var urls = ["http://www.getbootstrap.com",

            "http://www.reddit.com",

            "http://www.bbc.com/news",

            "http://www.google.com"

           ],

    c = 0,

    n = urls.length;

​

$('#forwards, #backwards').click(function( e ){

    e.preventDefault(); 

    c = this.id=='forwards' ? ++c : --c;

    c = c<0? n-1 : c%n ;

    $('p').text(urls[c]);

    //$('iframe')[0].src = urls[c];

});