Multiple distinct objects

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

Create a sequence (array, list, whatever) consisting of n distinct, initialized items of the same type. n should be determined at runtime.

By distinct we mean that if they are mutable, changes to one do not affect all others; if there is an appropriate equality operator they are considered unequal; etc. The code need not specify a particular kind of distinction, but do not use e.g. a numeric-range generator which does not generalize.

By initialized we mean that each item must be in a well-defined state appropriate for its type, rather than e.g. arbitrary previous memory contents in an array allocation. Do not show only an initialization technique which initializes only to "zero" values (e.g. calloc() or int a[n] = {}; in C), unless user-defined types can provide definitions of "zero" for that type.

This task was inspired by the common error of intending to do this, but instead creating a sequence of n references to the same mutable object; it might be informative to show the way to do that as well, both as a negative example and as how to do it when that's all that's actually necessary.

This task is most relevant to languages operating in the pass-references-by-value style (most object-oriented, garbage-collected, and/or 'dynamic' languages).

Ada

A : array (1..N) of T;

Here N can be unknown until run-time. T is any constrained type. In Ada all objects are always initialized, though some types may have null initialization. When T requires a non-null initialization, it is done for each array element. For example, when T is a task type, N tasks start upon initialization of A. Note that T can be a limited type like task. Limited types do not have predefined copy operation. Arrays of non-limited types can also be initialized by aggregates of:

A : array (1..N) of T := (others => V);

Here V is some value or expression of the type T. As an expression V may have side effects, in that case it is evaluated exactly N times, though the order of evaluation is not defined. Also an aggregate itself can be considered as a solution of the task:

(1..N => V)

Aime

void
show_sublist(list l)
{
    integer i;
 
    i = 0;
    while (i < l_length(l)) {
        if (i) {
            o_space(1);
        }
        o_integer(l_q_integer(l, i));
        i += 1;
    }
}
 
void
show_list(list l)
{
    integer i;
 
    i = 0;
    while (i < l_length(l)) {
        o_text(" [");
        show_sublist(l_q_list(l, i));
        o_text("]");
        i += 1;
    }
 
    o_byte('\n');
}
 
list
multiple_distinct(integer n, object o)
{
    list l;
 
    while (n) {
        l_append(l, o);
        n -= 1;
    }
 
    return l;
}
 
integer
main(void)
{
    list l, z;
 
    # create a list of integers - `3' will serve as initializer
    l = multiple_distinct(8, 3);
 
    l_clear(l);
 
    # create a list of distinct lists - `z' will serve as initializer
    l_append(z, 4);
    l = multiple_distinct(8, z);
 
    # modify one of the sublists
    l_r_integer(l_q_list(l, 3), 0, 7);
 
    # display the list of lists
    show_list(l);
 
    return 0;
}

Output:

 [4] [4] [4] [7] [4] [4] [4] [4]

ALGOL 68

Translation of: python

Works with: ALGOL 68 version Standard - no extensions to language used

Works with: ALGOL 68G version Any - tested with release mk15-0.8b.fc9.i386

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

MODE FOO = STRUCT(CHAR u,l);
INT n := 26;
[n]FOO f;
 
# Additionally each item can be initialised #
FOR i TO UPB f DO f[i] := (REPR(ABS("A")-1+i), REPR(ABS("a")-1+i))  OD;
 
print((f, new line))

Output:

AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz

AutoHotkey

Works with: AutoHotkey_L

a := []
Loop, %n%
   a[A_Index] := new Foo()

BBC BASIC

Works with: BBC BASIC for Windows

      REM Determine object count at runtime:
      n% = RND(1000)
 
      REM Declare an array of structures; all members are initialised to zero:
      DIM objects{(n%) a%, b$}
 
      REM Initialise the objects to distinct values:
      FOR i% = 0 TO DIM(objects{()},1)
        objects{(i%)}.a% = i%
        objects{(i%)}.b$ = STR$(i%)
      NEXT
 
      REM This is how to create an array of pointers to the same object:
      DIM objects%(n%), object{a%, b$}
      FOR i% = 0 TO DIM(objects%(),1)
        objects%(i%) = object{}
      NEXT

Brat

The wrong way, which creates an array of n references to the same new foo:

n.of foo.new

The right way, which calls the block n times and creates an array of new foos:

n.of { foo.new }

C

foo *foos = malloc(n * sizeof(*foos));
for (int i = 0; i < n; i++)
  init_foo(&foos[i]);

(Or if no particular initialization is needed, skip that part, or use calloc.)

