for-in loop

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Revision as of 03:25, 24 January 2011 by AndrewH (talk | contribs) (Declaring enumerators: added missing function to example)

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"for-in" loop exists in delphi starting from 2005 version. It is implemented now in FPC 2.4.2.

Delphi and FPC implementation

It has the next syntax:

String loop

<delphi> procedure StringLoop(S: String); var

 C: Char;

begin

 for C in S do
   DoSomething(C);

end; </delphi>

Array loop

<delphi> procedure ArrayLoop(A: Array of Byte); var

 B: Byte;

begin

 for B in A do
   DoSomething(B);

end; </delphi>

Set loop

<delphi> 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; </delphi>

Traversing container

To traverse some container class you need to add an enumerator for it. Enumerator is a class built by the next template:

<delphi> TSomeEnumerator = class public

 function MoveNext: Boolean;
 property Current: TSomeType;

end; </delphi>

There are only 2 things required for the enumerator: MoveNext method which asks enumerator to step forward and property Current which can return any desired type.

Next thing is to add magic GetEnumerator method to the container class which returns an enumerator instance.

For example: <delphi> 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

 // some logic to get the next node from a tree
 if FCurrent = nil then
   FCurrent := FTree.GetFirstNode
 else
   FCurrent := FTree.GetNextNode(FCurrent);
 Result := FCurrent <> FTree.GetLastNode;

end;

function TEnumerableTree.GetEnumerator: TTreeEnumerator; begin

 Result := TTreeEnumerator.Create(Self);

end;

</delphi>

After this you are able to execute the next code:

<delphi> procedure TreeLoop(ATree: TEnumerableTree); var

 ANode: TNode;

begin

 for ANode in ATree do
   DoSomething(ANode);

end; </delphi>

Of course enumerator support is built into the basic classes: TList, TStrings, TCollection, TComponent, ...

It is also possible to make some class enumerable if you implement the next interface for the container: <delphi>

 IEnumerable = interface(IInterface)
   function GetEnumerator: IEnumerator;
 end;

</delphi>

Where IEnumerator is declared as: <delphi>

 IEnumerator = interface(IInterface)
   function GetCurrent: TObject;
   function MoveNext: Boolean;
   procedure Reset;
   property Current: TObject read GetCurrent;
 end;

</delphi>

FPC extensions

The following examples are not supported by Delphi, and implemented in FPC only.

Traverse enumerations and ranges

In Delphi, it is impossible to traverse neither enumerated nor ranged type:

<delphi> 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. </delphi>

Declaring enumerators

It is also not possible in Delphi to add an enumerator without modifying the class, as well as add an enumerator to the non-class/object/record/interface type. It is possible in FPC by adding new operator type Enumerator. Like in the next example:

<delphi> 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. </delphi>

As a particularly useful example, the above extension would allow to traverse UTF-8 strings efficiently:

<delphi> type

 TUTF8StringEnumerator = class
 private
   FByteIndex: Integer;
   FCharIndex: Integer;
   function GetCurrent: UTF8Char;
 public
   constructor Create(const A: UTF8String);
   property Current: UTF8Char read GetCurrent;
   function MoveNext: Boolean;
 end;
 operator Enumerator(A: UTF8String): TUTF8StringEnumerator;
 begin
   Result := TUTF8String.Create(A);
 end;

var

 s: UTF8String;
 ch: UTF8Char;
 i: Integer;

begin

 // This requires O(N^2) operations
 for i := 1 to Length(s) do
   DoSomething(ch[i]);
 // This requires only O(N) operations
 for ch in s do
   DoSomething(ch);

end. </delphi>

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 solved this by using 'enumerator MoveNext' and 'enumerator Current' modifiers. Like in the next example:

<delphi> 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. </delphi>

Proposed extensions

Select which enumerator to use

It is impossible to choose among different possible enumerators. For example you can traverse a tree using different orders. The well known algorithms are: preorder, postorder, inorder and breadth‑first traversals. Therefore it would be useful to have an ability to choose an enumerator. For example using the following syntax:

<delphi> type

 TTreeEnumeratorType = (tePreOrder, tePostOrder, teInOrder, teBreadthFirst)

procedure TraverseTree(Tree: TTree); var

 Node: TNode;

begin

 // Variant1. For the class instances we can call the method Tree.GetEnumerator(teInOrder). 
 // For the classes we can call a class method
 for Node in Tree using GetEnumerator(teInOrder) do
   Dosomething(Node);
 // Variant2. Or we can call the global function
 for Node in Tree using GetEnumerator(Tree, teInOrder) do
   Dosomething(Node);
 // Variant3. In the previous variant 'in Tree' is useless so the next code is a simplified form:
 for Node using GetEnumerator(Tree, teInOrder) do
   Dosomething(Node);
 // Variant4. We can try to avoid new context key-word 'using' by calling method:
 for Node in Tree.GetSomeEnumerator(teInOrder) do
   Dosomething(Node);
 // but this brings ambiguity to the compiler since Tree.GetSomeEnumerator(teInOrder) can be translated into
 // Tree.GetSomeEnumerator(teInOrder).GetEnumerator
 // This ambiguity might be resolvable by checking whether the class implements IEnumerator interface

end;

// for basic type we will call only the apropriate function procedure TraverseString(S: String); var

 C: Char;

begin

 for C in S using GetReverseStringEnumerator(S) do
   DoSomething(C);

end; </delphi>

Get enumerator Position if available

Finally, it is impossible to extract any information from the iterator except the current item. Sometimes other data, such as current index, may be useful:

<delphi> 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);

end. </delphi>

Note that index might return arbitrary type, not necessarily integer. For example, in the case of tree traversal, the index might return an array of nodes from on the path from the tree root to the current node.


Reference