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{{task|Data Structures}}[[Category:Iteration]]
[[Category:Iteration]]
Traverse from the beginning of a doubly-linked list to the end, and from the end to the beginning.
 
Traverse from the beginning of a [[Doubly-linked list/Definition|doubly-linked list]] to the end, and from the end to the beginning.
 
 
{{Template:See also lists}}
<br><br>
 
=={{header|Action!}}==
The user must type in the monitor the following command after compilation and before running the program!<pre>SET EndProg=*</pre>
{{libheader|Action! Tool Kit}}
<syntaxhighlight lang="action!">CARD EndProg ;required for ALLOCATE.ACT
 
INCLUDE "D2:ALLOCATE.ACT" ;from the Action! Tool Kit. You must type 'SET EndProg=*' from the monitor after compiling, but before running this program!
 
DEFINE PTR="CARD"
DEFINE NODE_SIZE="6"
TYPE ListNode=[INT data PTR prv,nxt]
 
ListNode POINTER listBegin,listEnd
 
PROC Append(INT 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 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 ForwardTraverse()
ListNode POINTER n
 
n=listBegin
PrintE("Forward traverse:")
Print("(")
WHILE n
DO
PrintI(n.data)
IF n.nxt THEN
Print(", ")
FI
n=n.nxt
OD
PrintE(")")
RETURN
 
PROC BackwardTraverse()
ListNode POINTER n
 
n=listEnd
PrintE("Backward traverse")
Print("(")
WHILE n
DO
PrintI(n.data)
IF n.prv THEN
Print(", ")
FI
n=n.prv
OD
PrintE(")")
RETURN
 
PROC Main()
INT i
Put(125) PutE() ;clear screen
AllocInit(0)
listBegin=0
listEnd=0
 
FOR i=0 TO 50
DO
Append(i*i)
OD
ForwardTraverse()
PutE()
BackwardTraverse()
 
Clear()
RETURN</syntaxhighlight>
{{out}}
[https://gitlab.com/amarok8bit/action-rosetta-code/-/raw/master/images/Doubly-linked_list_traversal.png Screenshot from Atari 8-bit computer]
<pre>
Forward traverse:
(0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361,
400, 441, 484, 529, 576, 625, 676, 729, 784, 841, 900, 961, 1024, 1089, 1156, 1225, 1296,
1369, 1444, 1521, 1600, 1681, 1764, 1849, 1936, 2025, 2116, 2209, 2304, 2401, 2500)
 
Backward traverse
(2500, 2401, 2304, 2209, 2116, 2025, 1936, 1849, 1764, 1681, 1600, 1521, 1444, 1369, 1296,
1225, 1156, 1089, 1024, 961, 900, 841, 784, 729, 676, 625, 576, 529, 484, 441, 400, 361,
324, 289, 256, 225, 196, 169, 144, 121, 100, 81, 64, 49, 36, 25, 16, 9, 4, 1, 0)
</pre>
 
=={{header|Ada}}==
{{works with|Ada 2005}}
<syntaxhighlight lang="ada">with Ada.Containers.Doubly_Linked_Lists;
with Ada.Text_IO;
 
procedure Traversing is
package Char_Lists is new Ada.Containers.Doubly_Linked_Lists (Character);
 
procedure Print (Position : in Char_Lists.Cursor) is
begin
Ada.Text_IO.Put (Char_Lists.Element (Position));
end Print;
 
My_List : Char_Lists.List;
begin
My_List.Append ('R');
My_List.Append ('o');
My_List.Append ('s');
My_List.Append ('e');
My_List.Append ('t');
My_List.Append ('t');
My_List.Append ('a');
 
My_List.Iterate (Print'Access);
Ada.Text_IO.New_Line;
 
My_List.Reverse_Iterate (Print'Access);
Ada.Text_IO.New_Line;
end Traversing;</syntaxhighlight>
 
=={{header|ALGOL 68}}==
LinkedList.alg:<syntaxhighlight lang="algol68"># Node struct - contains next and prev NODE pointers and DATA #
MODE NODE = STRUCT(
DATA data,
REF NODE prev,
REF NODE next
);
 
# List structure - contains head and tail NODE pointers #
MODE LIST = STRUCT(
REF NODE head,
REF NODE tail
);
 
# --- PREPEND - Adds a node to the beginning of the list ---#
PRIO PREPEND = 1;
OP PREPEND = (REF LIST list, DATA data) VOID:
(
HEAP NODE n := (data, NIL, NIL);
IF head OF list IS REF NODE(NIL) THEN
head OF list := tail OF list := n
ELSE
next OF n := head OF list;
prev OF head OF list := head OF list := n
FI
);
#--- APPEND - Adds a node to the end of the list ---#
PRIO APPEND = 1;
OP APPEND = (REF LIST list, DATA data) VOID:
(
HEAP NODE n := (data, NIL, NIL);
IF head OF list IS REF NODE(NIL) THEN
head OF list := tail OF list := n
ELSE
prev OF n := tail OF list;
next OF tail OF list := tail OF list := n
FI
);
 
#--- REMOVE_FIRST - removes & returns node at end of the list ---#
PRIO REMOVE_FIRST = 1;
OP REMOVE_FIRST = (REF LIST list) DATA:
(
IF head OF list ISNT REF NODE(NIL) THEN
DATA d := data OF head OF list;
prev OF next OF head OF list := NIL;
head OF list := next OF head OF list;
d # return d #
FI
);
#--- REMOVE_LAST: removes & returns node at front of list --- #
PRIO REMOVE_LAST = 1;
OP REMOVE_LAST = (REF LIST list) DATA:
(
IF head OF list ISNT REF NODE(NIL) THEN
DATA d := data OF tail OF list;
next OF prev OF tail OF list := NIL;
tail OF list := prev OF tail OF list;
d # return d #
FI
);
#--- PURGE - removes all elements from the list ---#
PRIO PURGE = 2;
OP PURGE = (REF LIST list) VOID:
(
head OF list := tail OF list := NIL
);
 
#--- returns the data at the end of the list ---#
PRIO LAST_IN = 2;
OP LAST_IN = (REF LIST list) DATA: (
IF head OF list ISNT REF NODE(NIL) THEN
data OF tail OF list
FI
);
 
#--- returns the data at the front of the list ---#
PRIO FIRST_IN = 2;
OP FIRST_IN = (REF LIST list) DATA: (
IF head OF list ISNT REF NODE(NIL) THEN
data OF head OF list
FI
);
 
#--- Traverses through the list forwards ---#
PROC forward traversal = (LIST list) VOID:
(
REF NODE travel := head OF list;
WHILE travel ISNT REF NODE(NIL) DO
list visit(data OF travel);
travel := next OF travel
OD
);
 
#--- Traverses through the list backwards ---#
PROC backward traversal = (LIST list) VOID:
(
REF NODE travel := tail OF list;
WHILE travel ISNT REF NODE(NIL) DO
list visit(data OF travel);
travel := prev OF travel
OD
)</syntaxhighlight>
 
main.alg:<syntaxhighlight lang="algol68">PR READ "LinkedList.alg" PR;
 
MODE EMPLOYEE = STRUCT(STRING name, INT salary, INT years);
MODE DATA = EMPLOYEE; #Sets the data type that is in the list#
 
# Function that traversals call for each node in list #
PROC list visit = (REF DATA data) VOID:
(
print((
"EMPLOYEE NAME : ", name OF data , newline,
" SALARY: " , salary OF data, newline,
" YEARS : " , years OF data, newline
))
);
 
#***************************************************************#
main:
(
EMPLOYEE empl;
name OF empl := "one";
salary OF empl := 100;
years OF empl := 10;
LIST list := (NIL, NIL);
 
list PREPEND empl;
name OF empl := "two";
salary OF empl := 200;
years OF empl := 20;
list APPEND empl;
name OF empl := "three";
salary OF empl := 300;
years OF empl := 30;
list APPEND empl;
salary OF empl := 400;
years OF empl := 40;
name OF empl := "four";
list APPEND empl;
 
forward traversal(list);
PURGE list;
forward traversal(list)
)</syntaxhighlight>
{{out}}
<pre>
EMPLOYEE NAME : one
SALARY: +100
YEARS : +10
EMPLOYEE NAME : two
SALARY: +200
YEARS : +20
EMPLOYEE NAME : three
SALARY: +300
YEARS : +30
EMPLOYEE NAME : four
SALARY: +400
YEARS : +40
</pre>
 