C++

By default C++ has value semantics, so this problem does not present itself unless the programmer deliberately choses to refer to objects though a pointer. Examples are given for both cases.

Using only language primitives:

// this assumes T is a default-constructible type (all built-in types are)
T* p = new T[n]; // if T is POD, the objects are uninitialized, otherwise they are default-initialized
 
//If default initialisation is not what you want, or if T is a POD type which will be uninitialized
for(size_t i = 0; i != n; ++i)
   p[i] = make_a_T(); //or some other expression of type T
 
// when you don't need the objects any more, get rid of them
delete[] p;

Using the standard library

#include <vector>
#include <algorithm>
#include <iterator>
 
// this assumes T is default-constructible
std::vector<T> vec1(n); // all n objects are default-initialized
 
// this assumes t is a value of type T (or a type which implicitly converts to T)
std::vector<T> vec2(n, t); // all n objects are copy-initialized with t
 
// To initialise each value differently 
std::generate_n(std::back_inserter(vec), n, makeT); //makeT is a function of type T(void)
 

In C++ reference semantics are achieved by holding objects by pointer. Here is an example of the error, and a correct way of achieving distinctness.

These examples assume T has a public copy constructor, and that p is a pointer to T;

#include <vector>
#include <tr1/memory>
using namespace std;
using namespace std::tr1;
 
typedef shared_ptr<T> TPtr_t;
// the following is NOT correct:
std::vector<TPtr_t > bvec_WRONG(n, p); // create n copies of p, which all point to the same opject p points to.
 
// nor is this:
std::vector<TPtr_t> bvec_ALSO_WRONG(n, TPtr_t(new T(*p)) ); // create n pointers to a single copy of *p
 
// the correct solution
std::vector<TPtr_t > bvec(n);
for (int i = 0; i < n; ++i)
  bvec[i] = TPtr_t(new T(*p); //or any other call to T's constructor
 
// another correct solution
// this solution avoids uninitialized pointers at any point
std::vector<TPtr_t> bvec2;
for (int i = 0; i < n; ++i)
  bvec2.push_back(TPtr_t(new T(*p)); 
 
 

Of course, also in this case one can use the other sequence containers or plain new/delete instead of vector.

C#

using System;
using System.Linq;
using System.Collections.Generic;
 
List<Foo> foos = Enumerable.Range(1, n).Select(x => new Foo()).ToList();

Clojure

An example using pseudo-random numbers:

user> (take 3 (repeat (rand))) ; repeating the same random number three times
(0.2787011365537204 0.2787011365537204 0.2787011365537204)
user> (take 3 (repeatedly rand)) ; creating three different random number
(0.8334795669220695 0.08405601245793926 0.5795448744634744)
user>

Common Lisp

The mistake is often written as one of these:

(make-list n :initial-element (make-the-distinct-thing))
(make-array n :initial-element (make-the-distinct-thing))

which are incorrect since the form (make-the-distinct-thing) is only evaluated once and the single object is put in every position of the sequence. A commonly used correct version is:

(loop repeat n collect (make-the-distinct-thing))

which evaluates (make-the-distinct-thing) n times and collects each result in a list.

It is also possible to use map-into, the destructive map operation, to do this since it may take zero input sequences; this method can produce any sequence type, such as a vector (array) rather than a list, and takes a function rather than a form to specify the thing created:

(map-into (make-list n) #'make-the-distinct-thing)
(map-into (make-array n) #'make-the-distinct-thing)

D

For reference types (classes):

auto fooArray = new Foo[n];
foreach (ref item; fooArray)
    item = new Foo();
 

For value types:

auto barArray = new Bar[n];
barArray[] = initializerValue;

Delphi

Same object accessed multiple times (bad)

var
  i: Integer;
  lObject: TMyObject;
  lList: TObjectList<TMyObject>;
begin
  lList := TObjectList<TMyObject>.Create;
  lObject := TMyObject.Create;
  for i := 1 to 10 do
    lList.Add(lObject);
  // ...

Distinct objects (good)

var
  i: Integer;
  lList: TObjectList<TMyObject>;
begin
  lList := TObjectList<TMyObject>.Create;
  for i := 1 to 10 do
    lList.Add(TMyObject.Create);
  // ...

