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:

Delphi-Quellcode:

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:

Zitat von jaenicke (Beitrag 1194030)

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:

Zitat von WojTec (Beitrag 1194035)

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:

Zitat von WojTec (Beitrag 1194035)

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. :mrgreen:)