=={{header|ALGOL W}}==
Using the element type and insertion routine from the Doubly Linked List/Eleemnt Insertion task.
<syntaxhighlight lang="algolw">begin
% record type to hold an element of a doubly linked list of integers %
record DListIElement ( reference(DListIElement) prev
; integer iValue
; reference(DListIElement) next
);
% additional record types would be required for other element types %
% inserts a new element into the list, before e %
reference(DListIElement) procedure insertIntoDListIBefore( reference(DListIElement) value e
; integer value v
);
begin
reference(DListIElement) newElement;
newElement := DListIElement( null, v, e );
if e not = null then begin
% the element we are inserting before is not null %
reference(DListIElement) ePrev;
ePrev := prev(e);
prev(newElement) := ePrev;
prev(e) := newElement;
if ePrev not = null then next(ePrev) := newElement
end if_e_ne_null ;
newElement
end insertIntoDListiAfter ;
 
begin
reference(DListIElement) head, e, last;
head := null;
head := insertIntoDListIBefore( head, 1701 );
head := insertIntoDListIBefore( head, 9000 );
e := insertIntoDListIBefore( next(head), 90210 );
e := insertIntoDListIBefore( next(e), 4077 );
e := head;
last := null;
write( "Forward:" );
while e not = null do begin
write( i_w := 1, s_w := 0, " ", iValue(e) );
last := e;
e := next(e)
end while_e_ne_null ;
write( "Backward:" );
e := last;
while e not = null do begin
write( i_w := 1, s_w := 0, " ", iValue(e) );
last := e;
e := prev(e)
end while_e_ne_null
end
end.</syntaxhighlight>
{{out}}
<pre>
Forward:
9000
90210
4077
1701
Backward:
1701
4077
90210
9000
</pre>
 
=={{header|Applesoft BASIC}}==
<syntaxhighlight lang="gwbasic"> 100 REM BUILD THE LIST
110 FOR I = 20 TO 40 STEP 10
120 LET S$ = STR$ (I): GOSUB 260"APPEND"
130 NEXT I
140 REM TRAVERSE FORWARDS
150 LET N = HEAD
160 FOR Q = 1 TO 1
170 IF N < > NIL THEN PRINT S$(N)" ";:N = N(N):Q = 0
180 NEXT Q
190 PRINT
200 REM TRAVERSE BACKWARDS
210 LET N = TAIL
220 FOR Q = 1 TO 1
230 IF N < > NIL THEN PRINT S$(N)" ";:N = P(N):Q = 0
240 NEXT Q
250 END
260 REM APPEND S$
270 LET NIL = - 1
280 LET P(L) = NIL
290 LET N(L) = NIL
300 REM TRAVERSE UNTIL LAST NODE FOUND
310 FOR Q = 1 TO 1
320 IF N(N) < > NIL THEN N = N(N):Q = 0
330 NEXT Q
340 REM NEW NODE
350 LET S$(L) = S$
360 LET N(L) = NIL
370 IF N < > L THEN P(L) = N
380 REM POINT THE LAST NODE AT THE NEW NODE
390 IF N < > L THEN N(N) = L
400 LET TAIL = L
410 LET N = TAIL
420 LET L = L + 1
430 RETURN</syntaxhighlight>
=={{header|ARM Assembly}}==
{{works with|as|Raspberry Pi}}
<syntaxhighlight lang="arm assembly">
/* ARM assembly Raspberry PI */
/* program transDblList.s */
/* REMARK 1 : this program use routines in a include file
see task Include a file language arm assembly
for the routine affichageMess conversion10S
see at end of this program the instruction include */
 
/* 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"
szMessListInv: .asciz "Display list inverse :\n"
szCarriageReturn: .asciz "\n"
szMessErreur: .asciz "Error detected.\n"
/* datas message display */
szMessResult: .ascii "Result value :"
sValue: .space 12,' '
.asciz "\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
ldr r0,iAdrdllist1 @ list address
mov r1,#20 @ value to after insered
mov r2,#25 @ new value
bl insertAfter
cmp r0,#-1
beq 99f
ldr r0,iAdrdllist1
bl transHeadTail @ display value head to tail
ldr r0,iAdrszMessListInv
bl affichageMess
ldr r0,iAdrdllist1
bl transTailHead @ display value tail to head
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
iAdrszMessListInv: .int szMessListInv
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 @ 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
bl searchValue @ search node with 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
/******************************************************************/
/* transversal for head to tail */
/******************************************************************/
/* r0 contains the address of the list structure */
transHeadTail:
push {r2,lr} @ save registers
ldr r2,[r0,#dllist_head] @ load first node
1:
ldr r0,[r2,#NDlist_value]
ldr r1,iAdrsValue
bl conversion10S
ldr r0,iAdrszMessResult
bl affichageMess
ldr r2,[r2,#NDlist_next]
cmp r2,#0
bne 1b
100:
pop {r2,lr} @ restaur registers
bx lr @ return
iAdrszMessResult: .int szMessResult
iAdrsValue: .int sValue
/******************************************************************/
/* transversal for tail to head */
/******************************************************************/
/* r0 contains the address of the list structure */
transTailHead:
push {r2,lr} @ save registers
ldr r2,[r0,#dllist_tail] @ load last node
1:
ldr r0,[r2,#NDlist_value]
ldr r1,iAdrsValue
bl conversion10S
ldr r0,iAdrszMessResult
bl affichageMess
ldr r2,[r2,#NDlist_prev]
cmp r2,#0
bne 1b
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
/***************************************************/
/* ROUTINES INCLUDE */
/***************************************************/
.include "../affichage.inc"
 
</syntaxhighlight>
 
=={{header|AutoHotkey}}==
see [[Doubly-linked list/AutoHotkey]]
 
=={{header|Axe}}==
<syntaxhighlight lang="axe">LINK(L₁,1)→A
LINK(L₁+10,2)→B
LINK(L₁+50,3)→C
INSERT(A,B)
INSERT(A,C)
 
A→I
While I≠0
Disp VALUE(I)▶Dec,i
NEXT(I)→I
End
 
Disp "-----",i
 
B→I
While I≠0
Disp VALUE(I)▶Dec,i
PREV(I)→I
End</syntaxhighlight>
 
=={{header|BBC BASIC}}==
{{works with|BBC BASIC for Windows}}
<syntaxhighlight lang="bbcbasic"> DIM node{pPrev%, pNext%, iData%}
DIM a{} = node{}, b{} = node{}, c{} = node{}
a.pNext% = b{}
a.iData% = 123
b.pPrev% = a{}
b.iData% = 789
c.iData% = 456
PROCinsert(a{}, c{})
PRINT "Traverse forwards:"
pnode% = a{}
REPEAT
!(^node{}+4) = pnode%
PRINT node.iData%
pnode% = node.pNext%
UNTIL pnode% = 0
PRINT "Traverse backwards:"
pnode% = b{}
REPEAT
!(^node{}+4) = pnode%
PRINT node.iData%
pnode% = node.pPrev%
UNTIL pnode% = 0
END
DEF PROCinsert(here{}, new{})
LOCAL temp{} : DIM temp{} = node{}
new.pNext% = here.pNext%
new.pPrev% = here{}
!(^temp{}+4) = new.pNext%
temp.pPrev% = new{}
here.pNext% = new{}
ENDPROC
</syntaxhighlight>
Output:
<pre>Traverse forwards:
123
456
789
Traverse backwards:
789
456
123</pre>
 
=={{header|C}}==
<langsyntaxhighlight lang="c">// A doubly linked list of strings;
#include <stdio.h>
#include <stdlib.h>
Line 133 ⟶ 899:
//uhhh-- delete list??
return 0;
}</langsyntaxhighlight>
 
