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Doubly-linked list/Definition

From Rosetta Code
Task
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



Action![edit]

The user must type in the monitor the following command after compilation and before running the program!
SET EndProg=*
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
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[edit]

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[edit]

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
# -*- coding: utf-8 -*- #
COMMENT REQUIRES:
MODE VALUE = ~;
# For example: #
MODE VALUE = UNION(INT, REAL, COMPL)
END COMMENT
 
MODE LINKNEW = STRUCT (
LINK next, prev,
VALUE value
);
 
MODE LINK = REF LINKNEW;
 
SKIP
File: prelude/Doubly-linked_list_Operator.a68
# -*- 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;
 
# 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
);
 
# insert before #
OP +=: = (LINK link, LINK cursor)LINK: prev OF cursor +:= link;
 
# 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
);
 
# 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
File: test/Doubly-linked_list_Operator_Usage.a68
#!/usr/bin/a68g --script #
# -*- 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;
 
# 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;
 
# 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);
 
# 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);
 
# 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)
)
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[edit]

Works with: as version Raspberry Pi
 
/* 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
 
 

ATS[edit]

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.

In the following, the doubly linked list and its nodes are not distinct types. The nodes form a circle, but they do not form a circular list, because one of the nodes is a "root" node that holds no value, cannot be deleted, and is marked as special.

Here is dllist.sats.

(********************************************************************)
(* The public interface *)
 
abstype dllist_t (t : [email protected]+, is_root : bool) (* An abstract type. *)
typedef dllist_t (t : [email protected]+) = [b : bool] dllist_t (t, b)
 
(* Make a new dllist_t, consisting of a root node. *)
fun {t : [email protected]}
dllist_t_make () : dllist_t (t, true)
 
(* Is this the root node, with no element stored? *)
fun {t : [email protected]}
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 : [email protected]}
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 : [email protected]}
dllist_t_root (dl : dllist_t t) : dllist_t (t, true)
 
(* Return the previous node. *)
fun {t : [email protected]}
dllist_t_previous (dl : dllist_t t) : dllist_t t
 
(* Return the next node. *)
fun {t : [email protected]}
dllist_t_next (dl : dllist_t t) : dllist_t t
 
(* Insert an element before the given node. *)
fun {t : [email protected]}
dllist_t_insert_before (dl : dllist_t t, elem : t) : void
 
(* Insert an element after the given node. *)
fun {t : [email protected]}
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 : [email protected]}
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 : [email protected]}
dllist_t_element (dl: dllist_t t) : t
 
fun {t : [email protected]}
dllist_t_make_generator (dl: dllist_t t, direction : int) :
() -<cloref1> Option t
 
fun {t : [email protected]}
dllist_t_to_list
(dl : dllist_t t) : [n : nat] list (t, n)
 
fun {t : [email protected]}
list_to_dllist_t
{n : int}
(dl : list (t, n)) : dllist_t t
 
(********************************************************************)


Here is dllist.dats.

#define ATS_DYNLOADFLAG 0
 
#include "share/atspre_staload.hats"
 
staload "dllist.sats"
staload UN = "prelude/SATS/unsafe.sats"
 
(********************************************************************)
(* The implementation in terms of linear types. *)
 
absprop NODEPTR (t : [email protected]+, is_root : bool, p : addr)
 
vtypedef nodeptr_vt (t : [email protected]+, is_root : bool, p : addr) =
@(NODEPTR (t, is_root, p) | ptr p)
vtypedef nodeptr_vt (t : [email protected]+, p : addr) =
[is_root : bool] nodeptr_vt (t, is_root, p)
vtypedef nodeptr_vt (t : [email protected]+, is_root : bool) =
[p : addr] nodeptr_vt (t, is_root, p)
vtypedef nodeptr_vt (t : [email protected]+) =
[is_root : bool] [p : addr] nodeptr_vt (t, is_root, p)
 
datavtype node_vt (t : [email protected]+, 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 : [email protected]+) =
[is_root : bool] node_vt (t, is_root)
 
extern castfn
node2nodeptr_consuming :
{t : [email protected]}
{is_root : bool}
node_vt (t, is_root) -<> nodeptr_vt (t, is_root)
 
extern castfn
nodeptr2node_consuming :
{t : [email protected]}
{is_root : bool}
nodeptr_vt (t, is_root) -<> node_vt (t, is_root)
 
extern castfn
node2nodeptr_preserving :
{t : [email protected]}
{is_root : bool}
(!node_vt (t, is_root) >> _) -<> nodeptr_vt (t, is_root)
 
extern castfn
nodeptr2node_preserving :
{t : [email protected]}
{is_root : bool}
(!nodeptr_vt (t, is_root) >> _) -<> node_vt (t, is_root)
 
fn {t : [email protected]}
node_refcopy {is_root : bool}
(node  : !node_vt (t, is_root)) :
node_vt (t, is_root) =
nodeptr2node_preserving (node2nodeptr_preserving node)
 
fn {t : [email protected]}
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} ([email protected] 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 _ = [email protected] node
in
node
end
 
fn {t : [email protected]}
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 : [email protected]}
isnot_root {is_root : bool}
(node : !node_vt (t, is_root)) :
[b : bool | b == ~is_root] bool b =
~is_root<t> node
 
fn {t : [email protected]}
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 : [email protected]}
get_prev (node : !node_vt t) : node_vt t =
let
val+ @ node_vt_object (_, prev, _, _) = node
val prev_node = nodeptr2node_preserving prev
prval _ = [email protected] node
in
prev_node
end
 
fn {t : [email protected]}
get_next (node : !node_vt t) : node_vt t =
let
val+ @ node_vt_object (_, _, next, _) = node
val next_node = nodeptr2node_preserving next
prval _ = [email protected] node
in
next_node
end
 
fn {t : [email protected]}
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 _ = [email protected] node
}
 
fn {t : [email protected]}
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 _ = [email protected] node
}
 
fn {t : [email protected]}
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 : [email protected]}
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 : [email protected]}
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 : [email protected]}
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.
 
