第三个函数哪里有问题？？？求大神帮忙看看。
<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<meta name="Generator" content="EditPlus®">
<meta name="Author" content="">
<meta name="Keywords" content="">
<meta name="Description" content="">
<title>Document</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
<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()">
</form>
<script type="text/javascript">
function checkall(){
var hobby = document.getElementsByTagName("input");
for(var i = 0; i < hobby.length - 4; i++)
{
hobby[i].checked = true;
}
// 任务1
}
function clearall(){
var hobby = document.getElementsByName("hobby");
for(var j = 0; j < hobby.length; j++)
{
hobby[j].checked = false;
}
// 任务2
}
function checkone(){
var j = document.getElementById("wb").value;
var hobby = document.getElementById("hobby"+j);
if(k < 1||k > 6)
{
alert(请输入(1-6)之间的整数。);
return;
}
hobby.checked = true;
// 任务3
/*var hobby = document.getElementsByName("hobby");
var j=document.getElementById("wb").value;
if(j<1||j>6)
{
alert("请输入正确的数字！");
return;
}
hobby[j-1].checked=true;
}*/
</script>
</body>
</html>