=={{header|JC sharp|C#}}==
<syntaxhighlight lang="csharp">using System;
using System.Collections.Generic;
 
namespace RosettaCode.DoublyLinkedList
<lang j>traverse=:1 :0
{
internal static class Program
{
private static void Main()
{
var list = new LinkedList<char>("hello");
 
var current = list.First;
do
{
Console.WriteLine(current.Value);
} while ((current = current.Next) != null);
 
Console.WriteLine();
 
current = list.Last;
do
{
Console.WriteLine(current.Value);
} while ((current = current.Previous) != null);
}
}
}</syntaxhighlight>
Output:
<pre>
h
e
l
l
o
 
o
l
l
e
h</pre>
 
=={{header|C++}}==
{{works with|C++11}}
<syntaxhighlight lang="cpp">#include <iostream>
#include <list>
 
int main ()
{
std::list<int> numbers {1, 5, 7, 0, 3, 2};
for(const auto& i: numbers)
std::cout << i << ' ';
std::cout << '\n';
}</syntaxhighlight>
{{out}}
<pre>
1 5 7 0 3 2
</pre>
 
=={{header|Clojure}}==
Given the definition in [[../Definition#Clojure]],
<syntaxhighlight lang="clojure">(def dl (double-list [:a :b :c :d]))
;=> #'user/dl
 
((juxt seq rseq) dl)
;=> [(:a :b :c :d) (:d :c :b :a)]
 
(take-while identity (iterate get-next (get-head dl)))
;=> (#:double_list.Node{:prev nil, :next #<Object...>, :data :a, :key #<Object...>}
;=> #:double_list.Node{:prev #<Object...>, :next #<Object...>, :data :b, :key #<Object...>}
;=> #:double_list.Node{:prev #<Object...>, :next #<Object...>, :data :c, :key #<Object...>}
;=> #:double_list.Node{:prev #<Object...>, :next nil, :data :d, :key #<Object...>})
(take-while identity (iterate get-prev (get-tail dl)))
 
;=> (#:double_list.Node{:prev #<Object...>, :next nil, :data :d, :key #<Object...>}
;=> #:double_list.Node{:prev #<Object...>, :next #<Object...>, :data :c, :key #<Object...>}
;=> #:double_list.Node{:prev #<Object...>, :next #<Object...>, :data :b, :key #<Object...>}
;=> #:double_list.Node{:prev nil, :next #<Object...>, :data :a, :key #<Object...>})</syntaxhighlight>
 
=={{header|D}}==
===Standard Doubly-linked List===
<syntaxhighlight lang="d">void main() {
import std.stdio, std.container, std.range;
 
auto dll = DList!dchar("DCBA"d.dup);
 
dll[].writeln;
dll[].retro.writeln;
}</syntaxhighlight>
{{out}}
<pre>ABCD
DCBA </pre>
 
===User-defined Doubly-linked list===
Same output.
<syntaxhighlight lang="d">struct Node(T) {
T data;
typeof(this)* prev, next;
}
 
void prepend(T)(ref Node!T* head, in T item) pure nothrow {
auto newNode = new Node!T(item, null, head);
if (head)
head.prev = newNode;
head = newNode;
}
 
void main() {
import std.stdio;
 
Node!char* head;
foreach (char c; "DCBA")
head.prepend(c);
 
auto last = head;
for (auto p = head; p; p = p.next) {
p.data.write;
last = p;
}
writeln;
 
for (auto p = last; p; p = p.prev)
p.data.write;
writeln;
}</syntaxhighlight>
 
=={{header|Delphi}}==
<syntaxhighlight lang="delphi">uses system ;
 
type
 
// declare the list pointer type
plist = ^List ;
 
// declare the list type, a generic data pointer prev and next pointers
List = record
data : pointer ;
prev : pList ;
next : pList ;
end;
 
// since this task is just showing the traversal I am not allocating the memory and setting up the root node etc.
// Note the use of the carat symbol for de-referencing the pointer.
 
begin
 
// beginning to end
while not (pList^.Next = NIL) do pList := pList^.Next ;
 
// end to beginning
while not (pList^.prev = NIL) do pList := pList^.prev ;
 
end;</syntaxhighlight>
 
=={{header|E}}==
{{incorrect|E|Doesn't work, probably due to a bug in the list definition: runs over the beginning of the list. Needs debugging.}}
Given the definition in [[../Definition#E]],
<syntaxhighlight lang="e">def traverse(list) {
var node := list.atFirst()
while (true) {
println(node[])
if (node.hasNext()) {
node := node.next()
} else {
break
}
}
while (true) {
println(node[])
if (node.hasPrev()) {
node := node.prev()
} else {
break
}
}
}</syntaxhighlight>
 
<syntaxhighlight lang="e">? def list := makeDLList()
# value: <>
 
? list.push(1)
? list.push(2)
? list.push(3)
 
? traverse(list)
1
2
3
3
2
1</syntaxhighlight>
 
=={{header|Erlang}}==
The task in [[Doubly-linked_list/Element_insertion]] uses traversal as proof that the insertion worked.
 
=={{header|F_Sharp|F#}}==
<syntaxhighlight lang="fsharp">
open System.Collections.Generic
 
let first (l: LinkedList<char>) = l.First
let last (l: LinkedList<char>) = l.Last
 
let next (l: LinkedListNode<char>) = l.Next
let prev (l: LinkedListNode<char>) = l.Previous
 
let traverse g f (ls: LinkedList<char>) =
let rec traverse (l: LinkedListNode<char>) =
match l with
| null -> ()
| _ ->
printf "%A" l.Value
traverse (f l)
traverse (g ls)
 
let traverseForward = traverse first next
let traverseBackward = traverse last prev
 
let cs = LinkedList(['a'..'z'])
 
traverseForward cs
printfn ""
traverseBackward cs
</syntaxhighlight>
{{out}}
<pre>
'a''b''c''d''e''f''g''h''i''j''k''l''m''n''o''p''q''r''s''t''u''v''w''x''y''z'
'z''y''x''w''v''u''t''s''r''q''p''o''n''m''l''k''j''i''h''g''f''e''d''c''b''a'
</pre>
 
=={{header|Fortran}}==
see [[Doubly-linked list/Definition#Fortran]]
 
 
=={{header|FreeBASIC}}==
<syntaxhighlight lang="freebasic">Dim As Integer i, MiLista()
 
For i = 0 To Int(Rnd * 100)+25
Redim Preserve MiLista(i)
MiLista(i) = Rnd * 314
Next
 
'Tour from the beginning
For i = Lbound(MiLista) To Ubound(MiLista)
Print MiLista(i)
Next i
 
Print
'Travel from the end
For i = Ubound(MiLista) To Lbound(MiLista) Step -1
Print MiLista(i)
Next i
Sleep</syntaxhighlight>
 
 
=={{header|Go}}==
Code is identical to that for task Doubly-linked list/Element insertion except for addition of section at the end of main noted "traverse from end to beginning." Traversal from beginning to end is adequately demonstrated by the String method of dlList.
 
