参差不齐的数组

    var grades = [ [ 89, 77 ], [ 76, 82, 81 ], [ 91, 94, 89, 99 ] ];
    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;
    }