请问下按照下面代码写为什么会出错啊？
<!DOCTYPE HTML>
<html >
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>流程控制语句</title>
<script type="text/javascript">
//第一步把之前的数据写成一个数组的形式,定义变量为 infos
var infos=[["小A","女",21,"大一"],["小B","男",23,"大三"],["小C","男",24,"大四"],["小D","女",21,"大一"],
["小E","女",22,"大四"],["小F","男",21,"大一"],["小G","女",22,"大二"],["小H","女",20,"大三"],
["小I","女",20,"大一"],["小J","男",20,"大三"]];
//第一次筛选，找出都是大一的信息
var infos2=new Array(5);
var a=0;
var b=0;
for(var i=0;i<10;i++){
if(infos[i][3]=="大一"){
infos2[a]=infos[i];
a++;
}
}
//第二次筛选，找出都是女生的信息
var infos3=new Array(3);
for(var j=0;j<infos2.length;j++){
if(infos2[j][1]=="女"){
infos3[b]=infos2[j];
b++;
}
}
document.write(infos3);
</script>
</head>
<body>
</body>
</html>我试了下如果最后一个if语句中换成document.write(infos2[j])的话是对的，不知道为什么按上面所写的会出错;
