Adding new elements to XML

Question

Here is my Xml

<Cars>
<Car id="1" name="Opel" picture="\File\JPEG\opel.jpg" />
<Car id="2" name="Ford" picture="\File\JPEG\ford.jpg"  />
<Car id="3" name="Volvo" picture="\File\JPEG\volvo.jpg" />
</Cars>

Now I want to add new cars

Listbox1     Listbox2 
(CarName)   (Car.Picture)
 BMW        \File\JPEG\bmw.jpg
Mercedes    \File\JPEG\Merdedes.jpg

Problem I have with car.Id. Each id must be unique (+1) Having already 3 cars in the file how to make every other car id +1

My code

    var
    lNewCar: IXMLCarType;
      i, NewID : Integer;
      begin
      i:= 0;
      NewID := 1 + MaxID (Form1.Memo1.Lines.Text);
      while ( i < ListBox1.Count) and  ( i < ListBox2.Count) 
      begin
        lNewCar := XMLIntf.Cars.add;
        lNewCar.id   := NewID;
        lNewCar.name := Listbox1.Items[i];
        lNewCar.jpeg := Listbox2.Items[i];
       Inc(i);
      end;
    end;

Answer 1

In view of the simple structure of your XML, just a series of Car nodes below a Cars root node, you can find the maximum existing id attribute value by iterating the Car nodes and examining their id attributes, like this:

Sample project:

type
  TForm1 = class(TForm)
    Memo1: TMemo;
    btnMaxId: TButton;
    procedure btnMaxIdClick(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  public
  end;

function MaxId(const XML : String) : Integer;
[...]
var
  Form1: TForm1;
implementation
[...]
procedure TForm1.btnMaxIdClick(Sender: TObject);
begin
  ShowMessage(IntToStr(MaxID(Memo1.Lines.Text)));
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Memo1.Lines.Add('<Cars>');
  Memo1.Lines.Add('  <Car id="1" name="Opel" picture="\File\JPEG\opel.jpg" />');
  Memo1.Lines.Add('  <Car id="98" name="Ford" picture="\File\JPEG\ford.jpg"  />');
  Memo1.Lines.Add('  <Car id="3" name="Volvo" picture="\File\JPEG\volvo.jpg" />');
  Memo1.Lines.Add('</Cars>');
end;

function MaxId(const XML : String) : Integer;
var
  XmlDoc: IXMLDOMDocument;
  NodeList : IXmlDOMNodeList;
  Node : IXMLDomNode;
  i : Integer;
  ID : Integer;
  ErrorCode : Integer;
  S : String;
begin
  Result := 0;
  XmlDoc := CoDOMDocument.Create;
  try
    XmlDoc.Async := False;
    XmlDoc.LoadXml(XML);

    NodeList := XmlDoc.DocumentElement.childNodes;
    for i := 0 to NodeList.Length - 1 do begin
      Node := NodeList.item[i];
      S := Node.attributes.GetNamedItem('id').nodeValue;
      Val(S, ID, ErrorCode);
      if ErrorCode = 0 then begin
        if ID > Result then
          Result := ID;
      end;
    end;
  finally
    XmlDoc := Nil;
  end;
end;

end.

You need to pass your XML document as a string to this MaxID function. So, if the XML in your q were in a TMemo component on a form, you could use it like this:

var
  NewID : integer;
begin
  NewID := 1 + MaxID (Form1.Memo1.Lines.Text);
  lNewExpression.id := NewID;

There is a more direct way of getting the maximum value of an attribute, see e.g. How to find the max attribute from an XML document using Xpath 1.0 but that requires some familiarity with XPath queries and you would need to note what it says about getting the maximum value of a multi-character id.