DOM的用法，以及window.onload……eval()

var c=window.onload||function (){var num = parseInt(document.getElementById("course").getAttribute("colspan"));//alert(num);var str="";for(var i=0;i<num;i++){var tag = "'c_'+i";//alert(eval(tag));此处注意eval的用法，i是个变值，var c =document.getElementsByName(eval(tag));var sum=0;for(var j=0;j<c.length;j++){sum+=parseInt(c[j].innerText);}str+="<td>"+sum+"</td>";}document.getElementById("single").outerHTML=str;};window.onload = countAll();function countAll(){var students = document.getElementsByName("all");var sum=0;for(var i=0;i<students.length;i++){sum+=parseInt(students[i].innerText);}document.getElementById("all_sum").innerText=sum;c();}

该js实现的效果是如下：