How to reverse array of any type?

[WojTec]

Hi, is possible to reverse (I actually need this, but I mean processing in any way) array of any type (numbers, string, booleans, etc.) by one procedure?

[Furtbichler]

By 'reverse' you mean: ABCD => DCBA?

Well, that's simple: All you need is the Address of the first element, the size of an element and the total number of elements (n), then you exchange elements 0 with n-1, 1 with n-2 and so on. To exchange an element you will need a temporary storage of ElementSize Bytes.

The signature of the function could be:
Procedure SwapArrayElements(aStartElement : Pointer; aElementSize, aElementCount : Integer);

[uligerhardt]

That is what generics are for, but they are available only from Delphi 2009 and up.

[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. )