jquery scroll, change navigation active class as the page is scrolling, relative to sections

If you wish a more generic function:

SEE DEMO

$(window).scroll(function() {
    var windscroll = $(window).scrollTop();
    if (windscroll >= 100) {
        $('nav').addClass('fixed');
        $('.wrapper section').each(function(i) {
            if ($(this).position().top <= windscroll - 100) {
                $('nav a.active').removeClass('active');
                $('nav a').eq(i).addClass('active');
            }
        });

    } else {

        $('nav').removeClass('fixed');
        $('nav a.active').removeClass('active');
        $('nav a:first').addClass('active');
    }

}).scroll();​