E

E needs development of better map/filter/stream facilities. The easiest way to do this so far is with the accumulator syntax, which is officially experimental because we're not satisfied with it as yet.

pragma.enable("accumulator")
...
 
accum [] for _ in 1..n { _.with(makeWhatever()) }

Factor

clone is the important word here to have distinct objects. This creates an array of arrays.

1000 [ { 1 } clone ] replicate

Fortran

 
program multiple
  ! Define a simple type
  type T
     integer :: a = 3
  end type T
 
  ! Define a type containing a pointer
  type S
     integer, pointer :: a
  end type S
 
  type(T), allocatable :: T_array(:)
  type(S), allocatable :: S_same(:)
  integer              :: i
  integer, target      :: v
  integer, parameter   :: N = 10
 
  ! Create 10 
  allocate(T_array(N))
 
  ! Set the fifth one to b something different
  T_array(5)%a = 1
 
  ! Print them out to show they are distinct
  write(*,'(10i2)') (T_array(i),i=1,N)
 
  ! Create 10 references to the same object
  allocate(S_same(N))
  v = 5
  do i=1, N
     allocate(S_same(i)%a)
     S_same(i)%a => v
  end do
 
  ! Print them out - should all be 5
  write(*,'(10i2)') (S_same(i)%a,i=1,N)
 
  ! Change the referenced object and reprint - should all be 3
  v = 3
  write(*,'(10i2)') (S_same(i)%a,i=1,N)  
 
end program multiple
 

F#

The wrong way:

>List.replicate 3 (System.Guid.NewGuid());;
 
val it : Guid list =
  [485632d7-1fd6-4d9e-8910-7949d7b2b485; 485632d7-1fd6-4d9e-8910-7949d7b2b485;
   485632d7-1fd6-4d9e-8910-7949d7b2b485]

The right way:

> List.init 3 (fun _ -> System.Guid.NewGuid());;
 
val it : Guid list =
  [447acb0c-092e-4f85-9c3a-d369e4539dae; 5f41c04d-9bc0-4e96-8165-76b41fe8cd93;
   1086400c-72ff-4763-9bb9-27e17bd4c7d2]

Go

Useful:

func nxm(n, m int) [][]int {
    d2 := make([][]int, n)
    for i := range d2 {
        d2[i] = make([]int, m)
    }
    return d2
}

Probably not what the programmer wanted:

func nxm(n, m int) [][]int {
    d1 := make([]int, m)
    d2 := make([][]int, n)
    for i := range d2 {
        d2[i] = d1
    }
    return d2
}

Haskell

Below, we are assuming that makeTheDistinctThing is a monadic expression (i.e. it has type m a where m is some monad, like IO or ST), and we are talking about distinctness in the context of the monad. Otherwise, this task is pretty meaningless in Haskell, because Haskell is referentially transparent (so two values that are equal to the same expression are necessarily not distinct) and all values are immutable.

replicateM n makeTheDistinctThing

in an appropriate do block. If it is distinguished by, say, a numeric label, one could write

mapM makeTheDistinctThing [1..n]

An incorrect version:

do x <- makeTheDistinctThing
   return (replicate n x)

Icon and Unicon

An incorrect approach uses, e.g., the list constructor procedure with an initial value:

 
  items_wrong := list (10, [])
  # prints '0' for size of each item
  every item := !items_wrong do write (*item)
  # after trying to add an item to one of the lists
  push (items_wrong[1], 2)
  # now prints '1' for size of each item
  every item := !items_wrong do write (*item)
 

A correct approach initialises each element separately:

 
  items := list(10)
  every i := 1 to 10 do items[i] := []
 

J

i.

J almost always uses pass-by-value, so this topic is not very relevant to J.

Java

Works with: Java version 1.5+

simple array:

Foo[] foos = new Foo[n]; // all elements initialized to null
for (int i = 0; i < foos.length; i++)
    foos[i] = new Foo();
 
// incorrect version:
Foo[] foos_WRONG = new Foo[n];
Arrays.fill(foos, new Foo());  // new Foo() only evaluated once

simple list:

List<Foo> foos = new ArrayList<Foo>();
for (int i = 0; i < n; i++)
    foos.add(new Foo());
 
// incorrect:
List<Foo> foos_WRONG = Collections.nCopies(n, new Foo());  // new Foo() only evaluated once

Generic version for class given at runtime:

