Question

I am trying to retrieve a large number of images from the web using a TidHttp component.

The problem is that there is a number of images that are missing (Example: 7403, 7412, etc)

How do i test for only those that exist and save those to file?

procedure TForm.Button1Click(Sender: TObject);
var
  MS : TMemoryStream;
  JPEGImage: TJPEGImage;
  Url, numString: String;
  I, Code: Integer;
begin
 for I := 7400 to 7500 do
 begin
 { 
   Url        :='http://www.mywebpage.com/images/DSC' + numString+  '.jpg';
   try
   idhttp1.Head(URL);
   code := idhttp1.ResponseCode;
   except on E: EIdHTTPProtocolException do
    code := idhttp1.ResponseCode;
   end;//try except
   if code = 200 then
   begin

   MS := TMemoryStream.Create;
   JPEGImage  := TJPEGImage.Create;
   try
     try
     idhttp1.Get(Url, MS); //Send the request and get the image
     code := idhttp1.ResponseCode;
     MS.Seek(0,soFromBeginning);
     JPEGImage.LoadFromStream(MS);//load the image in a Stream
     Image1.Picture.Assign(JPEGImage);//Load the image in a Timage component
     Image1.Picture.SaveToFile('C:\Museum_Data\DSC' + numString + '.jpg');
     Application.ProcessMessages;
     except
      on E: EIdHTTPProtocolException do
        code := idhttp1.ResponseCode; // or: code := E.ErrorCode;
     end; //try except
          finally
    MS.free;
    JPEGImage.Free;

  end; //try finally
  end; //if

end;
 end;

Answer 1

You don’t have to do anything extra for that. If you try to access a non-existant URL, the HTTP server will report an error that TIdHTTP than wraps into an EIdHTTPProtocolException exception. You do not have to bother with calling TIdHTTP.Head() first, since you are downloading the images to a TMemoryStream before saving them. You can catch the exception when calling TIdHTTP.Get() by itself, no need to check the ResponseCode at all.

Try this:

procedure TForm.Button1Click(Sender: TObject); 
var 
  MS: TMemoryStream; 
  JPEG: TJPEGImage; 
  Url: String; 
  I: Integer; 
begin 
  MS := TMemoryStream.Create;
  try
    JPEG := TJPEGImage.Create; 
    try 
      for I := 7400 to 7500 do 
      begin 
        Url := 'http://www.mywebpage.com/images/DSC' + IntToStr(I) +  '.jpg';
        MS.Clear;
        try
          IdHTTP1.Get(Url, MS);
        except
          on E: EIdHTTPProtocolException do 
            Continue;
        end;
        MS.Position := 0; 
        JPEG.LoadFromStream(MS);
        Image1.Picture.Assign(JPEG);
        JPEG.SaveToFile('C:\Museum_Data\DSC' + IntToStr(I) + '.jpg'); 
        Application.ProcessMessages; 
      end;
    finally
      JPEG.Free;
    end;
  finally 
    MS.Free;
  end;
end;

You do not actually need the TImage in order to save the data to file. If you can omit the TImage.Picture.Assign() stage, then the code a bit simpler by eliminating the TJPEGImage altogether (unless you are trying to validate the download files are valid), eg:

procedure TForm.Button1Click(Sender: TObject); 
var 
  MS: TMemoryStream; 
  Url: String; 
  I: Integer; 
begin 
  MS := TMemoryStream.Create;
  try
    for I := 7400 to 7500 do 
    begin 
      Url := 'http://www.mywebpage.com/images/DSC' + IntToStr(I) +  '.jpg';
      MS.Clear;
      try
        IdHTTP1.Get(Url, MS);
      except
        on E: EIdHTTPProtocolException do 
          Continue;
      end;
      MS.Position := 0; 
      MS.SaveToFile('C:\Museum_Data\DSC' + IntToStr(I) + '.jpg'); 
      Application.ProcessMessages; 
    end;
  finally 
    MS.Free;
  end;
end;

Or:

procedure TForm.Button1Click(Sender: TObject); 
var 
  FS: TFileStream; 
  Url, FileName: String; 
  I: Integer; 
begin 
  for I := 7400 to 7500 do 
  begin 
    Url := 'http://www.mywebpage.com/images/DSC' + IntToStr(I) +  '.jpg';
    FileName := 'C:\Museum_Data\DSC' + IntToStr(I) + '.jpg'; 
    FS := TFileStream.Create(FileName, fmCreate);
    try
      try
        try
          IdHTTP1.Get(Url, FS);
        except
          on E: EIdHTTPProtocolException do 
            Continue;
        end;
        Application.ProcessMessages; 
      finally
        Fs.Free;
      end;
    except
      DeleteFile(FileName);
    end;
  end;
end;