成功运行出来了！第三步我用的是switch..case...实现，这样用不知道有没有其他问题。
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>无标题文档</title>
</head>
<body>
<form>
请选择你爱好:<br>
<input type="checkbox" name="hobby" id="hobby1"> 音乐
<input type="checkbox" name="hobby" id="hobby2"> 登山
<input type="checkbox" name="hobby" id="hobby3"> 游泳<br/>
<input type="checkbox" name="hobby" id="hobby4"> 阅读
<input type="checkbox" name="hobby" id="hobby5"> 打球
<input type="checkbox" name="hobby" id="hobby6"> 跑步 <br>
<input type="button" value = "全选" onclick = "checkall();">
<input type="button" value = "全不选" onclick = "clearall();">
<p>请输入您要选择爱好的序号，序号为1-6:</p>
<input id="wb" name="wb" type="text" >
<input name="ok" type="button" value="确定" onclick = "checkone();">
<input name="cancel" type="button" value="取消" onclick = "clean();">
</form>
<script type="text/javascript">
// 任务1
function checkall(){
var hobby = document.getElementsByTagName("input");
for(var i=0;i<hobby.length;i++)
if(hobby[i].type=="checkbox")
hobby[i].checked=true;
}
// 任务2
function clearall(){
var hobby = document.getElementsByName("hobby");
for(var i=0;i<hobby.length;i++)
if(hobby[i].type=="checkbox")
hobby[i].checked=false;
}
// 任务3
function checkone(){
var j=document.getElementById("wb").value;
var hobby = document.getElementsByName("hobby");
switch(parseInt(j)){
case 1:hobby[0].checked=true; break;
case 2:hobby[1].checked=true; break;
case 3:hobby[2].checked=true; break;
case 4:hobby[3].checked=true; break;
case 5:hobby[4].checked=true; break;
case 6:hobby[5].checked=true; break;
default:alert("请重新输入(1-6)的数字！");
}
}
function clean(){
var hobby=document.getElementsByName("hobby");
for(var i=0;i<hobby.length;i++){
hobby[i].checked=false;
}
document.getElementById("wb").value="";
}
</script>
</body>
</html>