It's not pretty but it gets the job done. The first method here is the one that does the work. The second method is a convenience method so that you can pass in a String of the class name. When using the second method, be sure to use the full class name (ex: "java.lang.String" for "String"). InstantiationExceptions will be thrown when instantiating classes that you would not normally be able to call new on (abstract classes, interfaces, etc.). Also, this only works on classes that have a no-argument constructor, since we are using newInstance().

public static <E> List<E> getNNewObjects(int n, Class<? extends E> c){
	List<E> ans = new LinkedList<E>();
	try {
		for(int i=0;i<n;i++)
			ans.add(c.newInstance());//can't call new on a class object
	} catch (InstantiationException e) {
		e.printStackTrace();
	} catch (IllegalAccessException e) {
		e.printStackTrace();
	}
	return ans;
}
 
public static List<Object> getNNewObjects(int n, String className)
throws ClassNotFoundException{
	return getNNewObjects(n, Class.forName(className));
}

JavaScript

var a = new Array(n);
for (var i = 0; i < n; i++)
  a[i] = new Foo();

Mathematica

The mistake is often written as:

{x, x, x, x} /. x -> Random[]

Here Random[] can be any expression that returns a new value which is incorrect since Random[] is only evaluated once. e.g. {0.175125, 0.175125, 0.175125, 0.175125}

A correct version is:

{x, x, x, x} /. x :> Random[]

which evaluates Random[] each time e.g. ->{0.514617, 0.0682395, 0.609602, 0.00177382}

Maxima

a: [1, 2]$
 
b: makelist(copy(a), 3);
[[1,2],[1,2],[1,2]]
 
b[1][2]: 1000$
 
b;
[[1,1000],[1,2],[1,2]]

Modula-3

This example is incorrect. It does not initialize the sequence. Please fix the code and remove this message.

Similar to the Ada version above:

VAR a: ARRAY OF T

This creates an open array (an array who's size is not known until runtime) of distinct elements of type T.

Modula-3 does not define what values the elements of A have, but it does guarantee that they will be of type T.

OCaml

For arrays:

Incorrect:

Array.make n (new foo);;
(* here (new foo) can be any expression that returns a new object,
   record, array, or string *)

which is incorrect since new foo is only evaluated once. A correct version is:

Array.init n (fun _ -> new foo);;

ooRexx

 
-- get an array of directory objects
array = fillArrayWith(3, .directory)
say "each object will have a different identityHash"
say
loop d over array
    say d~identityHash
end
 
::routine fillArrayWith
  use arg size, class
 
  array = .array~new(size)
  loop i = 1 to size
      -- Note, this assumes this object class can be created with
      -- no arguments
      array[i] = class~new
  end
 
return array
 

Oz

With lists, it is difficult to do wrong.

declare
  Xs = {MakeList 5} %% a list of 5 unbound variables
in
  {ForAll Xs OS.rand} %% fill it with random numbers (CORRECT)
  {Show Xs}

With arrays on the other hand, it is easy to get wrong:

declare
  Arr = {Array.new 0 10 {OS.rand}} %% WRONG: contains ten times the same number
in
  %% CORRECT: fill it with ten (probably) different numbers
  for I in {Array.low Arr}..{Array.high Arr} do
     Arr.I := {OS.rand}
  end

Pascal

See Delphi

Perl

incorrect:

(Foo->new) x $n 
# here Foo->new can be any expression that returns a reference representing
# a new object

which is incorrect since Foo->new is only evaluated once.

A correct version is:

map { Foo->new } 1 .. $n;

which evaluates Foo->new $n times and collects each result in a list.

Perl 6

Just like in Perl 5, the list repetition operator evaluates the statement only once, so

# WRONG
my @a = Foo.new xx $n;

produces $n references to the same object. This works instead:

my @a = map { Foo.new }, ^$n

PicoLisp

Create 5 distinct (empty) objects:

: (make (do 5 (link (new))))
-> ($384717187 $384717189 $384717191 $384717193 $384717195)

Create 5 anonymous symbols with the values 1 .. 5:

: (mapcar box (range 1 5))
-> ($384721107 $384721109 $384721111 $384721113 $384721115)
: (val (car @))
-> 1
: (val (cadr @@))
-> 2

PureBasic

n=Random(50)+25
Dim A.i(n)  
; Creates a Array of n [25-75] elements depending on the outcome of Random().
; Each element will be initiated to zero.
 
For i=0 To ArraySize(A())
  A(i)=2*i
Next i
; Set each individual element at a wanted (here 2*i) value and 
; automatically adjust accordingly to the unknown length of the Array.
 
