procedure TForm1.ZeiButtonClick(Sender: TObject); // zeichenweise Zahleneingabe

var zei: Char;

begin

    if Sender = Button10 then zei := '0';
    if Sender = Button1 then zei := '1';
    if Sender = Button2 then zei := '2';
    if Sender = Button3 then zei := '3';
    if Sender = Button4 then zei := '4';
    if Sender = Button5 then zei := '5';
    if Sender = Button6 then zei := '6';
    if Sender = Button7 then zei := '7';
    if Sender = Button8 then zei := '8';
    if Sender = Button9 then zei := '9';
    if Sender = Button11 then zei := '.';


    if mode = 1 then
    begin
                strBuff1 := strBuff1 + zei;
          Label1.Caption := strBuff1;
    end

    else
    begin

          strBuff2 := strBuff2 + zei;
          Label1.Caption := strBuff1 + ' ' + op + ' ' + strBuff2;

    end;

end;