Traverse from the beginning of a doubly-linked list to the end, and from the end to the beginning.

You are encouraged to solve this task according to the task description, using any language you may know.

## Action!

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="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
Output:
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)

procedure Traversing is

procedure Print (Position : in Char_Lists.Cursor) is
begin
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);

My_List.Reverse_Iterate (Print'Access);
end Traversing;

## ALGOL 68

# 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 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;
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;
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
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
)

main.alg:

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)
)
Output:
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

## ALGOL W

Using the element type and insertion routine from the Doubly Linked List/Eleemnt Insertion task.

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
e    := insertIntoDListIBefore( next(head), 90210 );
e    := insertIntoDListIBefore( next(e),     4077 );
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.
Output:
Forward:
9000
90210
4077
1701
Backward:
1701
4077
90210
9000

## Applesoft BASIC

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
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

## ARM Assembly

Works with: as version Raspberry Pi
/* 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 WRITE,  4

/*******************************************/
/* Structures                               */
/********************************************/
.struct  0
dllist_tail:                    @ tail node
.struct  dllist_tail  + 4
dllist_fin:
.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:
bl newDList                      @ create new list
bl affichageMess
mov r1,#10                       @ value
cmp r0,#-1
beq 99f
mov r1,#20
bl insertTail                    @ insertion at tail
cmp r0,#-1
beq 99f
mov r1,#10                       @ value to after insered
mov r2,#15                       @ new value
bl insertAfter
cmp r0,#-1
beq 99f
mov r1,#20                       @ value to after insered
mov r2,#25                       @ new value
bl insertAfter
cmp r0,#-1
beq 99f
bl affichageMess
b 100f
99:
bl affichageMess
100:                                 @ standard end of the program
mov r7, #EXIT                    @ request to exit program
svc 0                            @ perform system call
/******************************************************************/
/*     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]
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
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 */
push {r1-r4,lr}                         @ save  registers
mov r0,r1                            @ value
bl createNode
cmp r0,#-1                           @ allocation error ?
beq 100f
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
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
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
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
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  */
searchValue:
push {r2,lr}                         @ save  registers
1:
moveq r0,#-1
beq 100f
ldr r2,[r0,#NDlist_value]            @ load node value
cmp r2,r1                            @ equal ?
beq 100f
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 */
push {r2,lr}                         @ save  registers
1:
ldr r0,[r2,#NDlist_value]
bl conversion10S
bl affichageMess
ldr r2,[r2,#NDlist_next]
cmp r2,#0
bne 1b
100:
pop {r2,lr}                          @ restaur registers
bx lr                                @ return
/******************************************************************/
/*     transversal for tail to head                                               */
/******************************************************************/
/* r0 contains the address of the list structure */
push {r2,lr}                         @ save  registers
ldr r2,[r0,#dllist_tail]             @ load last node
1:
ldr r0,[r2,#NDlist_value]
bl conversion10S
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"

## Axe

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

## BBC BASIC

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

Output:

Traverse forwards:
123
456
789
Traverse backwards:
789
456
123

## C

// A doubly linked list of strings;
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct sListEntry {
const char *value;
struct sListEntry *next;
struct sListEntry *prev;

typedef struct sListIterator{
} *LIterator;

ListEntry le = malloc(sizeof(struct sListEntry));
if (le) {
le->value = NULL;
le->next = le->prev = NULL;
}
return le;
}

int LL_Append(LinkedList ll, const char *newVal)
{
ListEntry le = malloc(sizeof(struct sListEntry));
if (le) {
le->value = strdup(newVal);
le->prev = ll->prev;
le->next = NULL;
if (le->prev)
le->prev->next = le;
else
ll->next = le;
ll->prev = le;
}
return (le!= NULL);
}

int LI_Insert(LIterator iter, const char *newVal)
{
ListEntry le = malloc(sizeof(struct sListEntry));
if (le) {
le->value = strdup(newVal);
if ( crnt == iter->head) {
le->prev = NULL;
le->next = crnt->next;
crnt->next = le;
if (le->next)
le->next->prev = le;
else
crnt->prev = le;
}
else {
le->prev = ( crnt == NULL)? iter->head->prev : crnt->prev;
le->next = crnt;
if (le->prev)
le->prev->next = le;
else
if (crnt)
crnt->prev = le;
else
}
}
return (le!= NULL);
}

