for-in loop
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"for-in" loop exists in delphi starting from 2005 version. It is implemented now in fpc trunk.
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 constructor Create(const A: TMyRecord); function Current: TMyType; 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; public constructor Create(const A: UTF8String); function Current: UTF8Char; function MoveNext: Boolean; end;
operator Enumerator(A: UF8String): TUF8StringEnumerator; begin Result := TUF8String.Create(A); end;
var
s: UF8String; ch: UF8Char; 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: UF8String): TUF8StringEnumerator; begin Result := TUF8String.Create(A); end;
var
s: UF8String; ch: UF8Char; 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.