Difference between revisions of "for-in loop"

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(Limitations)
m (Traversing container)
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</delphi>
 
</delphi>
  
Ofcource enumerator support is builtin to the basic classes: TList, TStrings, TCollection, TComponent, ...
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Of course enumerator support is built into the basic classes: TList, TStrings, TCollection, TComponent, ...
  
 
For-in loop can be easily translated into the while loop. Two next examples doing same things:
 
For-in loop can be easily translated into the while loop. Two next examples doing same things:

Revision as of 08:01, 20 October 2009

"for-in" loop exists in delphi starting from 2005 version.

Delphi 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, ...

For-in loop can be easily translated into the while loop. Two next examples doing same things:

Example 1. <delphi> procedure TraverseStrings(AStrings: TStrings); var

 S: String;

begin

 for S in AStrings do
   DoSomething(S);

end; </delphi>

Example 2. <delphi> procedure TraverseStrings(AStrings: TStrings); var

 S: String;
 Enumerator: TStringsEnumerator;

begin

 Enumerator := AStrings.GetEnumerator;
 while Enumerator.MoveNext do
   DoSomething(Enumerator.Current);

end; </delphi>

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>

Limitations

1. It is imposible to traverse a collection:

The next code is not valid: <delphi> type

 TColor = (clRed, clBlue, clBlack);

var

 Color: TColor;

begin

 for Color in TColor do
   DoSomething(Color);

end. </delphi>

Although you can traverse a set. So the next code is valid: <delphi> type

 TColor = (clRed, clBlue, clBlack);

var

 Color: TColor;

begin

 for Color in [clRed..clBlack] do
   DoSomething(Color);

end. </delphi>

2. It is imposible to choose among different possible enumerators. For example you can traverse a tree using different routs. 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 next syntax:

<delphi> type

 TTreeEnumeratorType = (tePreOrder, tePostOrder, teInOrder, teBreadthFirst)

var

 Node: TNode;

begin

 for Node in Tree using GetEnumerator(teInOrder) do // or 'using GetInOrderEnumerator' or ...
   Dosomething(Node);

end. </delphi>