使用js给二维数组创建不同id的问题

我想使用嵌套for循环给多个a标签赋予不同的id值，如

<a href = "#" id = "cell-00"></a>
<a href = "#" id = "cell-01"></a>
<a href = "#" id = "cell-10"></a>
<a href = "#" id = "cell-11"></a>

编写代码如下：

var num = 2;
for(var i = 0;i < num;i++){
    for(var j = 0;j < num;j++){
        var a = document.getElementsByTagName("a");
        a[j][i].setAttribute("id","cell-"+i+j);
    }
}

报错：Uncaught TypeError: Cannot read property 'setAttribute' of undefined
想请问下大家报错的原因是什么，该怎样解决呢？谢谢

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document.getElementsByTagName("a");//返回一维数组，length==4

---

可能没配置好

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var num = 2;
var a = document.getElementsByTagName("a");
for(var i = 0;i < num;i++){
    for(var j = 0;j < num;j++){
        a[i*2+j].setAttribute("id","cell-"+i+j);
    }
}