Also note that there is a doubly linked list in the Go standard library in package container/list.
<syntaxhighlight lang="go">package main
 
import "fmt"
 
type dlNode struct {
string
next, prev *dlNode
}
 
type dlList struct {
head, tail *dlNode
}
 
func (list *dlList) String() string {
if list.head == nil {
return fmt.Sprint(list.head)
}
r := "[" + list.head.string
for p := list.head.next; p != nil; p = p.next {
r += " " + p.string
}
return r + "]"
}
 
func (list *dlList) insertTail(node *dlNode) {
if list.tail == nil {
list.head = node
} else {
list.tail.next = node
}
node.next = nil
node.prev = list.tail
list.tail = node
}
 
func (list *dlList) insertAfter(existing, insert *dlNode) {
insert.prev = existing
insert.next = existing.next
existing.next.prev = insert
existing.next = insert
if existing == list.tail {
list.tail = insert
}
}
 
func main() {
dll := &dlList{}
fmt.Println(dll)
a := &dlNode{string: "A"}
dll.insertTail(a)
dll.insertTail(&dlNode{string: "B"})
fmt.Println(dll)
dll.insertAfter(a, &dlNode{string: "C"})
fmt.Println(dll)
 
// traverse from end to beginning
fmt.Print("From tail:")
for p := dll.tail; p != nil; p = p.prev {
fmt.Print(" ", p.string)
}
fmt.Println("")
}</syntaxhighlight>
Output:
<pre>
<nil>
[A B]
[A C B]
From tail: B C A
</pre>
 
=={{header|Groovy}}==
<syntaxhighlight lang="groovy">class DoubleLinkedListTraversing {
static void main(args) {
def linkedList = (1..9).collect() as LinkedList
 
linkedList.each {
print it
}
 
println()
 
linkedList.reverseEach {
print it
}
}
}</syntaxhighlight>
 
{{out}}
<pre>123456789
987654321</pre>
 
=={{header|Haskell}}==
<syntaxhighlight lang="haskell">main = print . traverse True $ create [10,20,30,40]
 
data DList a = Leaf | Node { prev::(DList a), elt::a, next::(DList a) }
 
create = go Leaf
where go _ [] = Leaf
go prev (x:xs) = current
where current = Node prev x next
next = go current xs
 
isLeaf Leaf = True
isLeaf _ = False
 
lastNode Leaf = Leaf
lastNode dl = until (isLeaf.next) next dl
 
traverse _ Leaf = []
traverse True (Node l v Leaf) = v : v : traverse False l
traverse dir (Node l v r) = v : traverse dir (if dir then r else l)</syntaxhighlight>
 
==Icon and {{header|Unicon}}==
Uses Unicon classes.
<syntaxhighlight lang="unicon">class DoubleLink (value, prev_link, next_link)
 
# insert given node after this one, removing its existing connections
method insert_after (node)
node.prev_link := self
if (\next_link) then next_link.prev_link := node
node.next_link := next_link
self.next_link := node
end
 
# use a generator to traverse
# - keep suspending the prev/next link until a null node is reached
method traverse_backwards ()
current := self
while \current do {
suspend current
current := current.prev_link
}
end
 
method traverse_forwards ()
current := self
while \current do {
suspend current
current := current.next_link
}
end
 
initially (value, prev_link, next_link)
self.value := value
self.prev_link := prev_link # links are 'null' if not given
self.next_link := next_link
end
 
procedure main ()
l1 := DoubleLink (1)
l2 := DoubleLink (2)
l1.insert_after (l2)
l1.insert_after (DoubleLink (3))
 
write ("Traverse from beginning to end")
every (node := l1.traverse_forwards ()) do
write (node.value)
 
write ("Traverse from end to beginning")
every (node := l2.traverse_backwards ()) do
write (node.value)
end</syntaxhighlight>
 
Output:
<pre>
Traverse from beginning to end
1
3
2
Traverse from end to beginning
2
3
1
</pre>
 
=={{header|J}}==
<syntaxhighlight lang="j">traverse=:1 :0
work=. result=. conew 'DoublyLinkedListHead'
current=.successor__y y
while.current y ~: ycurrent=. successor__current do.
work=. (work;result;<u data__current) conew 'DoublyLinkedListElement'
end.
result
)</langsyntaxhighlight>
 
This traverses a doubly linked list, applying the verb u to the data in each list element and creates a new doubly linked list containing the results. A reference to the new doubly linked list is returned.
 
=={{header|Java}}==
{{works with|Java|8 or higher}}
<lang java>import java.util.LinkedList;
 
<syntaxhighlight lang="java">
public static void main(){
package com.rosettacode;
LinkedList<String> LL = new LinkedList<String>();
traverse(LL.iterator());
traverse(LL.descendingIterator());
}
 
import java.util.LinkedList;
private static void traverse(Iterator<String> iter){
import java.util.stream.Collectors;
while(iter.hasNext()){
import java.util.stream.IntStream;
iter.next();
 
}
public class DoubleLinkedListTraversing {
}</lang>
 
public static void main(String[] args) {
 
final LinkedList<String> doubleLinkedList =
IntStream.range(1, 10)
.mapToObj(String::valueOf)
.collect(Collectors.toCollection(LinkedList::new));
 
doubleLinkedList.iterator().forEachRemaining(System.out::print);
System.out.println();
doubleLinkedList.descendingIterator().forEachRemaining(System.out::print);
}
}</syntaxhighlight>
 
{{out}}
<pre>123456789
987654321</pre>
 
=={{header|JavaScript}}==
See [[Doubly-Linked List (element)#JavaScript]]. The <code>traverse()</code> and <code>print()</code> functions have been inherited from [[Singly-Linked List (traversal)#JavaScript]].
<langsyntaxhighlight lang="javascript">DoublyLinkedList.prototype.getTail = function() {
var tail;
this.traverse(function(node){tail = node;});
Line 181 ⟶ 1,394:
var head = createDoublyLinkedListFromArray([10,20,30,40]);
head.print();
head.getTail().printBackward();</langsyntaxhighlight>
 
outputs:
Line 192 ⟶ 1,405:
20
10</pre>
 
Uses the <code>print()</code> function from [[Rhino]] or [[SpiderMonkey]].
 
=={{header|HaskellJulia}}==
<syntaxhighlight lang="julia">mutable struct DLNode{T}
<lang haskell>
value::T
main = print . traverse True $ create [10,20,30,40]
pred::Union{DLNode{T}, Nothing}
succ::Union{DLNode{T}, Nothing}
DLNode(v) = new{typeof(v)}(v, nothing, nothing)
end
 
function insertpost(prevnode, node)
data DList a = Leaf | Node (DList a) a (DList a)
succ = prevnode.succ
prevnode.succ = node
node.pred = prevnode
node.succ = succ
if succ != nothing
succ.pred = node
end
node
end
 
first(nd) = (while nd.pred != nothing nd = nd.prev end; nd)
create = worker Leaf
last(nd) = (while nd.succ != nothing nd = nd.succ end; nd)
where worker _ [] = Leaf
worker prev (x:xs) = current
where current = Node prev x next
next = worker current xs
 
function printconnected(nd; fromtail = false)
traverse _ Leaf = []
if fromtail
traverse True (Node l v Leaf) = v : v : traverse False l
nd = last(nd)
traverse dir (Node l v r) = v : traverse dir (if dir then r else l)
print(nd.value)
</lang>
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)
insertpost(node1, node2)
insertpost(node2, node3)
print("From beginning to end: "); printconnected(node1)
print("From end to beginning: "); printconnected(node1, fromtail = true)
</syntaxhighlight> {{output}} <pre>
From beginning to end: 1 -> 2 -> 3
From end to beginning: 3 -> 2 -> 1
</pre>
 
=={{header|Kotlin}}==
To complete this task, we just need to add a couple of traversal functions to the class we defined in the [[Doubly-linked list/Definition]] task:
<syntaxhighlight lang="scala">// version 1.1.2
 
class LinkedList<E> {
class Node<E>(var data: E, var prev: Node<E>? = null, var next: Node<E>? = null) {
override fun toString(): String {
val sb = StringBuilder(this.data.toString())
var node = this.next
while (node != null) {
sb.append(" -> ", node.data.toString())
node = node.next
}
return sb.toString()
}
}
 
var first: Node<E>? = null
var last: Node<E>? = null
 
fun addFirst(value: E) {
if (first == null) {
first = Node(value)
last = first
}
else {
val node = first!!
first = Node(value, null, node)
node.prev = first
}
}
 
fun addLast(value: E) {
if (last == null) {
last = Node(value)
first = last
}
else {
val node = last!!
last = Node(value, node, null)
node.next = last
}
}
 
fun insert(after: Node<E>?, value: E) {
if (after == null)
addFirst(value)
else if (after == last)
addLast(value)
else {
val next = after.next
val new = Node(value, after, next)
after.next = new
if (next != null) next.prev = new
}
}
 
override fun toString() = first.toString()
 
fun firstToLast() = first?.toString() ?: ""
 
fun lastToFirst(): String {
if (last == null) return ""
val sb = StringBuilder(last.toString())
var node = last!!.prev
while (node != null) {
sb.append(" -> ", node.data.toString())
node = node.prev
}
return sb.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("First to last : ${ll.firstToLast()}")
println("Last to first : ${ll.lastToFirst()}")
}</syntaxhighlight>
 
