for-in loop/fr
│
English (en) │
français (fr) │
日本語 (ja) │
русский (ru) │
La construction de boucle "for-in" est pris en charge dans Delphi de Delphi 2005. Il a été mis en œuvre en FPC 2.4.2.
Implémentation Delphi et FPC
Une boucle for in respecte la syntaxe suivante:
Boucle sur une chaîne
procedure StringLoop(S: String);
var
C: Char;
begin
for C in S do
DoSomething(C);
end;
Boucle sur un tableau
procedure ArrayLoop(A: Array of Byte);
var
B: Byte;
begin
for B in A do
DoSomething(B);
end;
Boucle sur un ensemble
type
TColor = (cRed, cGren, cBlue);
TColors = set of TColor;
procedure SetLoop(Colors: TColors);
var
Color: TColor;
begin
for Color in Colors do
DoSomething(Color);
end;
Traverser un conteneur
Pour travrser une classe conteneur, vous avez besoin d'ajouter un énumérateur. Un Enumérateur est une classe construite selon le modèle suivant :
TSomeEnumerator = class
public
function MoveNext: Boolean;
property Current: TSomeType;
end;
Il faut seulement 2 choses pour définir une classe Enumerateur: une méthode MoveNext qui demande à l'énumérateur d'avancer d'un pas et une propriété Current qui peut retourner tout type approprié.
Par la suite, vous devez ajouter la méthode magique GetEnumerator de la classe conteneur qui retourne un énumérateur de l'instance.
Par exemple:
type
TEnumerableTree = class;
TTreeEnumerator = class
private
FTree: TEnumerableTree;
FCurrent: TNode;
public
constructor Create(ATree: TEnumerableTree);
function MoveNext: Boolean;
property Current: TNode read FCurrent;
end;
TEnumerableTree = class
public
function GetEnumerator: TTreeEnumerator;
end;
constructor TTreeEnumerator.Create(ATree: TEnumerableTree);
begin
inherited Create;
FTree := ATree;
FCurrent := nil;
end;
function TTreeEnumerator.MoveNext: Boolean;
begin
// pour obtenir le noeud suivant dans l'arbre
if FCurrent = nil then
FCurrent := FTree.GetFirstNode
else
FCurrent := FTree.GetNextNode(FCurrent);
Result := FCurrent <> nil;
end;
function TEnumerableTree.GetEnumerator: TTreeEnumerator;
begin
Result := TTreeEnumerator.Create(Self);
// Important: l'objet créé est automatiquement libéré par le compilateur après la boucle.
end;
Ensuite, cela vous permet de d'exécuter le code suivant:
procedure TreeLoop(ATree: TEnumerableTree);
var
ANode: TNode;
begin
for ANode in ATree do
DoSomething(ANode);
end;
Vos trouverez que plusieurs classes de bases (telles que TList, TStrings, TCollection, TComponent ...) ont des énumérateurs intégrés.
Il est aussi possible de rendre une classe énumérable si vous implémentez l'interface suivante dans votre classe conteneur enumérable:
IEnumerable = interface(IInterface)
function GetEnumerator: IEnumerator;
end;
ooù IEnumerator est déclarée comme :
IEnumerator = interface(IInterface)
function GetCurrent: TObject;
function MoveNext: Boolean;
procedure Reset;
property Current: TObject read GetCurrent;
end;
Enumérateurs multiple pour une classe
Vous pouvez ajouter des énumérateur supplémentaires à des classes, des objets et des enregistrements.
Voici un exemple d'ajout d'énumérateur qui traverse un TEnumerableTree en ordre inverse:
type
TEnumerableTree = class;
TTreeEnumerator = class
...pour la traversée dans l'ordre, voir au dessus...
end;
TTreeReverseEnumerator = class
private
FTree: TEnumerableTree;
FCurrent: TNode;
public
constructor Create(ATree: TEnumerableTree);
function MoveNext: Boolean;
property Current: TNode read FCurrent;
function GetEnumerator: TTreeReverseEnumerator; // se retourne lui-même
end;
TEnumerableTree = class
public
function GetEnumerator: TTreeEnumerator;
function GetReverseEnumerator: TTreeReverseEnumerator;
end;
...voir au dessus l'implémentation de TTreeEnumerator...
constructor TTreeReverseEnumerator.Create(ATree: TEnumerableTree);
begin
inherited Create;
FTree := ATree;
end;
function TTreeReverseEnumerator.MoveNext: Boolean;
begin
// some logic to get the next node from a tree in reverse order
if FCurrent = nil then
FCurrent := FTree.GetLastNode
else
FCurrent := FTree.GetPrevNode(FCurrent);
Result := FCurrent <> nil;
end;
function TTreeReverseEnumerator.GetEnumerator: TTreeReverseEnumerator;
begin
Result := Self;
end;
function TEnumerableTree.GetReverseEnumerator: TTreeReverseEnumerator;
begin
Result := TTreeReverseEnumerator.Create(Self);
// Note: l'objet est automatiquement supprimé par le compilateur après la boucle.
end;
Après, vous êtes en mesure d'exécuter le code suivant:
procedure TreeLoop(ATree: TEnumerableTree);
var
ANode: TNode;
begin
for ANode in ATree.GetReverseEnumerator do
DoSomething(ANode);
end;
FPC extensions
The following code examples illustrate constructs that are implemented only by FPC, constructs which are not supported by Delphi.
Traversing enumeration and subrange types
In Delphi, it is not possible to traverse either enumerated types or subrange types, whereas in Free Pascal we can write the following:
type
TColor = (clRed, clBlue, clBlack);
TRange = 'a'..'z';
var
Color: TColor;
ch: Char;
begin
for Color in TColor do
DoSomething(Color);
for ch in TRange do
DoSomethingOther(ch);
end.
