Singly-linked list/Element definition

Singly-linked list/Element definition
You are encouraged to solve this task according to the task description, using any language you may know.

Define the data structure for a singly-linked list element. Said element should contain a data member capable of holding a numeric value, and the link to the next element should be mutable.

Ada

<lang ada> type Link;

type Link_Access is access Link;
type Link is record
   Next : Link_Access := null;
   Data : Integer;
end record;</lang>

ALGOL 68

MODE DATA = STRUCT ( ... );

MODE LINK = STRUCT (
  REF LINK next,
  DATA value
);

C

<lang c> struct link {

  struct link *next;
  int data;
};</lang>

C++

The simplest C++ version looks basically like the C version:

<lang cpp> struct link

{
  link* next;
  int data;
};</lang>

Initialization of links on the heap can be simplified by adding a constructor:

<lang cpp> struct link

{
  link* next;
  int data;
  link(int a_data, link* a_next = 0): next(a_next), data(a_data) {}
};</lang>

With this constructor, new nodes can be initialized directly at allocation; e.g. the following code creates a complete list with just one statement:

<lang cpp> link* small_primes = new link(2, new link(3, new link(5, new link(7))));</lang>

However, C++ also allows to make it generic on the data type (e.g. if you need large numbers, you might want to use a larger type than int, e.g. long on 64-bit platforms, long long on compilers that support it, or even a bigint class).

<lang cpp> template<typename T> struct link

{
  link* next;
  T data;
  link(T a_data, link* a_next = 0): next(a_next), data(a_data) {}
};</lang>

Note that the generic version works for any type, not only integral types.

Clean

import StdMaybe

:: Link t = { next :: Maybe (Link t), data :: t }

D

Generic template-based node element.

<lang D>class Node(T) { public:

   T data;
   Node next;
   this(T d, Node n = null) { data=d; next=n; }

}</lang>

Delphi

A simple one way list. I use a generic pointer for the data that way it can point to any structure, individual variable or whatever. Note that in Standard Pascal, there are no generic pointers, therefore one has to settle for a specific data type there.

<lang delphi> Type

    pOneWayList = ^OneWayList;
    OneWayList = record
                  pData : pointer ;
                  Next  : pOneWayList ;
                 end;</lang>

E

<lang e> interface LinkedList guards LinkedListStamp {}

def empty implements LinkedListStamp {
    to null() { return true }
}
def makeLink(value :int, var next :LinkedList) {
    def link implements LinkedListStamp {
        to null() { return false }
        to value() { return value }
        to next() { return next }
        to setNext(new) { next := new }
    }
    return link
}</lang>

Forth

Forth has no "struct" facility, but you can easily implement a single linked list with a data cell using a double-cell value.

: >cell-link ( a -- a )  ;
: >cell-data ( a -- b )  cell+ ;

As an example of usage, here is a word to link 'a' after 'b'

: chain ( a b -- )  \ links 'a' after 'b'
   over >r
   dup >cell-link @  r> >cell-link !   \ a points at b's old link
   >cell-link ! ;

Or with named parameters:

: chain { a b -- }
   b >cell-link @  a >cell-link !
   a  b >cell-link ! ;

Due to Forth's lack of typechecking, 'b' in the above examples does not have to be an actual cell, but can instead be the head pointer of the list.

Fortran

In ISO Fortran 95 or later: <lang fortran> type node

     real :: data
     type( node ), pointer :: next => null() 
  end type node
  !
  !. . . .
  !
  type( node ) :: head</lang>

Haskell

This task is not idiomatic for Haskell. Usually, all data in pure functional programming is immutable, and deconstructed through Pattern Matching. The Prelude already contains a parametrically polymorphic list type that can take any data member type, including numeric values. These lists are then used very frequently. Because of this, lists have additional special syntactic sugar.

An equivalent declaration for such a list type without the special syntax would look like this:

<lang haskell> data List a = Nil | Cons a (List a)</lang>

A declaration like the one required in the task, with an integer as element type and a mutable link, would be

<lang haskell> data IntList s = Nil | Cons Integer (STRef s (IntList s))</lang>

but that would be really awkward to use.

Java

The simplest Java version looks basically like the C++ version:

<lang java> class Link

{
    Link next;
    int data;
}</lang>

Initialization of links on the heap can be simplified by adding a constructor:

<lang java> class Link

{
    Link next;
    int data;
    Link(int a_data, Link a_next) { next = a_next; data = a_data; }
}</lang>

With this constructor, new nodes can be initialized directly at allocation; e.g. the following code creates a complete list with just one statement:

<lang java> Link small_primes = new Link(2, new Link(3, new Link(5, new Link(7, null))));</lang>

Works with: Java version 1.5+

However, Java also allows to make it generic on the data type. This will only work on reference types, not primitive types like int or float (wrapper classes like Integer and Float are available).

<lang java> class Link<T>

{
  Link<T> next;
  T data;
  Link(T a_data, Link<T> a_next) { next = a_next; data = a_data; }
}</lang>

Logo

As with other list-based languages, simple lists are represented easily in Logo.

fput item list ; add item to the head of a list

first list  ; get the data
butfirst list ; get the remainder
bf list       ; contraction for "butfirst"

These return modified lists, but you cal also destructively modify lists. These are normally not used because you might accidentally create cycles in the list.

