处理二维数组的元素

一、按列访问：外层循环对应行， 内层循环对应列。

    var grades = [ [ 89, 77, 78 ], [ 76, 82, 81 ], [ 91, 94, 89 ] ];
    var total = 0;
    var average = 0.0;
    for ( var row = 0; row < grades.length; ++row) {
        for ( var col = 0; col < grades[row].length; ++col) {
            total += grades[row][col];
        }
        average = total / grades[row].length;
        document.write("Student " + parseInt(row + 1) + " average: "
                + average.toFixed(2) + "<br />");
        total = 0;
        average = 0.0;
    }

二、按行访问：外层循环对应列， 内层循环对应行。

    var grades = [ [ 89, 77, 78 ], [ 76, 82, 81 ], [ 91, 94, 89 ] ];
    var total = 0;
    var average = 0.0;
    for ( var col = 0; col < grades.length; ++col) {
        for ( var row = 0; row < grades[col].length; ++row) {
            total += grades[row][col];
        }
        average = total / grades[col].length;
        document.write("Test " + parseInt(col + 1) + " average: "
                + average.toFixed(2) + "<br />");
        total = 0;
        average = 0.0;
    }