请问一下，为什么不能直接输出infos长度

Question

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"]];var arr = [];var j = 0;for(var i=0;i<infos.length;i++){ if(infos[i][3]=='大一'){ arr[j]=infos[i]; document.write(arr[j]+'<br/>')        j++ } } document.write(arr.length+'<br/>') var arr1 = [];var k = 0;for(var i = 0;i<arr.length;i++){ if(arr[i][1]=='女'){ arr1[k]=arr[i] document.write(arr1[k]+'<br/>') k++; } }document.write(arr1.length)到了定义新的arr[n]数组储存infos[i]=="大一"还能理解,为什么不能直接输出infos.length正确的值。后面的n++这个意思是什么倒是不懂了,求教或者用另一种容易读的写法求教可以吗.

Answer 1

哦不好意思，是arr[j]储存infos[i]