{{out}}
<pre>
First to last : 1 -> 2 -> 3 -> 4
Last to first : 4 -> 3 -> 2 -> 1
</pre>
 
=={{header|Lua}}==
Begin with this: [[Doubly-linked_list/Definition#Lua]], then extend with this:
<syntaxhighlight lang="lua">--------------
-- TRAVERSAL:
--------------
List.iterateForward = function(self)
local function iter(self, node)
if node then return node.next else return self.head end
end
return iter, self, nil
end
List.iterateReverse = function(self)
local function iter(self, node)
if node then return node.prev else return self.tail end
end
return iter, self, nil
end
 
---------
-- TEST:
---------
local list = List()
for i = 1, 5 do list:insertTail(i) end
io.write("Forward: ") for node in list:iterateForward() do io.write(node.data..",") end print()
io.write("Reverse: ") for node in list:iterateReverse() do io.write(node.data..",") end print()</syntaxhighlight>
{{out}}
<pre>Forward: 1,2,3,4,5,
Reverse: 5,4,3,2,1,</pre>
 
=={{header|Liberty BASIC}}==
<syntaxhighlight lang="lb">
struct block,nxt as ulong,prev as ulong,nm as char[20],age as long'Our structure of the blocks in our list.
 
global hHeap
global hFirst
global hLast
global blockCount
global blockSize
blockSize=len(block.struct)
 
 
call Init
if hHeap=0 then
print "Error occured! Could not create heap, exiting..."
end
end if
 
FirstUser=New("David",20)
notImportant=New("Jessica",35)
notImportant=New("Joey",38)
MiddleUser=New("Jack",56)
notImportant=New("Amy",17)
notImportant=New("Bob",28)
LastUser=New("Kenny",56)
 
 
print "-Traversing the list forwards"
 
hCurrent=hFirst
while hCurrent<>0
print tab(2);dechex$(hCurrent);" ";Block.name$(hCurrent);" ";Block.age(hCurrent)
hCurrent=Block.next(hCurrent)
wend
 
print
print "-Deleting first, middle, and last person."
 
call Delete FirstUser'1
call Delete MiddleUser'2
call Delete LastUser'3
 
print
print "-Traversing the list backwards"
hCurrent=hLast
while hCurrent<>0
print tab(2);dechex$(hCurrent);" ";Block.name$(hCurrent);" ";Block.age(hCurrent)
hCurrent=Block.prev(hCurrent)
wend
 
call Uninit
 
end
 
 
function Block.next(hBlock)
calldll #kernel32,"RtlMoveMemory",block as struct,hBlock as ulong,blockSize as long,ret as void
Block.next=block.nxt.struct
end function
 
function Block.prev(hBlock)
calldll #kernel32,"RtlMoveMemory",block as struct,hBlock as ulong,blockSize as long,ret as void
Block.prev=block.prev.struct
end function
 
function Block.age(hBlock)
calldll #kernel32,"RtlMoveMemory",block as struct,hBlock as ulong,blockSize as long,ret as void
Block.age=block.age.struct
end function
 
function Block.name$(hBlock)
calldll #kernel32,"RtlMoveMemory",block as struct,hBlock as ulong,blockSize as long,ret as void
Block.name$=block.nm.struct
end function
 
sub Block.age hBlock,age
calldll #kernel32,"RtlMoveMemory",block as struct,hBlock as ulong,blockSize as long,ret as void
block.age.struct=age
calldll #kernel32,"RtlMoveMemory",hBlock as ulong,block as struct,blockSize as long,ret as void
end sub
 
sub Block.name hBlock,name$
calldll #kernel32,"RtlMoveMemory",block as struct,hBlock as ulong,blockSize as long,ret as void
block.nm.struct=name$
calldll #kernel32,"RtlMoveMemory",hBlock as ulong,block as struct,blockSize as long,ret as void
end sub
 
sub Block.next hBlock,nxt
calldll #kernel32,"RtlMoveMemory",block as struct,hBlock as ulong,blockSize as long,ret as void
block.nxt.struct=nxt
calldll #kernel32,"RtlMoveMemory",hBlock as ulong,block as struct,blockSize as long,ret as void
end sub
 
sub Block.prev hBlock,prev
calldll #kernel32,"RtlMoveMemory",block as struct,hBlock as ulong,blockSize as long,ret as void
block.prev.struct=prev
calldll #kernel32,"RtlMoveMemory",hBlock as ulong,block as struct,blockSize as long,ret as void
end sub
 
function New(name$,age)
calldll #kernel32,"HeapAlloc",hHeap as ulong,_HEAP_ZERO_MEMORY as ulong,blockSize as long,New as ulong
if New<>0 then
blockCount=blockCount+1
if hFirst=0 then
hFirst=New
hLast=New
else
call Block.next hLast,New
call Block.prev New,hLast
hLast=New
end if
call Block.name New,name$
call Block.age New,age
end if
end function
 
sub Delete hBlock
if hBlock<>0 then
blockCount=blockCount-1
if blockCount=0 then
hFirst=0
hLast=0
else
if hBlock=hFirst then
hFirst=Block.next(hBlock)
call Block.prev hFirst,0
else
if hBlock=hLast then
hLast=Block.prev(hBlock)
call Block.next hLast,0
else
call Block.next Block.prev(hBlock),Block.next(hBlock)
call Block.prev Block.next(hBlock),Block.prev(hBlock)
end if
end if
end if
calldll #kernel32,"HeapFree",hHeap as ulong,0 as long,hBlock as ulong,ret as void
end if
end sub
 
 
sub Init
calldll #kernel32,"HeapCreate",0 as long,10000 as long,0 as long,hHeap as ulong
end sub
 
sub Uninit
calldll #kernel32,"HeapDestroy",hHeap as ulong,ret as void
end sub
</syntaxhighlight>
 
=={{header|Nim}}==
<syntaxhighlight lang="nim">type
List[T] = object
head, tail: Node[T]
 
Node[T] = ref TNode[T]
 
TNode[T] = object
next, prev: Node[T]
data: T
 
proc initList[T](): List[T] = discard
 
proc newNode[T](data: T): Node[T] =
new(result)
result.data = data
 
proc prepend[T](l: var List[T], n: Node[T]) =
n.next = l.head
if l.head != nil: l.head.prev = n
l.head = n
if l.tail == nil: l.tail = n
 
proc append[T](l: var List[T], n: Node[T]) =
n.next = nil
n.prev = l.tail
if l.tail != nil:
l.tail.next = n
l.tail = n
if l.head == nil:
l.head = n
 
proc insertAfter[T](l: var List[T], r, n: Node[T]) =
n.prev = r
n.next = r.next
n.next.prev = n
r.next = n
if r == l.tail: l.tail = n
 
proc remove[T](l: var List[T], n: Node[T]) =
if n == l.tail: l.tail = n.prev
if n == l.head: l.head = n.next
if n.next != nil: n.next.prev = n.prev
if n.prev != nil: n.prev.next = n.next
 
proc `$`[T](l: var List[T]): string =
result = ""
var n = l.head
while n != nil:
if result.len > 0: result.add(" -> ")
result.add($n.data)
n = n.next
 
iterator traverseForward[T](l: List[T]): T =
var n = l.head
while n != nil:
yield n.data
n = n.next
 
iterator traverseBackward[T](l: List[T]): T =
var n = l.tail
while n != nil:
yield n.data
n = n.prev
 
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)
 
for i in l.traverseForward():
echo "> ", i
 
for i in l.traverseBackward():
echo "< ", i</syntaxhighlight>
 
=={{header|Oberon-2}}==
<syntaxhighlight lang="oberon2">
MODULE Collections;
IMPORT Box;
 
TYPE
Action = PROCEDURE (o: Box.Object);
 