{
LIterator liter = malloc(sizeof(struct sListIterator));
return liter;
}

#define LLI_Delete( iter ) \
{free(iter); \
iter = NULL;}

int LLI_AtEnd(LIterator iter)
{
}
const char *LLI_Value(LIterator iter)
{
}
int LLI_Next(LIterator iter)
{
}
int LLI_Prev(LIterator iter)
{
}

int main()
{
static const char *contents[] = {"Read", "Orage", "Yeller",
"Glean", "Blew", "Burple"};
int ix;
LIterator iter;

for (ix=0; ix<6; ix++)        //insert contents
LL_Append(ll, contents[ix]);

iter = LL_GetIterator(ll);    //get an iterator
printf("forward\n");
while(LLI_Next(iter))         //iterate forward
printf("value=%s\n", LLI_Value(iter));
LLI_Delete(iter);             //delete iterator

printf("\nreverse\n");
iter = LL_GetIterator(ll);
while(LLI_Prev(iter))         //iterate reverse
printf("value=%s\n", LLI_Value(iter));
LLI_Delete(iter);
//uhhh-- delete list??
return 0;
}

## C#

using System;
using System.Collections.Generic;

{
internal static class Program
{
private static void Main()
{

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);
}
}
}

Output:

h
e
l
l
o

o
l
l
e
h

## C++

Works with: C++11
#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';
}
Output:
1 5 7 0 3 2

## Clojure

Given the definition in Doubly-linked list/Definition#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...>})

## D

void main() {
import std.stdio, std.container, std.range;

auto dll = DList!dchar("DCBA"d.dup);

dll[].writeln;
dll[].retro.writeln;
}
Output:
ABCD
DCBA

Same output.

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);
}

void main() {
import std.stdio;

foreach (char c; "DCBA")

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;
}

## 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;

## E

 This example is incorrect. Please fix the code and remove this message.Details: 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 Doubly-linked list/Definition#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
}
}
}
? def list := makeDLList()
# value: <>

? list.push(1)
? list.push(2)
? list.push(3)

? traverse(list)
1
2
3
3
2
1

## Erlang

The task in Doubly-linked_list/Element_insertion uses traversal as proof that the insertion worked.

## F#

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

traverseForward cs
printfn ""
traverseBackward cs
Output:
'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'

## 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

## 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.

package main

import "fmt"

type dlNode struct {
string
next, prev *dlNode
}

type dlList struct {
}

func (list *dlList) String() 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 {
} 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("")
}

Output:

<nil>
[A B]
[A C B]
From tail: B C A

## Groovy

static void main(args) {

print it
}

println()

print it
}
}
}
Output:
123456789
987654321

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)

## Icon and Unicon

Uses Unicon classes.

# insert given node after this one, removing its existing connections
method insert_after (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
}
end

method traverse_forwards ()
current := self
while \current do {
suspend current
}
end

self.value := value
end

procedure main ()
l1.insert_after (l2)

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

Output:

Traverse from beginning to end
1
3
2
Traverse from end to beginning
2
3
1

## J

traverse=:1 :0
current=. y
while. y ~: current=. successor__current do.
end.
result
)

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.

## Java

Works with: Java version 8 or higher
package com.rosettacode;

import java.util.stream.Collectors;
import java.util.stream.IntStream;

public static void main(String[] args) {

IntStream.range(1, 10)
.mapToObj(String::valueOf)

System.out.println();
}
}
Output:
123456789
987654321

## JavaScript

See Doubly-Linked List (element)#JavaScript. The traverse() and print() functions have been inherited from Singly-Linked List (traversal)#JavaScript.

var tail;
this.traverse(function(node){tail = node;});
return tail;
}
func(this);
if (this.prev() != null)
this.prev().traverseBackward(func);
}
this.traverseBackward( function(node) {print(node.value())} );
}

outputs:

10
20
30
40
40
30
20
10

Uses the print() function from Rhino or SpiderMonkey.

## Julia

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

function insertpost(prevnode, node)
succ = prevnode.succ
prevnode.succ = node
node.pred = prevnode
node.succ = succ
if succ != nothing
succ.pred =  node
end
node
end

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)
insertpost(node1, node2)
insertpost(node2, node3)
print("From beginning to end: "); printconnected(node1)
print("From end to beginning: "); printconnected(node1, fromtail = true)
Output:

From beginning to end: 1 -> 2 -> 3 From end to beginning: 3 -> 2 -> 1

## 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:

// version 1.1.2

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

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

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)
else if (after == last)
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>) {
ll.insert(ll.first, 2)
ll.insert(ll.last!!.prev, 3)
println("First to last : \${ll.firstToLast()}")
println("Last to first : \${ll.lastToFirst()}")
}
Output:
First to last : 1 -> 2 -> 3 -> 4
Last to first : 4 -> 3 -> 2 -> 1

## Lua

Begin with this: Doubly-linked_list/Definition#Lua, then extend with this:

--------------
-- 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()
Output:
Forward: 1,2,3,4,5,
Reverse: 5,4,3,2,1,

## Liberty BASIC

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

## Nim

type
List[T] = object

Node[T] = ref TNode[T]

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

proc newNode[T](data: T): Node[T] =
new(result)
result.data = data

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

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.next != nil: n.next.prev = n.prev
if n.prev != nil: n.prev.next = n.next

proc `\$`[T](l: var List[T]): string =
result = ""
while n != nil:
if result.len > 0: result.add(" -> ")
n = n.next

iterator traverseForward[T](l: List[T]): T =
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

## Oberon-2

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.