.setfirst list value
.setbf list remainder

Objective-C

This implements a class which has the primitive basic Objective-C class Object as parent.

<lang objc>#import <objc/Object.h>

@interface RCListElement : Object {

 RCListElement *next;
 id datum;

} + (RCListElement *)new; - (RCListElement *)next; - (id)datum; - (RCListElement *)setNext: (RCListElement *)nx; - (void)setDatum: (id)d; @end

@implementation RCListElement + (RCListElement *)new {

 RCListElement *m = [super new];
 [m setNext: nil];
 [m setDatum: nil];
 return m;

} - (RCListElement *)next {

 return next;

} - (id)datum {

 return datum;

} - (RCListElement *)setNext: (RCListElement *)nx {

 RCListElement *p;
 p = next;
 next = nx;
 return p;

} - (void)setDatum: (id)d {

 datum = d;

} @end</lang>

OCaml

This task is not idiomatic for OCaml. OCaml already contains a built-in parametrically polymorphic list type that can take any data member type, including numeric values. These lists are then used very frequently. Because of this, lists have additional special syntactic sugar. OCaml's built-in lists, like most functional data structures, are immutable, and are deconstructed through Pattern Matching.

An equivalent declaration for such a list type without the special syntax would look like this:

<lang ocaml> type 'a list = Nil | Cons of 'a * 'a list</lang>

A declaration like the one required in the task, with an integer as element type and a mutable link, would be

<lang ocaml> type int_list = Nil | Cons of int * int_list ref</lang>

but that would be really awkward to use.

Perl

Just use an array. You can traverse and splice it any way. Linked lists are way too low level.

However, if all you got is an algorithm in a foreign language, you can use references to accomplish the translation. <lang perl> my %node = (

    data => 'say what',
    next => \%foo_node,
);
$node{next} = \%bar_node;  # mutable</lang>

Pop11

List are built in into Pop11, so normally on would just use them:

;;; Use shorthand syntax to create list.
lvars l1 = [1 2 three 'four'];
;;; Allocate a single list node, with value field 1 and the link field
;;; pointing to empty list
lvars l2 = cons(1, []);
;;; print first element of l1
front(l1) =>
;;; print the rest of l1
back(l1) =>
;;; Use index notation to access third element
l1(3) =>
;;; modify link field of l2 to point to l1
l1 -> back(l2);
;;; Print l2
l2 =>

If however one wants to definite equivalent user-defined type, one can do this:

uses objectclass;
define :class ListNode;
    slot value = [];
    slot next = [];
enddefine;
;;; Allocate new node and assign to l1
newListNode() -> l1;
;;; Print it
l1 =>
;;; modify value
1 -> value(l1);
l1 =>
;;; Allocate new node with initialized values and assign to link field
;;; of l1
consListNode(2, []) -> next(l1);
l1 =>

Python

The Node class implements also iteration for more Pythonic iteration over linked lists.

<lang python>

  class LinkedList(object):
       """USELESS academic/classroom example of a linked list implemented in Python.
          Don't ever consider using something this crude!  Use the built-in list() type!
       """

class Node(object): def __init__(self, item): self.value = item self.next = None def __init__(self, item=None): if item is not None: self.head = Node(item); self.tail = self.head else: self.head = None; self.tail = None def append(self, item): if not self.head: self.head = Node(item) self.tail = self.head elif self.tail: self.tail.next = Node(item) self.tail = self.tail.next else: self.tail = Node(item) def __iter__(self): cursor = self.head while cursor: yield cursor.value cursor = cursor.next </lang>

Note: As explained in this class' docstring implementing linked lists and nodes in Python is an utterly pointless academic exercise. It may give on the flavor of the elements that would be necessary in some other programming languages (e.g. using Python as "executable psuedo-code"). Adding methods for finding, counting, removing and inserting elements is left as an academic exercise to the reader. For any practical application use the built-in list() or dict() types as appropriate.

Ruby

<lang ruby> class ListNode

   attr_accessor :value, :succ
   def initialize(v,s=nil)
     self.value=v
     self.succ=s
   end
   def each(&b)
     yield value
     succ.each(&b) if succ
   end
   include Enumerable
 end</lang>

Scheme

Scheme, like other Lisp dialects, has extensive support for singly-linked lists. The element of such a list is known as a cons-pair, because you use the cons function to construct it: <lang scheme>(cons value next)</lang>

The value and next-link parts of the pair can be deconstructed using the car and cdr functions, respectively: <lang scheme>(car my-list) ; returns the first element of the list (cdr my-list) ; returns the remainder of the list</lang>

Each of these parts are mutable and can be set using the set-car! and set-cdr! functions, respectively: <lang scheme>(set-car! my-list new-elem) (set-cdr! my-list new-next)</lang>

Tcl

While it is highly unusual to implement linked lists in Tcl, since the language has a built-in list type (that internally uses arrays of references), it is possible to simulate it with objects.

Works with: Tcl version 8.6

<lang tcl>oo::class create List {

   variable content next
   constructor {value {list ""}} {
       set content $value
       if {$list ne ""} {
           set next $list
       }
   }
   method value args {
       set content {*}$args
   }
   method attach {list} {
       set next $list
   }
   method detach {} {
       unset next
   }
   method next {} {
       return $next
   }

}</lang>