js二维数组

//二维数组  
    var gradeTable=[[80,90,90],[90,80,70],[100,60,90],[95,75,80]];  //二维数组对应的是甲乙丙的语数外三科成绩
    alert(gradeTable.length);  //4
    alert(gradeTable[0][0]);   //80
    //计算每个人的三门课程的平均成绩
    var personTotal=0,average=0; 
    for(var row=0;row<gradeTable.length;row++){   
        for(var col=0;col<gradeTable[row].length;col++){
            personTotal+=gradeTable[row][col];
        }
        average=personTotal/gradeTable[row].length;
        alert(average);
        personTotal=0;
        average=0;
    }

    //计算所有人的每门课程的平均成绩
    var subjectTotal=0,average=0;
    for(var col=0;col<gradeTable[0].length;col++){
        for(var row=0;row<gradeTable.length;row++){
            subjectTotal+=gradeTable[row][col];
        }   
        average=subjectTotal/gradeTable.length;
        alert(average);
        subjectTotal=0;
        average=0;
    }


    //批量生成二维数组并初始化
    var matrix=function twoD (colNum,rowNum,init) {
        var arr=[];
        for(var i=0;i<rowNum;i++){
            var columns=[];
            for(var j=0;j<colNum;j++){
                columns[j]=init;
            }
            arr[i]=columns;
        }
        return arr;
    }

    var array = matrix(3,3,0);
    alert(array[0][0]);