第三个练习的效果图不动,在这更新

但是第三个练习,我没有办法循环,还请大神帮忙。
最后一张的时候,不知道该怎么操作,加清掉class的话,总是飘来飘去的。
代码是:
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>妙味课堂视频自学练习-vip群活动-图片滑动切换扩展课后练习</title>
</head>
<style>
body { background:#BF7274;}
#box { margin:30px auto; width:310px; height:450px; position: relative; border:15px solid #BC383A; border-radius:24px; }
#box a { font-size:15px; color:#000; width:30px; height:30px; line-height:30px; text-align:center; background:#F1F1F1; text-decoration:none; position:absolute; top:210px; border-radius:24px; opacity:0.8; z-index:5}
#box a:hover { background:#FFFFFF; opacity:1;}
#pic { width:310; height:450px; position:absolute; top:0; left:0; }
#box img { position:absolute; top:0; left:0; width:310px; height:450px; border:none; transition:1s;}
#box .ileft { left:-310px; opacity:0;}
#box .iright { left:310px ;opacity:0;}
#prev { left:-15px;}
#next { right:-15px;}
</style>
<script>
window.onload = function(){
var oBox = document.getElementById(*box*);
var oPic = document.getElementById(*pic*);
var oPrev = document.getElementById(*prev*);
var oNext = document.getElementById(*next*);
var aImg = document.getElementsByTagName(*img*);
var arrP = [*img/1.jpg*,*img/2.jpg*,*img/3.jpg*,*img/4.jpg*,*img/5.jpg*];
var num = 0;
var iZ=0;
for(var i=0; i<arrP.length*2; i++){
oPic.innerHTML += *<img src=""/>*;
}
for(var i=0; i<arrP.length*2; i++){
iZ++;
aImg[i].src = arrP[i%arrP.length];
aImg[i].style.zIndex=arrP.length-iZ;
}
oPrev.onclick = tabl;
oNext.onclick = tabr;
function tabl(){
if (num ==arrP.length){
//alert(*亲,最后一张了,还重看不?*); //怎么重新开始循环
num = 0;
tabl();
}
aImg[num].className = *ileft*;
num++ ;
};
function tabr(){
if (num ==arrP.length){
num = 0;
}
aImg[num].className = *iright*;
num++ ;
};
};
</script>
<body>
<div id="box">
<div id="pic"></div>
<a id="prev" href="javascript:;"><</a>
<a id="next" href="javascript:;">></a>
</div>
</body>
</html>

但是第三个练习,我没有办法循环,还请大神帮忙。
最后一张的时候,不知道该怎么操作,加清掉class的话,总是飘来飘去的。
代码是:
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>妙味课堂视频自学练习-vip群活动-图片滑动切换扩展课后练习</title>
</head>
<style>
body { background:#BF7274;}
#box { margin:30px auto; width:310px; height:450px; position: relative; border:15px solid #BC383A; border-radius:24px; }
#box a { font-size:15px; color:#000; width:30px; height:30px; line-height:30px; text-align:center; background:#F1F1F1; text-decoration:none; position:absolute; top:210px; border-radius:24px; opacity:0.8; z-index:5}
#box a:hover { background:#FFFFFF; opacity:1;}
#pic { width:310; height:450px; position:absolute; top:0; left:0; }
#box img { position:absolute; top:0; left:0; width:310px; height:450px; border:none; transition:1s;}
#box .ileft { left:-310px; opacity:0;}
#box .iright { left:310px ;opacity:0;}
#prev { left:-15px;}
#next { right:-15px;}
</style>
<script>
window.onload = function(){
var oBox = document.getElementById(*box*);
var oPic = document.getElementById(*pic*);
var oPrev = document.getElementById(*prev*);
var oNext = document.getElementById(*next*);
var aImg = document.getElementsByTagName(*img*);
var arrP = [*img/1.jpg*,*img/2.jpg*,*img/3.jpg*,*img/4.jpg*,*img/5.jpg*];
var num = 0;
var iZ=0;
for(var i=0; i<arrP.length*2; i++){
oPic.innerHTML += *<img src=""/>*;
}
for(var i=0; i<arrP.length*2; i++){
iZ++;
aImg[i].src = arrP[i%arrP.length];
aImg[i].style.zIndex=arrP.length-iZ;
}
oPrev.onclick = tabl;
oNext.onclick = tabr;
function tabl(){
if (num ==arrP.length){
//alert(*亲,最后一张了,还重看不?*); //怎么重新开始循环
num = 0;
tabl();
}
aImg[num].className = *ileft*;
num++ ;
};
function tabr(){
if (num ==arrP.length){
num = 0;
}
aImg[num].className = *iright*;
num++ ;
};
};
</script>
<body>
<div id="box">
<div id="pic"></div>
<a id="prev" href="javascript:;"><</a>
<a id="next" href="javascript:;">></a>
</div>
</body>
</html>