NewList *PointersToA()
For i=0 To ArraySize(A())
  AddElement(*PointersToA())
  *PointersToA()=@A(i)
Next
; Create a linked list of the same length as A() above.
; Each element is then set to point to the Array element
; of the same order.
 
ForEach *PointersToA()
  Debug PeekI(*PointersToA())
Next
; Verify by sending each value of A() via *PointersToA()
; to the debugger's output.

Python

The mistake is often written as:

[Foo()] * n # here Foo() can be any expression that returns a new object

which is incorrect since Foo() is only evaluated once. A common correct version is:

[Foo() for i in range(n)]

which evaluates Foo() n times and collects each result in a list. This last form is also discussed here, on the correct construction of a two dimensional array.

R

The mistake is often written as:

rep(foo(), n)        # foo() is any code returning a value

A common correct version is:

replicate(n, foo())

which evaluates foo() n times and collects each result in a list. (Using simplify=TRUE lets the function return an array, where possible.)

Racket

 
#lang racket
 
;; a list of 10 references to the same vector
(make-list 10 (make-vector 10 0))
 
;; a list of 10 distinct vectors
(build-list 10 (λ (n) (make-vector 10 0)))
 

Ruby

The mistake is often written as one of these:

[Foo.new] * n         # here Foo.new can be any expression that returns a new object
Array.new(n, Foo.new)

which are incorrect since Foo.new is only evaluated once, and thus you now have n references to the same object. A common correct version is:

Array.new(n) { Foo.new }

which evaluates Foo.new n times and collects each result in an Array. This last form is also discussed here, on the correct construction of a two dimensional array.

Scala

Yielding a normal class instance here (rather than a case class instance), as case objects are identical if created with the same constructor arguments.

for (i <- (0 until n)) yield new Foo()

Seed7

The example below defines the local array variable fileArray. The times operator creates a new array value with a specified size. Finally multiple distinct objects are assigned to the array elements.

$ include "seed7_05.s7i";
 
const func array file: openFiles (in array string: fileNames) is func
  result
    var array file: fileArray is 0 times STD_NULL;  # Define array variable
  local
    var integer: i is 0;
  begin
    fileArray := length(fileNames) times STD_NULL;  # Array size computed at run-time
    for key i range fileArray do
      fileArray[i] := open(fileNames[i], "r");      # Assign multiple distinct objects
    end for;
  end func;
 
const proc: main is func
  local
    var array file: files is 0 times STD_NULL;
  begin
    files := openFiles([] ("abc.txt", "def.txt", "ghi.txt", "jkl.txt"));
  end func;

Smalltalk

|c|
"Create an ordered collection that will grow while we add elements"
c := OrderedCollection new.
"fill the collection with 9 arrays of 10 elements; elements (objects)
 are initialized to the nil object, which is a well-defined 'state'"
1 to: 9 do: [ :i | c add: (Array new: 10) ].
"However, let us show a way of filling the arrays with object number 0"
c := OrderedCollection new.
1 to: 9 do: [ :i | c add: ((Array new: 10) copyReplacing: nil withObject: 0) ].
"demonstrate that the arrays are distinct: modify the fourth of each"
1 to: 9 do: [ :i | (c at: i) at: 4 put: i ].
"show it"
c do: [ :e | e printNl ].

Tcl

Tcl values are implemented using copy-on-write reference semantics with no (exposed) mechanism for determining whether two values are really references to the same value, which makes this task relatively moot. However, in the case where there is a collection of objects it becomes important to perform the construction correctly (i.e., repeatedly) otherwise it is just the name of the object that will be copied when it is written to.

Works with: Tcl version 8.6

or

Library: TclOO

package require TclOO
 
# The class that we want to make unique instances of
set theClass Foo
 
# Wrong version; only a single object created
set theList [lrepeat $n [$theClass new]]
 
# Right version; objects distinct
set theList {}
for {set i 0} {$i<$n} {incr i} {
    lappend theList [$theClass new]
}

XPL0

code Reserve=3, IntIn=10;
char A;  int  N, I;  
[N:= IntIn(8);                  \get number of items from command line
A:= Reserve(N);                 \create array of N bytes
for I:= 0 to N-1 do A(I):= I*3; \initialize items with different values
for I:= 0 to N-1 do A:= I*3;    \error: "references to the same mutable object"
]