我无限滚动了一轮后会出现图片位置不一样，第一轮的情况正常，但是到了第二轮的时候，比如按下向右箭头原来的index=1的图片变到了index=3的位置或者其他位置，按下左箭头也会出现这样的情况.

window.onload=function(){
            var container=document.getElementById('container');
            var list=document.getElementById('list');
            var buttons=document.getElementById('buttons').getElementsByTagName('span');
            var next=document.getElementById('next');
            var prev=document.getElementById('prev');
            var index=1;
            function animate(offset){
                var newLeft=parseInt(list.style.left)+offset;
                list.style.left=newLeft+'px';
                if(newLeft<-6000){
                    list.style.left=-1000+'px';
                }
                if(newLeft>-1000){
                    list.style.left=-6000+'px';
                }

            }
            function showButton(){
                for(var i=0;i<buttons.length;i++){
                    if(buttons[i].className=='on'){
                        buttons[i].className='';
                        break;
                    }
                }
                buttons[index-1].className='on';
            }
            next.onclick=function(){
               
                if(index==5){
                    index=1;
                }
                else{
                    index+=1;
                }
                animate(-1000);
                showButton();
            }
            prev.onclick=function(){
               
                if(index==1){
                    index=5;
                }
                else{
                    index-=1;
                }
                animate(1000);
                showButton();
            }
            for(var i=0;i<buttons.length;i++){
                buttons[i].onclick=function(){
                    if(this.className=='on'){
                        return;
                    }
                    var myIndex=parseInt(this.getAttribute('index'));
                    var offset=-1000*(myIndex-index);
                    animate(offset);
                    index=myIndex;
                    showButton();
                }
            }
        }