PROCEDURE (dll: DLList) GoForth*(do: Action);
VAR
iter: Node;
BEGIN
iter := dll.first;
WHILE iter # NIL DO
do(iter.value);
iter := iter.next
END
END GoForth;
 
PROCEDURE (dll: DLList) GoBack*(do: Action);
VAR
iter: Node;
BEGIN
ASSERT(dll.last # NIL);
iter := dll.last;
WHILE iter # NIL DO
do(iter.value);
iter := iter.prev
END
END GoBack;
 
END Collections.
</syntaxhighlight>
 
=={{header|Objeck}}==
<syntaxhighlight lang="objeck">
class Traverse {
function : Main(args : String[]) ~ Nil {
list := Collection.IntList->New();
list->Insert(100);
list->Insert(50);
list->Insert(25);
list->Insert(10);
list->Insert(5);
 
"-- forward --"->PrintLine();
list->Rewind();
while(list->More()) {
list->Get()->PrintLine();
list->Next();
};
 
"-- backward --"->PrintLine();
list->Forward();
while(list->More()) {
list->Get()->PrintLine();
list->Previous();
};
}
}
</syntaxhighlight>
 
<pre>
-- forward --
100
5
10
25
50
-- backward --
50
25
10
5
100
</pre>
 
=={{header|Oforth}}==
 
Complete definition is here : [[../Definition#Oforth]]
 
Defining #forEachNext and #forEachPrev allow to traverse this double linked list using #forEach: and #revEach: syntax
 
<syntaxhighlight lang="oforth">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
| dl n |
DList new ->dl
dl insertFront("A")
dl insertBack("B")
dl head insertAfter(DNode new("C", null , null))
 
"Traversal (beginning to end) : " println
dl forEach: n [ n . ]
 
"\nTraversal (end to beginning) : " println
dl revEach: n [ n . ] ;</syntaxhighlight>
 
{{out}}
<pre>
>test
Traversal (beginning to end) :
A C B
Traversal (end to beginning) :
B C A ok
</pre>
 
=={{header|Oz}}==
Warning: Highly unidiomatic code. (Use built-in lists instead.)
<langsyntaxhighlight lang="oz">declare
proc {Walk Node Action}
case Node of nil then skip
Line 267 ⟶ 1,978:
{InsertAfter A C}
{Walk A Show}
{WalkBackwards A Show}</langsyntaxhighlight>
 
=={{header|Pascal}}==
See [[Doubly-linked_list/Traversal#Delphi | Delphi]]
 
=={{header|Phix}}==
<!--<syntaxhighlight lang="phix">(phixonline)-->
<span style="color: #008080;">enum</span> <span style="color: #000000;">NEXT</span><span style="color: #0000FF;">,</span><span style="color: #000000;">PREV</span><span style="color: #0000FF;">,</span><span style="color: #000000;">DATA</span>
<span style="color: #008080;">constant</span> <span style="color: #000000;">empty_dll</span> <span style="color: #0000FF;">=</span> <span style="color: #0000FF;">{{</span><span style="color: #000000;">1</span><span style="color: #0000FF;">,</span><span style="color: #000000;">1</span><span style="color: #0000FF;">}}</span>
<span style="color: #004080;">sequence</span> <span style="color: #000000;">dll</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">deep_copy</span><span style="color: #0000FF;">(</span><span style="color: #000000;">empty_dll</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">procedure</span> <span style="color: #000000;">insert_after</span><span style="color: #0000FF;">(</span><span style="color: #004080;">object</span> <span style="color: #000000;">data</span><span style="color: #0000FF;">,</span> <span style="color: #004080;">integer</span> <span style="color: #000000;">pos</span><span style="color: #0000FF;">=</span><span style="color: #000000;">1</span><span style="color: #0000FF;">)</span>
<span style="color: #004080;">integer</span> <span style="color: #000000;">prv</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">dll</span><span style="color: #0000FF;">[</span><span style="color: #000000;">pos</span><span style="color: #0000FF;">][</span><span style="color: #000000;">PREV</span><span style="color: #0000FF;">]</span>
<span style="color: #000000;">dll</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">append</span><span style="color: #0000FF;">(</span><span style="color: #000000;">dll</span><span style="color: #0000FF;">,{</span><span style="color: #000000;">pos</span><span style="color: #0000FF;">,</span><span style="color: #000000;">prv</span><span style="color: #0000FF;">,</span><span style="color: #000000;">data</span><span style="color: #0000FF;">})</span>
<span style="color: #008080;">if</span> <span style="color: #000000;">prv</span><span style="color: #0000FF;">!=</span><span style="color: #000000;">0</span> <span style="color: #008080;">then</span>
<span style="color: #000000;">dll</span><span style="color: #0000FF;">[</span><span style="color: #000000;">prv</span><span style="color: #0000FF;">][</span><span style="color: #000000;">NEXT</span><span style="color: #0000FF;">]</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">length</span><span style="color: #0000FF;">(</span><span style="color: #000000;">dll</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">if</span>
<span style="color: #000000;">dll</span><span style="color: #0000FF;">[</span><span style="color: #000000;">pos</span><span style="color: #0000FF;">][</span><span style="color: #000000;">PREV</span><span style="color: #0000FF;">]</span> <span style="color: #0000FF;">=</span> <span style="color: #7060A8;">length</span><span style="color: #0000FF;">(</span><span style="color: #000000;">dll</span><span style="color: #0000FF;">)</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">procedure</span>
<span style="color: #000000;">insert_after</span><span style="color: #0000FF;">(</span><span style="color: #008000;">"ONE"</span><span style="color: #0000FF;">)</span>
<span style="color: #000000;">insert_after</span><span style="color: #0000FF;">(</span><span style="color: #008000;">"TWO"</span><span style="color: #0000FF;">)</span>
<span style="color: #000000;">insert_after</span><span style="color: #0000FF;">(</span><span style="color: #008000;">"THREE"</span><span style="color: #0000FF;">)</span>
<span style="color: #0000FF;">?</span><span style="color: #000000;">dll</span>
<span style="color: #008080;">procedure</span> <span style="color: #000000;">show</span><span style="color: #0000FF;">(</span><span style="color: #004080;">integer</span> <span style="color: #000000;">d</span><span style="color: #0000FF;">)</span>
<span style="color: #004080;">integer</span> <span style="color: #000000;">idx</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">dll</span><span style="color: #0000FF;">[</span><span style="color: #000000;">1</span><span style="color: #0000FF;">][</span><span style="color: #000000;">d</span><span style="color: #0000FF;">]</span>
<span style="color: #008080;">while</span> <span style="color: #000000;">idx</span><span style="color: #0000FF;">!=</span><span style="color: #000000;">1</span> <span style="color: #008080;">do</span>
<span style="color: #0000FF;">?</span><span style="color: #000000;">dll</span><span style="color: #0000FF;">[</span><span style="color: #000000;">idx</span><span style="color: #0000FF;">][</span><span style="color: #000000;">DATA</span><span style="color: #0000FF;">]</span>
<span style="color: #000000;">idx</span> <span style="color: #0000FF;">=</span> <span style="color: #000000;">dll</span><span style="color: #0000FF;">[</span><span style="color: #000000;">idx</span><span style="color: #0000FF;">][</span><span style="color: #000000;">d</span><span style="color: #0000FF;">]</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">while</span>
<span style="color: #008080;">end</span> <span style="color: #008080;">procedure</span>
<span style="color: #000000;">show</span><span style="color: #0000FF;">(</span><span style="color: #000000;">NEXT</span><span style="color: #0000FF;">)</span>
<span style="color: #0000FF;">?</span><span style="color: #008000;">"=="</span>
<span style="color: #000000;">show</span><span style="color: #0000FF;">(</span><span style="color: #000000;">PREV</span><span style="color: #0000FF;">)</span>
<!--</syntaxhighlight>-->
{{out}}
<pre>
{{2,4},{3,1,"ONE"},{4,2,"TWO"},{1,3,"THREE"}}
"ONE"
"TWO"
"THREE"
"=="
"THREE"
"TWO"
"ONE"
</pre>
 