## 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();
};
}
}
-- forward --
100
5
10
25
50
-- backward --
50
25
10
5
100

## Oforth

Complete definition is here : Doubly-linked list/Definition#Oforth

Defining #forEachNext and #forEachPrev allow to traverse this double linked list using #forEach: and #revEach: syntax

DList method: forEachNext
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 . ] ;
Output:
>test
Traversal (beginning to end) :
A C B
Traversal (end to beginning) :
B C A ok

## Oz

Warning: Highly unidiomatic code. (Use built-in lists instead.)

declare
proc {Walk Node Action}
case Node of nil then skip
[] node(value:V next:N ...) then
{Action V}
{Walk @N Action}
end
end

proc {WalkBackwards Node Action}
Tail = {GetLast Node}
proc {Loop N}
case N of nil then skip
[] node(value:V prev:P ...) then
{Action V}
{Loop @P}
end
end
in
{Loop Tail}
end

fun {GetLast Node}
case @(Node.next) of nil then Node
[] NextNode=node(...) then {GetLast NextNode}
end
end

fun {CreateNewNode Value}
node(prev:{NewCell nil}
next:{NewCell nil}
value:Value)
end

proc {InsertAfter Node NewNode}
Next = Node.next
in
(NewNode.next) := @Next
(NewNode.prev) := Node
case @Next of nil then skip
[] node(prev:NextPrev ...) then
NextPrev := NewNode
end
Next := NewNode
end

A = {CreateNewNode a}
B = {CreateNewNode b}
C = {CreateNewNode c}
in
{InsertAfter A B}
{InsertAfter A C}
{Walk A Show}
{WalkBackwards A Show}

See Delphi

## PascalABC.NET

begin
var current := lst.First;
while current <> nil do
begin
Print(current.Value);
current := current.Next;
end;
end.
Output:
1 2 3 4 5 6 7 8 9 10

## Phix

enum NEXT,PREV,DATA
constant empty_dll = {{1,1}}
sequence dll = deep_copy(empty_dll)

procedure insert_after(object data, integer pos=1)
integer prv = dll[pos][PREV]
dll = append(dll,{pos,prv,data})
if prv!=0 then
dll[prv][NEXT] = length(dll)
end if
dll[pos][PREV] = length(dll)
end procedure

insert_after("ONE")
insert_after("TWO")
insert_after("THREE")

?dll

procedure show(integer d)
integer idx = dll[1][d]
while idx!=1 do
?dll[idx][DATA]
idx = dll[idx][d]
end while
end procedure
show(NEXT)
?"=="
show(PREV)
Output:
{{2,4},{3,1,"ONE"},{4,2,"TWO"},{1,3,"THREE"}}
"ONE"
"TWO"
"THREE"
"=="
"THREE"
"TWO"
"ONE"

## 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) )

Output for the example data produced in Doubly-linked list/Definition#PicoLisp and Doubly-linked list/Element definition#PicoLisp:

: (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

Output for the example data produced in Doubly-linked list/Element insertion#PicoLisp:

: (2print *DL)                   # Print the list
A C B

: (2printReversed *DL)           # Print it in reversed order
B C A

## PL/I

/* To implement a doubly-linked list -- i.e., a 2-way linked list. */

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

declare (head, tail, t) handle (node);
declare null builtin;
declare i fixed binary;

do i = 1 to 10; /* Add ten items to the tail of the queue. */
if head = bind(:node, null:) then
do;
put skip list (head => value);
end;
else
do;
t = new(:node:);
t => back_pointer = tail; /* Point the new tail back to the old */
/* tail.                              */
tail => fwd_pointer = t;  /* Point the tail to the new node.    */
t => back_pointer = tail; /* Point the new tail back to the old */
/*  tail.                             */
tail = t;                 /* Point at the new tail.             */
tail => fwd_pointer = bind(:node, null:);
/* Set the tail link to NULL          */
get list (tail => value) copy;
put skip list (tail => value);
end;
end;

if head = bind(:node, null:) then return; /* Empty list. */

/* Traverse the list from the head. */
put skip list ('In a forwards direction, the list has:');
do while (t ^= bind(:node, null:));
put skip list (t => value);
t = t => fwd_pointer;
end;
/* Traverse the list from the tail to the head. */
put skip list ('In the reverse direction, the list has:');
t = tail;
do while (t ^= bind(:node, null:));
put skip list (t => value);
t = t => back_pointer;
end;

Output:

In a forwards direction, the list has:
1
2
3
4
5
16
7
8
9
10
In the reverse direction, the list has:
10
9
8
7
16
5
4
3
2
1

