works with: R7RS

<lang scheme>(cond-expand

 (r7rs)
 (chicken (import r7rs)))

(import (scheme base)) (import (scheme write)) (import (scheme case-lambda)) (import (scheme process-context))

A doubly-linked list will be represented by a reference to any of

its nodes. This is possible because the "root node" is marked as

such.

(define-record-type <dllist>

 (%dllist root? prev next element)
 dllist?
 (root? dllist-root?)
 (prev dllist-prev %dllist-set-prev!)
 (next dllist-next %dllist-set-next!)
 (element %dllist-element))

(define (dllist-element node)

 ;; Get the element in a node. It is an error if the node is the
 ;; root.
 (when (dllist-root? node)
   (display "dllist-element of a root node\n" (current-error-port))
   (exit 1))
 (%dllist-element node))

(define (dllist . elements)

 ;; Make a doubly-linked list from the given elements.
 (list->dllist elements))

(define (make-dllist)

 ;; Return a node marked as being a root, and which points to itself.
 (let ((root (%dllist #t #f #f #f)))
   (%dllist-set-prev! root root)
   (%dllist-set-next! root root)
   root))

(define (dllist-root node)

 ;; Given a reference to any node of a <dllist>, return a reference
 ;; to the list's root node.
 (if (dllist-root? node)
     node
     (dllist-root (dllist-prev node))))

(define (dllist-insert-after! node element)

 ;; Insert an element after the given node, which may be either the
 ;; root or some other node.
 (let* ((next-node (dllist-next node))
        (new-node (%dllist #f node next-node element)))
   (%dllist-set-next! node new-node)
   (%dllist-set-prev! next-node new-node)))

(define (dllist-insert-before! node element)

 ;; Insert an element before the given node, which may be either the
 ;; root or some other node.
 (let* ((prev-node (dllist-prev node))
        (new-node (%dllist #f prev-node node element)))
   (%dllist-set-next! prev-node new-node)
   (%dllist-set-prev! node new-node)))

(define (dllist-remove! node)

 ;; Remove a node from a <dllist>. It is an error if the node is the
 ;; root.
 (when (dllist-root? node)
   (display "dllist-remove! of a root node\n" (current-error-port))
   (exit 1))
 (let ((prev (dllist-prev node))
       (next (dllist-next node)))
   (%dllist-set-next! prev next)
   (%dllist-set-prev! next prev)))

(define dllist-make-generator

 ;; Make a thunk that returns the elements of the list, starting at
 ;; the root and going in either direction.
 (case-lambda
   ((node) (dllist-make-generator node 1))
   ((node direction)
    (if (negative? direction)
        (let ((p (dllist-prev (dllist-root node))))
          (lambda ()
            (and (not (dllist-root? p))
                 (let ((element (dllist-element p)))
                   (set! p (dllist-prev p))
                   element))))
        (let ((p (dllist-next (dllist-root node))))
          (lambda ()
            (and (not (dllist-root? p))
                 (let ((element (dllist-element p)))
                   (set! p (dllist-next p))
                   element))))))))

(define (list->dllist lst)

 ;; Make a doubly-linked list from the elements of an ordinary list.
 (let loop ((node (make-dllist))
            (lst lst))
   (if (null? lst)
       (dllist-next node)
       (begin
         (dllist-insert-after! node (car lst))
         (loop (dllist-next node) (cdr lst))))))

(define (dllist->list node)

 ;; Make an ordinary list from the elements of a doubly-linked list.
 (let loop ((lst '())
            (node (dllist-prev (dllist-root node))))
   (if (dllist-root? node)
       lst
       (loop (cons (dllist-element node) lst) (dllist-prev node)))))

Some demonstration.