=={{header|PicoLisp}}==
<syntaxhighlight lang="picolisp"># Print the elements a doubly-linked list
(de 2print (DLst)
(for (L (car DLst) L (cddr L))
(printsp (car L)) )
(prinl) )
 
# Print the elements a doubly-linked list in reverse order
(de 2printReversed (DLst)
(for (L (cdr DLst) L (cadr L))
(printsp (car L)) )
(prinl) )</syntaxhighlight>
Output for the example data produced in
[[Doubly-linked list/Definition#PicoLisp]] and
[[Doubly-linked list/Element definition#PicoLisp]]:
<pre>: (2print *DLst) # Print the list
not was it a cat I saw
 
: (2printReversed *DLst) # Print it in reversed order
saw I cat a it was not</pre>
Output for the example data produced in
[[Doubly-linked list/Element insertion#PicoLisp]]:
<pre>: (2print *DL) # Print the list
A C B
 
: (2printReversed *DL) # Print it in reversed order
B C A</pre>
 
=={{header|PL/I}}==
<syntaxhighlight lang="pl/i">/* To implement a doubly-linked list -- i.e., a 2-way linked list. */
<lang PL/I>
/* To implement a doubly-linked list -- i.e., a 2-way linked list. */
doubly_linked_list: proc options (main);
 
Line 303 ⟶ 2,088:
t => back_pointer = tail; /* Point the new tail back to the old */
/* tail. */
tail = t; /* Point at tehthe new tail. */
tail => fwd_pointer = bind(:node, null:);
/* Set the tail link to NULL */
Line 327 ⟶ 2,112:
t = t => back_pointer;
end;
end doubly_linked_list;</syntaxhighlight>
</lang>
 
Output:
<pre>
 
<lang>
In a forwards direction, the list has:
1
Line 354 ⟶ 2,136:
3
2
1 </pre>
</lang>
 
=={{header|PicoLisp}}==
<lang PicoLisp># Print the elements a doubly-linked list
(de 2print (DLst)
(for (L (car DLst) L (cddr L))
(printsp (car L)) )
(prinl) )
 
# Print the elements a doubly-linked list in reverse order
(de 2printReversed (DLst)
(for (L (cdr DLst) L (cadr L))
(printsp (car L)) )
(prinl) )</lang>
Output for the example data produced in
[[Doubly-linked list/Definition#PicoLisp]] and
[[Doubly-linked list/Element definition#PicoLisp]]:
<pre>: (2print *DLst) # Print the list
not was it a cat I saw
 
: (2printReversed *DLst) # Print it in reversed order
saw I cat a it was not</pre>
Output for the example data produced in
[[Doubly-linked list/Element insertion#PicoLisp]]:
<pre>: (2print *DL) # Print the list
A C B
 
: (2printReversed *DL) # Print it in reversed order
B C A</pre>
 
=={{header|PureBasic}}==
<langsyntaxhighlight PureBasiclang="purebasic">NewList MyData.i() ; Create a double linked list holding a single value (integer)
 
;Set up a randomly long linked list in the range 25-125 elements
Line 404 ⟶ 2,157:
Repeat
Debug MyData() ; Present the value in the current cell
Until Not PreviousElement(MyData())</langsyntaxhighlight>
 
=={{header|Python}}==
This provides two solutions. One that explicitly builds a linked list and traverses it two ways, and another which uses pythons combined list/array class. Unless two lists explicitly needed to be spliced together in O(1) time, or a double linked list was needed for some other reason, most python programmers would probably use the second solution.
<syntaxhighlight lang="python">class List:
def __init__(self, data, next=None, prev=None):
self.data = data
self.next = next
self.prev = prev
 
def append(self, data):
if self.next == None:
self.next = List(data, None, self)
return self.next
else:
return self.next.append(data)
 
# Build the list
tail = head = List(10)
for i in [ 20, 30, 40 ]:
tail = tail.append(i)
 
# Traverse forwards
node = head
while node != None:
print(node.data)
node = node.next
 
# Traverse Backwards
node = tail
while node != None:
print(node.data)
node = node.prev</syntaxhighlight>
 
This produces the desired output. However, I expect most python programmers would do the following instead:
 
<syntaxhighlight lang="python">l = [ 10, 20, 30, 40 ]
for i in l:
print(i)
for i in reversed(l): # reversed produces an iterator, so only O(1) memory is used
print(i)</syntaxhighlight>
 
Double-ended queues in python are provided by the collections.deque class and the array/list type can perform all the operations of a C++ vector (and more), so building one's own doubly-linked list would be restricted to very specialized situations.
 
=={{header|Racket}}==
See [[Doubly-Linked List]] for the structure definitions.
 
These functions traverse the list in the two directions.
They create a native (singly-linked) list by adding to the front, so they traverse in the reverse order from the desired result order.
<syntaxhighlight lang="racket">(define (dlist-elements dlist)
(let loop ([elements '()] [dlink (dlist-tail dlist)])
(if dlink
(loop (cons (dlink-content dlink) elements) (dlink-prev dlink))
elements)))
 
(define (dlist-elements/reverse dlist)
(let loop ([elements '()] [dlink (dlist-head dlist)])
(if dlink
(loop (cons (dlink-content dlink) elements) (dlink-next dlink))
elements)))</syntaxhighlight>
 
=={{header|Raku}}==
(formerly Perl 6)
 
Since the list routines are supplied by the generic roles defined in [[Doubly-linked_list/Definition#Raku]], it suffices to say:
<syntaxhighlight lang="raku" line>say $dll.list;
say $dll.reverse;</syntaxhighlight>
These automatically return just the payloads, hiding the elements containing the forward and backward pointers.
 
=={{header|REXX}}==
REXX doesn't have linked lists, as there are no pointers (or handles).
However, linked lists can be simulated with lists in REXX.
<syntaxhighlight lang="rexx">/*REXX program that implements various List Manager functions. */
/*┌────────────────────────────────────────────────────────────────────┐
┌─┘ Functions of the List Manager └─┐
│ │
│ @init ─── initializes the List. │
│ │
│ @size ─── returns the size of the List [could be 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. │
└─┐ ┌─┘
└────────────────────────────────────────────────────────────────────┘*/
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 6th & 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 done.*/
/*──────────────────────────────────subroutines─────────────────────────*/
p: return word(arg(1),1)
sy: say; say left('',30) "───" arg(1) '───'; return
@hasopt: arg o; return pos(o,opt)\==0
@size: return $.#
@init: $.@=''; $.#=0; return 0
@adjust: $.@=space($.@); $.#=words($.@); return 0
 
@parms: arg opt
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
 
@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 0
 
@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(_)
 
@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 0
 
@del: procedure expose $.; arg k,m
call @parms 'km'
_=subword($.@,k,k-1) subword($.@,k+m)
$.@=_
call @adjust
return</syntaxhighlight>
'''output'''
<pre style="height:30ex;overflow:scroll">
─── initializing the list. ───
 
─── building list: Was it a cat I saw ───
 
─── displaying list size. ───
list size=6
 
─── forward list ───
Was it a cat I saw
 
─── backward list ───
saw I cat a it Was
 
─── showing 4th item ───
cat
 
─── showing 6th & 6th items ───
I saw
 
─── adding item before item 4: black ───
 
─── showing list ───
Was it a black cat I saw
 
─── adding to tail: there, in the ... ───
 
─── showing list ───
Was it a black cat I saw there, in the shadows, stalking its prey (and next meal
 
─── adding to head: Oy! ───
 
─── showing list ───
Oy! Was it a black cat I saw there, in the shadows, stalking its prey (and next
</pre>
 
=={{header|Ring}}==
<syntaxhighlight lang="ring">
# Project : Doubly-linked list/Traversal
 
trav = [123, 789, 456]
travfor = sort(trav)
see "Traverse forwards:" + nl
see travfor
see nl
travback = reverse(travfor)
see "Traverse backwards:" + nl
see travback
see nl
</syntaxhighlight>
Output:
<pre>
Traverse forwards:
123
456
789
 