## 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
For i=0 To (Random(100)+25)
AddElement(MyData())        ; Create a new tailing element
MyData()=Random(314)        ; Inert a vale into it
Next
;
;Traverse from the beginning of a doubly-linked list to the end.
FirstElement(MyData())
Repeat
Debug MyData()              ; Present the value in the current cell
Until Not NextElement(MyData())
;
;Traverse from the end to the beginning.
LastElement(MyData())
Repeat
Debug MyData()             ; Present the value in the current cell
Until Not PreviousElement(MyData())

## 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.

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
for i in [ 20, 30, 40 ]:
tail = tail.append(i)

# Traverse forwards
while node != None:
print(node.data)
node = node.next

# Traverse Backwards
node = tail
while node != None:
print(node.data)
node = node.prev

This produces the desired output. However, I expect most python programmers would do the following instead:

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)

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.

## 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.

(define (dlist-elements dlist)
(let loop ([elements '()] [dlink (dlist-tail dlist)])
elements)))

(define (dlist-elements/reverse dlist)
elements)))

## 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:

say \$dll.list;
say \$dll.reverse;

These automatically return just the payloads, hiding the elements containing the forward and backward pointers.

## REXX

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 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 '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

@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)
return 0

@del:    procedure expose \$.;   arg k,m
call @parms 'km'
_=subword(\$.@,k,k-1) subword(\$.@,k+m)
\$.@=_
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 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

─── showing list ───
Oy! Was it a black cat I saw there, in the shadows, stalking its prey (and next

## Ring

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

Output:

Traverse forwards:
123
456
789

Traverse backwards:
789
456
123

## Ruby

class DListNode
def get_tail
# parent class (ListNode) includes Enumerable, so the find method is available to us
self.find {|node| node.succ.nil?}
end

def each_previous(&b)
yield self
self.prev.each_previous(&b) if self.prev
end
end

## Salmon

Without explicit type information:

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;;

With explicit type information:

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;;

Both of these produce the following output:

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

## Scala

import java.util

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)
}

## Tcl

Assuming that the List class from this other task is already present...

# Modify the List class to add the iterator methods
oo::define List {
method foreach {varName script} {
upvar 1 \$varName v
for {set node [self]} {\$node ne ""} {set node [\$node next]} {
set v [\$node value]
uplevel 1 \$script
}
}
method revforeach {varName script} {
upvar 1 \$varName v
for {set node [self]} {\$node ne ""} {set node [\$node previous]} {
set v [\$node value]
uplevel 1 \$script
}
}
}

# Demonstrating...
set first [List new a [List new b [List new c [set last [List new d]]]]]
puts "Forward..."
\$first foreach char { puts \$char }
puts "Backward..."
\$last revforeach char { puts \$char }

Which produces this output:

Forward...
a
b
c
d
Backward...
d
c
b
a

## Wren

Library: Wren-llist
Library: Wren-fmt
import "./fmt" for Fmt

// create a new doubly-linked list and add the first 50 positive integers to it

for (i in dll) {
Fmt.write("\$4d ", i)
if (i % 10 == 0) System.print()
}
System.print()
for (i in dll.reversed) {
Fmt.write("\$4d ", i)
if (i % 10 == 1) System.print()
}
Output:
1    2    3    4    5    6    7    8    9   10
11   12   13   14   15   16   17   18   19   20
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
40   39   38   37   36   35   34   33   32   31
30   29   28   27   26   25   24   23   22   21
20   19   18   17   16   15   14   13   12   11
10    9    8    7    6    5    4    3    2    1

## XPL0

def \Node\ Prev, Data, Next;    \Element (Node) definition
def SizeofInt = 4;

proc Insert(NewNode, Node);     \Insert NewNode after Node
int  NewNode, Node, NextNode;
[NextNode:= Node(Next);
NextNode(Prev):= NewNode;
NewNode(Next):= NextNode;
NewNode(Prev):= Node;
Node(Next):= NewNode;
];

int N, NewNode, Node;
[\Further define (initialize) the doubly linked list
\Insert some Nodes containing square data
for N:= 1 to 10 do
[NewNode:= Reserve(3*SizeofInt);
NewNode(Data):= N*N;
];
\Traverse list from Head to Tail
while Node # Tail do
[IntOut(0, Node(Data));  ChOut(0, ^ );
Node:= Node(Next);
];
CrLf(0);
\Traverse list from Tail to Head
Node:= Tail(Prev);
[IntOut(0, Node(Data));  ChOut(0, ^ );
Node:= Node(Prev);
];
CrLf(0);
]
Output:
100 81 64 49 36 25 16 9 4 1
1 4 9 16 25 36 49 64 81 100

## 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
}
}
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();
Output:
a  b  c
c  b  a