How to reverse array of any type?

[WojTec]

Exactly, you are correct!

I thought about pointers, but I don't know how to access fields?

AData: Pointer
Inc(AData) - to get next one, usage not possible

[jaenicke]

As you have Delphi 2010 in your profile, generics should be not problem, are they?

Delphi-Quellcode:

  TArrayUtil = class
  public
    class procedure Swap<T>(var Value1, Value2: T);
    class procedure Reverse<T>(var Value: array of T);
  end;

{ TArrayUtil }

class procedure TArrayUtil.Reverse<T>(var Value: array of T);
var
  i: Integer;
begin
  if Length(Value) > 0 then
    for i := Low(Value) to High(Value) div 2 do
      Swap<T>(Value[i], Value[High(Value) - i]);
end;

class procedure TArrayUtil.Swap<T>(var Value1, Value2: T);
var
  Temp: T;
begin
  Temp := Value1;
  Value1 := Value2;
  Value2 := Temp;
end;

Example usage:

Delphi-Quellcode:

var
  Test: array of Integer;
  i: Integer;
begin
  SetLength(Test, 0);
  for i := Low(Test) to High(Test) do
    Test[i] := i;
  TArrayUtil.Reverse<Integer>(Test);
  for i := Low(Test) to High(Test) do
    ShowMessage(IntToStr(Test[i]));
end;

// EDIT:
Of course it would be faster to swap directly in procedure Reverse.

// EDIT2:
Ok, translated to English and I did not see Delphi version 7 in the post. Then this won't work.

[uligerhardt]

Zitat von jaenicke:

Da bei dir Delphi 2010 steht

Im Post selbst steht D7.

[WojTec]

Ok, nice, nice, but how to access array elements if parameter is Pointer only? Array can be in any type.

[uligerhardt]

Zitat von WojTec:

Ok, nice, nice, but how to access array elements if parameter is Pointer only? Array can be in any type.

What is a pointer? The parameter to your routine or the array element?

[WojTec]

Pointer is procedure parameter (see @Furtbichler post).

[uligerhardt]

Zitat von WojTec:

Ok, nice, nice, but how to access array elements if parameter is Pointer only? Array can be in any type.

With Furtbichler's routine signature you could have something like

Delphi-Quellcode:

procedure SwapArrayElements(aStartElement : Pointer; aElementSize, aElementCount : Integer);
var
  p: PByte;
begin
  p := AStartElement; // p points to first element
  Inc(p, aElementSize); // Now p points to second element
  Move(p^, ..., aElementSize); // Moves the element p points to
end;

(NB: These are only some building blocks of the routine, not the whole thing. )

[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