(define (print-it node)

 (let ((gen (dllist-make-generator node)))
   (do ((x (gen) (gen)))
       ((not x))
     (display " ")
     (write x))
   (newline)))

(define dl (dllist 10 20 30 40 50))

(let ((gen (dllist-make-generator dl)))

 (display "forwards generator: ")
 (do ((x (gen) (gen)))
     ((not x))
   (display " ")
   (write x))
 (newline))

(let ((gen (dllist-make-generator dl -1)))

 (display "backwards generator:")
 (do ((x (gen) (gen)))
     ((not x))
   (display " ")
   (write x))
 (newline))

(display "insertion after the root: ") (dllist-insert-after! dl 5) (print-it dl)

(display "insertion before the root:") (dllist-insert-before! dl 55) (print-it dl)

(display "insertion after the second element:") (dllist-insert-after! (dllist-next (dllist-next dl)) 15) (print-it dl)

(display "insertion before the second from last element:") (dllist-insert-before! (dllist-prev (dllist-prev dl)) 45) (print-it dl)

(display "removal of the element 30:") (let ((node (let loop ((node (dllist-next dl)))

             (if (= (dllist-element node) 30)
                 node
                 (loop (dllist-next node))))))
 (dllist-remove! node))

(print-it dl)

(display "any node can be used for the generator:") (print-it (dllist-next (dllist-next (dllist-next dl))))

(display "conversion to a list: ") (display (dllist->list dl)) (newline)

(display "conversion from a list:") (print-it (list->dllist (list "a" "b" "c")))</lang>

Output:

$ chibi dllist.scm
forwards generator:  10 20 30 40 50
backwards generator: 50 40 30 20 10
insertion after the root:  5 10 20 30 40 50
insertion before the root: 5 10 20 30 40 50 55
insertion after the second element: 5 10 15 20 30 40 50 55
insertion before the second from last element: 5 10 15 20 30 40 45 50 55
removal of the element 30: 5 10 15 20 40 45 50 55
any node can be used for the generator: 5 10 15 20 40 45 50 55
conversion to a list: (5 10 15 20 40 45 50 55)
conversion from a list: "a" "b" "c"

Swift

<lang>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
 }

}

struct DoublyLinkedList<T> {

 fileprivate var head: NodePtr<T>?
 fileprivate var tail: NodePtr<T>?

 @discardableResult
 mutating func insert(_ val: T, at: InsertLoc<T> = .last) -> Node<T> {
   let node = NodePtr<T>.allocate(capacity: 1)

   node.initialize(to: Node(value: val))

   if head == nil && tail == nil {
     head = node
     tail = node

     return node.pointee
   }

   switch at {
   case .first:
     node.pointee.next = head
     head?.pointee.prev = node
     head = node
   case .last:
     node.pointee.prev = tail
     tail?.pointee.next = node
     tail = node
   case let .after(insertNode):
     var n = head

     while n != nil {
       if n!.pointee !== insertNode {
         n = n?.pointee.next

         continue
       }

       node.pointee.prev = n
       node.pointee.next = n!.pointee.next
       n!.pointee.next = node

       if n == tail {
         tail = node
       }

       break
     }
   }

   return node.pointee
 }

 @discardableResult
 mutating func remove(_ node: Node<T>) -> T? {
   var n = head

   while n != nil {
     if n!.pointee !== node {
       n = n?.pointee.next

       continue
     }

     if n == head {
       n?.pointee.next?.pointee.prev = nil
       head = n?.pointee.next
     } else if n == tail {
       n?.pointee.prev?.pointee.next = nil
       tail = n?.pointee.prev
     } else {
       n?.pointee.prev?.pointee.next = n?.pointee.next
       n?.pointee.next?.pointee.prev = n?.pointee.prev
     }

     break
   }

   if n == nil {
     return nil
   }

   defer {
     n?.deinitialize(count: 1)
     n?.deallocate()
   }

   return n?.pointee.value
 }

 enum InsertLoc<T> {
   case first
   case last
   case after(Node<T>)
 }

}

extension DoublyLinkedList: CustomStringConvertible {

 var description: String {
   var res = "["
   var node = head

   while node != nil {
     res += "\(node!.pointee.value), "
     node = node?.pointee.next
   }

   return (res.count > 1 ? String(res.dropLast(2)) : res) + "]"
 }

}

var list = DoublyLinkedList<Int>() var node: Node<Int>!

for i in 0..<10 {

 let insertedNode = list.insert(i)

 if i == 5 {
   node = insertedNode
 }

}

print(list)

list.insert(100, at: .after(node!))

print(list)

list.remove(node!)

print(list)</lang>

Output:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
[0, 1, 2, 3, 4, 5, 100, 6, 7, 8, 9]
[0, 1, 2, 3, 4, 100, 6, 7, 8, 9]

Tcl

This task was earlier marked as unfeasible for Tcl. Tcl lists are compact arrays of pointers to values. However, on very long lists, insertions and deletions (if not at end) may require copying a large amount of data. In such cases, the implementation below may be helpful. It provides a single dl command, which is called with the name of a DList, a method name, and possibly more arguments as required. The testcases below should give a good idea. The asList and asList2 methods demonstrate forward and backward traversal.

