How to reverse array of any type?

[himitsu]

I submitted an expansion proposal at Embarcadero.
http://qc.embarcadero.com/wc/qcmain.aspx?d=110391
You may like to vote for it, so it will soon be integrated.
For all Delphi versions from 2009, you can upgrade this as a class helper.

Otherwise, these methods can of course also copy out and use directly.

Zitat:

procedure SwapArrayElements(aStartElement: Pointer; aElementSize, aElementCount: Integer);

Or you get the necessary information about the RTTI/TypeInfo.

Delphi-Quellcode:

procedure SwapArrayElements(var TheArray; ArrTypeInfo: Pointer);

var MyArr: array of ...;
SwapArrayElements(MyArr, TypeInfo(MyArr));

[WojTec]

Delphi-Quellcode:

procedure ReverseArray(AData: Pointer; const ASize, ACount: Integer);
var
  Left, Right, Temp: Integer;
  P: PCardinal;
begin
  P := AData;

  Left := 0;
  Right := ACount;

  Result := Left < Right;

  if Result then
  begin
    while (Left < Right) do
    begin
      {Temp := AValues[Left];
      AValues[Left]  := AValues[Right];
      AValues[Right] := Temp;}  // Don't know how to exchange with pointers :(
      //Move(P^, ..., ASize);  // Don't know how if I don't know types ????????

      Inc(P);
      Inc(Left);
      Dec(Right);
    end;
  end;
end;

I don't know how replace part should works. Above is form reversing knowed type array.

[Medium]

You need to understand, that you can not operate in a array-like fashion anymore, if you want a fully generic function. You have to go down an abstraction layer, and work on the actual memory.
An array simply is a sequence of values written behind each other in memory. Every element has a length (in bytes), but the length will be determined by the data type in the array. You employ pointer operations instead of the compiler magic (pascal arrays) to gain the possibility to work with data of arbitrary size. Without Generics, there just is no other way. So instead of array indexes, you point to elements using a memory address and the size of the elements. Traversing one element ahead "increase index by 1" then becomes "take pointer of the current element, and add the element-size to it". Copying "myArray[i] := myArray[m]" becomes a raw memory move-operation "Move(DestinationPointer, SourcePointer, ElementSizeInBytes)". As long as you have anything with square brackets "[]" in your reverse-function, you can be sure to think too high-level

[WojTec]

Yes yes, I know So I'm asking how to do it? Because I don't know how do it on pointers only?

BTW: @himitsu I voted for your extended TArray

[uligerhardt]

Zitat von WojTec:

Delphi-Quellcode:

procedure ReverseArray(AData: Pointer; const ASize, ACount: Integer);
var
  Left, Right, Temp: Integer;
  P: PCardinal;
begin
  P := AData;

  Left := 0;
  Right := ACount;

  Result := Left < Right;

  if Result then
  begin
    while (Left < Right) do
    begin
      {Temp := AValues[Left];
      AValues[Left]  := AValues[Right];
      AValues[Right] := Temp;}  // Don't know how to exchange with pointers :(
      //Move(P^, ..., ASize);  // Don't know how if I don't know types ????????

      Inc(P);
      Inc(Left);
      Dec(Right);
    end;
  end;
end;

I don't know how replace part should works. Above is form reversing knowed type array.

Don't use PCardinal and Inc(P) because this would limit you to elements with the same size as Cardinal: if P is of type PCardinal , then it points to an element of SizeOf(Cardinal) bytes. If you then do Inc(P) you advance P by SizeOf(Cardinal) bytes. But you want P to point to elements of size ASize bytes, so use PByte and Inc(P, ASize) (as I already wrote ).

[himitsu]

Delphi-Quellcode:

procedure ReverseArray(AData: Pointer; const ASize, ACount: Integer);
var
  Temp: array of Byte;
  Data: PAnsiChar absolute AData;  // PAnsiChar verfügt über Zeiger-Arithmetik
  i, j: Integer;
begin
  SetLength(Temp, ASize);
  i := ACount div 2 - 1;
  while i >= 0 do begin
    j := ACount - i - 1;
    Move(Data[i * ASize], Temp[0], ASize);
    Move(Data[j * ASize], Data[i * ASize], ASize);
    Move(Temp[0], Data[j * ASize], ASize);
    Dec(i);
  end;
end;

[WojTec]

Thanks Now I'll try to transform Shuffle() in this way