Multiple version of same app (Mobile)

[eluser]bapobap[/eluser]
I think I'll just go with different versions on different URL's, I can then setup the exceptions to my catch-all controller in routes and just have different controllers for those parts. I think my main problem is I haven't thought anything through!

One quick question if anyone knows, a lot of my app is based around the URI being requested, is there a better way of doing this:

if ((isset($this->uri->router->segments[4])) && ($this->uri->router->segments[4]=='filter')):

If I don't check if it's set, I get an undefined offset notice, which is fair enough but maybe there is a better way to test this.

Thanks!