See also Doubly-Linked List (element) for a TclOO-based version.

<lang Tcl>package require Tcl 8.4 proc dl {_name cmd {where error} {value ""}} {

   upvar 1 $_name N
   switch -- $cmd {
       insert {
           if ![info exists N()] {set N() {"" "" 0}}
           set id [lindex $N() 2]
           lset N() 2 [incr id]
           switch -- $where {
               head {
                   set prev {}
                   set next [lindex $N() 0]
                   lset N() 0 $id
               }
               end {
                   set prev [lindex $N() 1]
                   set next {}
                   lset N() 1 $id
               }
               default {
                   set prev $where
                   set next [lindex $N($where) 1]
                   lset N($where) 1 $id
               }
           }
           if {$prev ne ""} {lset N($prev) 1 $id}
           if {$next ne ""} {lset N($next) 0 $id}
           if {[lindex $N() 1] eq ""} {lset N() 1 $id}
           set N($id) [list $prev $next $value]
           return $id
       }
       delete {
           set i $where
           if {$where eq "head"} {set i [dl N head]}
           if {$where eq "end"}  {set i [dl N end]}
           foreach {prev next} $N($i) break
           if {$prev ne ""} {lset N($prev) 1 $next}
           if {$next ne ""} {lset N($next) 0 $prev}
           if {[dl N head] == $i} {lset N() 0 $next} 
           if {[dl N end] == $i}  {lset N() 1 $prev}
           unset N($i)
       }
       findfrom {
           if {$where eq "head"} {set where [dl N head]}
           for {set i $where} {$i ne ""} {set i [dl N next $i]} {
               if {[dl N get $i] eq $value} {return $i}
           }
       } 
       get    {lindex $N($where) 2}
       set    {lset   N($where) 2 $value; set value}
       head   {lindex $N() 0}
       end    {lindex $N() 1}
       next   {lindex $N($where) 1}
       prev   {lindex $N($where) 0}
       length {expr {[array size N]-1}}
       asList {
           set res {}
           for {set i [dl N head]} {$i ne ""} {set i [dl N next $i]} {
               lappend res [dl N get $i]
           }
           return $res
       } 
       asList2 {
           set res {}
           for {set i [dl N end]} {$i ne ""} {set i [dl N prev $i]} {
               lappend res [dl N get $i]
           }
           return $res
       } 
   }

}</lang> <lang tcl># Testing code set testcases [split {

   dl D insert head foo
   dl D insert end  bar
   dl D insert head hello
   dl D set [dl D head] hi
   dl D insert end  grill
   set i [dl D findfrom head bar]
   dl D set    $i BAR
   dl D insert $i and
   dl D length
   dl D asList2
   dl D delete $i
   dl D findfrom head nix
   dl D delete head
   dl D delete end
   dl D delete end
   dl D delete head
   dl D length

} \n] foreach case $testcases {

   if {[string trim $case] ne ""} {
       puts " $case -> [eval $case] : [dl D asList]"
       if {[lsearch $argv -p] >= 0} {parray D}
   }

}</lang>

Visual Basic .NET

<lang vbnet>Public Class DoubleLinkList(Of T)

  Private m_Head As Node(Of T)
  Private m_Tail As Node(Of T)

  Public Sub AddHead(ByVal value As T)
      Dim node As New Node(Of T)(Me, value)

      If m_Head Is Nothing Then
          m_Head = Node
          m_Tail = m_Head
      Else
          node.Next = m_Head
          m_Head = node
      End If

  End Sub

  Public Sub AddTail(ByVal value As T)
      Dim node As New Node(Of T)(Me, value)

      If m_Tail Is Nothing Then
          m_Head = node
          m_Tail = m_Head
      Else
          node.Previous = m_Tail
          m_Tail = node
      End If
  End Sub