Traverse backwards:
789
456
123
</pre>
 
=={{header|Ruby}}==
<langsyntaxhighlight lang="ruby">class DListNode
def get_tail
# parent class (ListNode) includes Enumerable, so the find method is available to us
Line 421 ⟶ 2,387:
head = DListNode.from_array([:a, :b, :c])
head.each {|node| p node.value}
head.get_tail.each_previous {|node| p node.value}</langsyntaxhighlight>
 
=={{header|Salmon}}==
Without explicit type information:
<syntaxhighlight lang="salmon">class item(init_data)
{
variable next,
previous,
data := init_data;
};
 
function prepend(tail, new_data)
{
immutable result := item(new_data);
result.next := tail;
result.previous := null;
if (tail != null)
tail.previous := result;;
return result;
};
 
variable my_list := null;
my_list := prepend(my_list, "R");
my_list := prepend(my_list, "o");
my_list := prepend(my_list, "s");
my_list := prepend(my_list, "e");
my_list := prepend(my_list, "t");
my_list := prepend(my_list, "t");
my_list := prepend(my_list, "a");
 
"Items in the list going forward:"!
variable follow := my_list;
while (true)
{
follow.data!
if (follow.next == null)
break;;
}
step
follow := follow.next;;
 
"Items in the list going backward:"!
while (follow != null)
follow.data!
step
follow := follow.previous;;</syntaxhighlight>
 
With explicit type information:
<syntaxhighlight lang="salmon">class item(init_data : string)
{
variable next: item | {null},
previous : item | {null},
data : string := init_data;
};
 
function prepend(tail : item | {null}, new_data : string) returns item
{
immutable result := item(new_data);
result.next := tail;
result.previous := null;
if (tail != null)
tail.previous := result;;
return result;
};
 
variable my_list : item | {null} := null;
my_list := prepend(my_list, "R");
my_list := prepend(my_list, "o");
my_list := prepend(my_list, "s");
my_list := prepend(my_list, "e");
my_list := prepend(my_list, "t");
my_list := prepend(my_list, "t");
my_list := prepend(my_list, "a");
 
"Items in the list going forward:"!
variable follow : item | {null} := my_list;
while (true)
{
follow.data!
if (follow.next == null)
break;;
}
step
follow := follow.next;;
 
"Items in the list going backward:"!
while (follow != null)
follow.data!
step
follow := follow.previous;;</syntaxhighlight>
 
Both of these produce the following output:
<pre>
Items in the list going forward:
a
t
t
e
s
o
R
Items in the list going backward:
R
o
s
e
t
t
a
</pre>
 
=={{header|Scala}}==
<syntaxhighlight lang="scala">import java.util
 
object DoublyLinkedListTraversal extends App {
 
private val ll = new util.LinkedList[String]
 
private def traverse(iter: util.Iterator[String]) =
while (iter.hasNext) iter.next
 
traverse(ll.iterator)
traverse(ll.descendingIterator)
}</syntaxhighlight>
 
=={{header|Tcl}}==
Assuming that the <code>List</code> class from [[Doubly-Linked List (element)#Tcl|this other task]] is already present...
<langsyntaxhighlight lang="tcl"># Modify the List class to add the iterator methods
oo::define List {
method foreach {varName script} {
Line 448 ⟶ 2,537:
$first foreach char { puts $char }
puts "Backward..."
$last revforeach char { puts $char }</langsyntaxhighlight>
Which produces this output:
<pre>Forward...
Line 461 ⟶ 2,550:
a</pre>
 
=={{header|PythonWren}}==
{{libheader|Wren-llist}}
{{libheader|Wren-fmt}}
<syntaxhighlight lang="wren">import "./llist" for DLinkedList
import "./fmt" for Fmt
 
// create a new doubly-linked list and add the first 50 positive integers to it
This provides two solutions. One that explicitly builds a linked list and traverses it two ways, and another which uses pythons combined list/array class. Unless two lists explicitly needed to be spliced together in O(1) time, or a double linked list was needed for some other reason, most python programmers would probably use the second solution.
var dll = DLinkedList.new(1..50)
<lang python>class List:
def __init__(self, data, next=None, prev=None):
self.data = data
self.next = next
self.prev = prev
 
// traverse the doubly-linked list from head to tail
def append(self, data):
for (i in dll) {
if self.next == None:
selfFmt.next = Listwrite(data,"$4d None", selfi)
if (i % 10 == 0) return selfSystem.nextprint()
}
else:
System.print()
return self.next.append(data)
// traverse the doubly-linked list from tail to head
for (i in dll.reversed) {
Fmt.write("$4d ", i)
if (i % 10 == 1) System.print()
}</syntaxhighlight>
 
{{out}}
# Build the list
<pre>
tail = head = List(10)
1 2 3 4 5 6 7 8 9 10
for i in [ 20, 30, 40 ]:
11 12 13 14 15 16 17 18 19 20
tail = tail.append(i)
21 22 23 24 25 26 27 28 29 30
31 32 33 34 35 36 37 38 39 40
41 42 43 44 45 46 47 48 49 50
 
50 49 48 47 46 45 44 43 42 41
# Traverse forwards
40 39 38 37 36 35 34 33 32 31
node = head
30 29 28 27 26 25 24 23 22 21
while node != None:
20 19 18 17 16 15 14 13 12 11
print(node.data)
10 9 8 7 6 5 4 3 2 1
node = node.next
</pre>
 
=={{header|XPL0}}==
# Traverse Backwards
<syntaxhighlight lang "XPL0">def \Node\ Prev, Data, Next; \Element (Node) definition
node = tail
def SizeofInt = 4;
while node != None:
print(node.data)
node = node.prev</lang>
 
proc Insert(NewNode, Node); \Insert NewNode after Node
This produces the desired output. However, I expect most python programmers would do the following instead:
int NewNode, Node, NextNode;
[NextNode:= Node(Next);
NextNode(Prev):= NewNode;
NewNode(Next):= NextNode;
NewNode(Prev):= Node;
Node(Next):= NewNode;
];
 
int Head(3), Tail(3); \Doubly linked list definition
<lang python>l = [ 10, 20, 30, 40 ]
int N, NewNode, Node;
for i in l:
[\Further define (initialize) the doubly linked list
print(i)
Head(Next):= Tail;
for i in reversed(l): # reversed produces an iterator, so only O(1) memory is used
Tail(Prev):= Head;
print(i)</lang>
\Insert some Nodes containing square data
for N:= 1 to 10 do
[NewNode:= Reserve(3*SizeofInt);
NewNode(Data):= N*N;
Insert(NewNode, Head);
];
\Traverse list from Head to Tail
Node:= Head(Next);
while Node # Tail do
[IntOut(0, Node(Data)); ChOut(0, ^ );
Node:= Node(Next);
];
CrLf(0);
\Traverse list from Tail to Head
Node:= Tail(Prev);
while Node # Head do
[IntOut(0, Node(Data)); ChOut(0, ^ );
Node:= Node(Prev);
];
CrLf(0);
]</syntaxhighlight>
{{out}}
<pre>
100 81 64 49 36 25 16 9 4 1
1 4 9 16 25 36 49 64 81 100
</pre>
 
=={{header|zkl}}==
Double-ended queues in python are provided by the collections.deque class and the array/list type can perform all the operations of a C++ vector (and more), so building one's own doubly-linked list would be restricted to very specialized situations.
<syntaxhighlight lang="zkl">class Node{
fcn init(_value,_prev=Void,_next=Void)
{ var value=_value, prev=_prev, next=_next; }
fcn toString{ value.toString() }
fcn append(value){
b,c := Node(value,self,next),next;
next=b;
if(c) c.prev=b;
b
}
}</syntaxhighlight>
<syntaxhighlight lang="zkl">a,c := Node("a"), a.append("b").append("c");
n:=a; while(n){ print(n," "); n=n.next }
println();
n:=c; while(n){ print(n," "); n=n.prev }
println();</syntaxhighlight>
{{out}}
<pre>
a b c
c b a
</pre>
 
{{omit from|ACL2}}
{{omit from|GUISS}}
PureFox
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