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Frequently Asked Questions

How can I create a dynamic array of TPoints to draw a polygon?

Question:

How can I create a dynamic array of TPoints to draw a polygon?

Answer:

The easist way is to declare a type of array that has only one
element, turn range checking off, access the array through the use of
a variable, and call the Windows API function Polygon().

Example:

type
  TPtArray = array[0..0] of TPoint;
  PPtArray = ^TPtArray;

procedure DrawDynamicPolyArray(NumPoints : integer;
                               Canvas : TCanvas);
var
  p : PPtArray;
  i : integer;
begin
{$IFOPT R+}
  {$DEFINE CKRANGE}
  {$R-}
{$ENDIF}
  GetMem(p, sizeof(TPoint) * NumPoints);
  Randomize;
  for i := 0 to (NumPoints -1) do begin
    p^[i].x := Random(Form1.Width);
    p^[i].y := Random(Form1.Height);
  end;
  Polygon(Canvas.Handle, p, NumPoints);
  FreeMem(p, sizeof(TPoint) * NumPoints);
{$IFDEF CKRANGE}
  {$UNDEF CKRANGE}
  {$R+}
{$ENDIF}
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  Form1.Canvas.Brush.Color := clWhite;
  Form1.Canvas.FillRect(Rect(0, 0, Form1.Width, Form1.Height));
  Form1.Canvas.Brush.Color := clRed;
  DrawDynamicPolyArray(SpinEdit1.Value,
                       Form1.Canvas);
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  SpinEdit1.MinValue := 2;
  SpinEdit1.MaxValue := 1000;
  SpinEdit1.Value := 100;
end;

end.

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