Declaring enumerators
It is also not possible in Delphi to add an enumerator without modifying the class, nor to add an enumerator to the non-class/object/record/interface type. FPC makes this possible using the new syntax operator type Enumerator. As in the following example:
type
TMyRecord = record F1: Integer; F2: array of TMyType; end;
TMyArrayEnumerator = class
private
function GetCurrent: TMyType;
public
constructor Create(const A: TMyRecord);
property Current: TMyType read GetCurrent;
function MoveNext: Boolean;
end;
// This is new built-in operator.
operator Enumerator(const A: TMyRecord): TMyArrayEnumerator;
begin
Result := TMyArrayEnumerator.Create(A);
end;
var
A: MyRecord;
V: TMyType
begin
for V in A do
DoSomething(V);
end.
Traversing UTF-8 strings
As a particularly useful example, the above extension allows very efficient traversal of UTF-8 strings:
uses
LazUTF8;
interface
type
{ TUTF8StringAsStringEnumerator
Traversing UTF8 codepoints as strings is useful when you want to know the
exact encoding of the UTF8 character or if you like to use string
constants in your code.
For security reasons you should use the codepoints values (cardinals) instead.
If speed matters, don't use enumerators. Instead use the PChar directly as
shown in the MoveNext method and read about UTF8. It has some interesting features. }
TUTF8StringAsStringEnumerator = class
private
fCurrent: UTF8String;
fCurrentPos, fEndPos: PChar;
function GetCurrent: UTF8String;
public
constructor Create(const A: UTF8String);
property Current: UTF8String read GetCurrent;
function MoveNext: Boolean;
end;
operator Enumerator(A: UTF8String): TUTF8StringAsStringEnumerator;
var
Form1: TForm1;
implementation
operator Enumerator(A: UTF8String): TUTF8StringAsStringEnumerator;
begin
Result := TUTF8StringAsStringEnumerator.Create(A);
end;
{ TUTF8StringAsStringEnumerator }
function TUTF8StringAsStringEnumerator.GetCurrent: UTF8String;
begin
Result:=fCurrent;
end;
constructor TUTF8StringAsStringEnumerator.Create(const A: UTF8String);
begin
fCurrentPos:=PChar(A); // Note: if A='' then PChar(A) returns a pointer to a #0 string
fEndPos:=fCurrentPos+length(A);
end;
function TUTF8StringAsStringEnumerator.MoveNext: Boolean;
var
l: Integer;
begin
if fCurrentPos<fEndPos then
begin
l:=UTF8CharacterLength(fCurrentPos);
SetLength(fCurrent,l);
Move(fCurrentPos^,fCurrent[1],l);
inc(fCurrentPos,l);
Result:=true;
end else
Result:=false;
end;
{ TForm1 }
procedure TForm1.FormCreate(Sender: TObject);
var
s, ch: UTF8String;
i: SizeInt;
begin
s:='mäßig';
// using UTF8Length and UTF8Copy this way is slow, requiring O(n)^2
for i:=1 to UTF8Length(s) do
writeln('ch=',UTF8Copy(s,i,1));
// using the above enumerator is shorter and quite fast, requiring O(n)
for ch in s do
writeln('ch=',ch);
end;
Using any identifiers instead of builtin MoveNext and Current
In Delphi you must use a function with the name 'MoveNext' and a property with the name 'Current' in enumerators. With FPC you can choose whatever names you wish. This is enabled by the use of the enumerator modifier, with the syntax 'enumerator MoveNext;' and 'enumerator Current;' modifiers. As in the following example:
type
{ TMyListEnumerator }
TMyListEnumerator = object
private
FCurrent: Integer;
public
constructor Create;
destructor Destroy;
function StepNext: Boolean; enumerator MoveNext;
property Value: Integer read FCurrent; enumerator Current;
end;
TMyList = class
end;
{ TMyListEnumerator }
constructor TMyListEnumerator.Create;
begin
FCurrent := 0;
end;
destructor TMyListEnumerator.Destroy;
begin
inherited;
end;
function TMyListEnumerator.StepNext: Boolean;
begin
inc(FCurrent);
Result := FCurrent <= 3;
end;
operator enumerator (AList: TMyList): TMyListEnumerator;
begin
Result.Create;
end;
var
List: TMyList;
i: integer;
begin
List := TMyList.Create;
for i in List do
WriteLn(i);
List.Free;
end.
Proposed extensions
Get enumerator Position if available
It is impossible to extract any information from the iterator except the current item. Sometimes other data, such as the current index, might be useful:
type
TUTF8StringEnumerator = class
private
FByteIndex: Integer;
FCharIndex: Integer;
public
constructor Create(const A: UTF8String);
function Current: UTF8Char;
function CurrentIndex: Integer;
function MoveNext: Boolean;
end;
operator GetEnumerator(A: UTF8String): TUTF8StringEnumerator;
begin
Result := TUTF8String.Create(A);
end;
var
s: UTF8String;
ch: UTF8Char;
i: Integer;
begin
// Inefficient, as discussed above
for i := 1 to Length(s) do
Writeln(i, ': ', ch[i]);
// Ok, but ugly
i := 1;
for ch in s do begin
Writeln(i, ': ', ch);
Inc(i);
end;
// Proposed extension
for ch in s index i do
Writeln(i, ': ', ch);
// Proposed extension for traversing backwards (equivalent to downto)
for ch in reverse s do
Writeln(i, ': ', ch);
// With proposed index extension
for ch in reverse s index i do
Writeln(i, ': ', ch);
end.
Note that index could be designed to return an arbitrary type (i. e. not necessarily an integer). For example, in the case of tree traversal, the index might return an array of nodes describing the path from the tree root to the current node.