  Public ReadOnly Property Head() As Node(Of T)
      Get
          Return m_Head
      End Get
  End Property

  Public ReadOnly Property Tail() As Node(Of T)
      Get
          Return m_Tail
      End Get
  End Property

  Public Sub RemoveTail()
      If m_Tail Is Nothing Then Return

      If m_Tail.Previous Is Nothing Then 'empty
          m_Head = Nothing
          m_Tail = Nothing
      Else
          m_Tail = m_Tail.Previous
          m_Tail.Next = Nothing
      End If
  End Sub

  Public Sub RemoveHead()
      If m_Head Is Nothing Then Return

      If m_Head.Next Is Nothing Then 'empty
          m_Head = Nothing
          m_Tail = Nothing
      Else
          m_Head = m_Head.Next
          m_Head.Previous = Nothing
      End If
  End Sub

End Class

Public Class Node(Of T)

  Private ReadOnly m_Value As T
  Private m_Next As Node(Of T)
  Private m_Previous As Node(Of T)
  Private ReadOnly m_Parent As DoubleLinkList(Of T)

  Public Sub New(ByVal parent As DoubleLinkList(Of T), ByVal value As T)
      m_Parent = parent
      m_Value = value
  End Sub

  Public Property [Next]() As Node(Of T)
      Get
          Return m_Next
      End Get
      Friend Set(ByVal value As Node(Of T))
          m_Next = value
      End Set
  End Property

  Public Property Previous() As Node(Of T)
      Get
          Return m_Previous
      End Get
      Friend Set(ByVal value As Node(Of T))
          m_Previous = value
      End Set
  End Property

  Public ReadOnly Property Value() As T
      Get
          Return m_Value
      End Get
  End Property

  Public Sub InsertAfter(ByVal value As T)
      If m_Next Is Nothing Then
          m_Parent.AddTail(value)
      ElseIf m_Previous Is Nothing Then
          m_Parent.AddHead(value)
      Else
          Dim node As New Node(Of T)(m_Parent, value)
          node.Previous = Me
          node.Next = Me.Next
          Me.Next.Previous = node
          Me.Next = node
      End If
  End Sub

  Public Sub Remove()
      If m_Next Is Nothing Then
          m_Parent.RemoveTail()
      ElseIf m_Previous Is Nothing Then
          m_Parent.RemoveHead()
      Else
          m_Previous.Next = Me.Next
          m_Next.Previous = Me.Previous
      End If
  End Sub

End Class</lang>

Wren

Library: Wren-llist

The DLinkedList class in the above module is a generic doubly-linked list and is implemented in such a way that circular loops are not possible. We therefore use it here. <lang ecmascript>import "/llist" for DLinkedList

var dll = DLinkedList.new() for (i in 1..3) dll.add(i) System.print(dll) for (i in 1..3) dll.remove(i) System.print(dll)</lang>

Output:

[1 <-> 2 <-> 3]
[]

zkl

<lang zkl>class Node{

  fcn init(_value,_prev=Void,_next=Void)
     { var value=_value, prev=_prev, next=_next; }
  fcn toString{ value.toString() }
  fcn append(value){  // loops not allowed: create a new Node
     b,c := Node(value,self,next),next;
     next=b;
     if(c) c.prev=b;
     b
  }
  fcn delete{ 
     if(prev) prev.next=next;
     if(next) next.prev=prev; 
     self 
  }
  fcn last  { n,p := self,self; while(n){ p,n = n,n.next } p }
  fcn first { n,p := self,self; while(n){ p,n = n,n.prev } p }
  fcn walker(forward=True){
     dir:=forward and "next" or "prev";
     Walker(fcn(rn,dir){ 
        if(not (n:=rn.value)) return(Void.Stop);

rn.set(n.setVar(dir));

        n.value;
     }.fp(Ref(self),dir))
  }

}</lang> <lang zkl>a:=Node("a"); a.append("c").append("d"); a.last().append("e"); a.last().first().append("b"); foreach n in (a){ print(n," ") } println(); foreach n in (a.last().walker(False)){ print(n," ") } println();</lang>

Output:

a  b  c  d  e  
e  d  c  b  a