A garbage collector, of course, is necessary for 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. *)
 
assume dllist_t (t, is_root) = ptr
 
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
 
(********************************************************************)


And here is dllist-demo.dats.

#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! ()
}


Output:
$ patscc -O2 -DATS_MEMALLOC_GCBDW dllist-demo.dats dllist.dats dllist.sats -lgc && ./a.out
doubly linked list: 10 20 30 40 50
conversion to a regular list: 10, 20, 30, 40, 50
traversal backwards: 50 40 30 20 10
traversal forwards, given a non-root node: 10 20 30 40 50
traversal backwards, given a non-root node: 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

Aside: This is a form of doubly linked list I first implemented in Fortran 2018 for a mark-and-sweep garbage collector. I used the list to store the mutator roots.

AutoHotkey[edit]

see Doubly-linked list/AutoHotkey

C[edit]

/* double linked list */
#include <stdio.h>
#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;
}

Simple test:

/* 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);
}
}

C#[edit]

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.

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;
}
}
}
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++[edit]

Works with: C++11
#include <iostream>
#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';
}
Output:
9 1 5 7 6 0 3 2 4 

Clojure[edit]

(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) [email protected])))
 
(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))

Usage:

(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 ...>}

Common Lisp[edit]

(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))))

The following produces (1 2 3 4).

(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)))

D[edit]

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;
}
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[edit]

[[1]]
Translation of: C#
 
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 can''t 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.

E[edit]

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
}
? def list := makeDLList()
# value: <>
 
? list.push(1)
? list
# value: <1>
 
? list.push(10)
? list.push(100)
? list
# value: <1, 10, 100>
 
? list.atFirst().insertAfter(5)
? list
# value: <1, 5, 10, 100>
 
? list.insertFirst(0)
? list
# value: <0, 1, 5, 10, 100>
 
? list.atLast().prev().remove()
? list
# value: <0, 1, 5, 100>
 
? list.atLast()[] := 10
? list
# value: <0, 1, 5, 10>
 
? for x in 11..20 { list.push(x) }
? list
# value: <0, 1, 5, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20>

Erlang[edit]

As with Singly-linked_list/Element_insertion a process is used to get mutability in Erlang's single assignment world.

 
-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}.
 

F#[edit]

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

Fortran[edit]

Tested with g95 and gfortran v. 4.6.

 
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
 
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[edit]

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.

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
}

Or, just use the container/list package:

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)
}
}

Haskell[edit]

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.

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

An example of use:

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

Icon and Unicon[edit]

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

 
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
 

An insert_before method was added to the DoubleLink class:

 
# 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
 

To test the double-linked list:

 
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
 
Output:
1
2
3
11
4
5
6
7
8
9
10

J[edit]

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[edit]

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.

 
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();
}
 
}
 
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[edit]

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

Julia[edit]

Regarding the avoidance or circular loops part of this task, a call to
show(DLNode)
reveals that Julia considers all of the nodes of doubly linked lists of this kind to contain circular references to their adjacent nodes.
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)
 
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[edit]

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

// 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)
}
Output:
1 -> 2 -> 3 -> 4

Lua[edit]

-- 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")
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[edit]

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).


 
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
 

Mathematica / Wolfram Language[edit]

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"]
Output:
{9, 1, 5, 14, 0, 3, 2, 4, 7}

Nim[edit]

Nim has a doubly linked list already in the lists module of the standard library.

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
Output:
15 -> 14 -> 12
hello -> world

Oberon-2[edit]

 
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;
 

Objeck[edit]

 
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);
}
}

Oforth[edit]

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

Phix[edit]

See Doubly-linked_list/Traversal for a complete example.

PicoLisp[edit]

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
        +-----+-----+
# 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))

For output of the example data, see Doubly-linked list/Traversal#PicoLisp.

PL/I[edit]

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

PowerShell[edit]

Create and populate the list:

 
$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
 
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:

 
$list.AddFirst([PSCustomObject]@{ID=123; X=123;Y=123}) | Out-Null
 
$list
 
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:

 
$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
 
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:

 
$list.AddLast([PSCustomObject]@{ID=789; X=789;Y=789}) | Out-Null
 
$list
 
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[edit]

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() <> ""
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[edit]

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[edit]

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.

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

Alternative[edit]

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.

 
"""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
 

Racket[edit]

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

 
#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))
 

Raku[edit]

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

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
Output:
0 1 2 40 41 42
42 41 40 2 1 0

REXX[edit]

        ╔═════════════════════════════════════════════════════════════════════════╗
        ║        ☼☼☼☼☼☼☼☼☼☼☼ 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.

/*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

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[edit]

 
# 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
 

Output:

9 1 5 7 6 0 3 2 4

Ruby[edit]

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

Scheme[edit]

Works with: R7RS


See also ATS.


(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")))
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[edit]

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)
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[edit]

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.

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
}
}
}
# 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}
}
}

Visual Basic .NET[edit]

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

Wren[edit]

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.

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)
Output:
[1 <-> 2 <-> 3]
[]

zkl[edit]

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))
}
}
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();
Output:
a  b  c  d  e  
e  d  c  b  a