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hobby[(j.charAt(n))-1].checked=true; 【n 和 -1 的概念？】

<!DOCTYPE?HTML>
<html>
<head>
????<meta?http-equiv="Content-Type"?content="text/html;?charset=gb2312">
????<title>無標(biāo)題文檔</title>
</head>

<body>
<form>
????請選擇你愛好:<br>
????<input?type="checkbox"?name="hobby"?id="hobby1">??音樂
????<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">??閱讀
????<input?type="checkbox"?name="hobby"?id="hobby5">??打球
????<input?type="checkbox"?name="hobby"?id="hobby6">??跑步?<br>
????<input?type="button"?value?=?"全選"?onclick?=?"checkall();">
????<input?type="button"?value?=?"全不選"?onclick?=?"clearall();">
????<p>請輸入您要選擇愛好的序號，序號為1-6:</p>
????<input?id="wb"?name="wb"?type="text"?>
????<input?name="ok"?type="button"?value="確定"?onclick?=?"checkone();">
</form>
<script?type="text/javascript">
????function?checkall(){
????????var?hobby?=?document.getElementsByTagName("input");
????????for(var?i=0;i<hobby.length;i++){
????????????hobby[i].checked=true;

????????}
????????//?任務(wù)1

????}
????function?clearall(){
????????var?hobby?=?document.getElementsByName("hobby");
????????//alert(hobby.length);
????????for(var?i=0;i<hobby.length;i++){
????????????hobby[i].checked=false;
????????}
????????//?任務(wù)2

????}

????function?checkone(){??//確定
????????clearall();
????????var?hobby?=?document.getElementsByName("hobby");
????????var?j=document.getElementById("wb").value;
????????for(var?i=0;i<j.length;i++){
????????????if(j.charAt(i)>6?||?j.charAt(i)<1){
????????????????alert("越界,請重新輸入！");
????????????????document.getElementById("wb").value="";
????????????}
????????}
????????for(var?n=0;n<j.length;n++){

????????????hobby[(j.charAt(n))-1].checked=true;
????????}

????}

</script>
</body>
</html>