创建二维数组的方法

一、对于小规模数据

    var grades = [ [ 89, 77, 78 ], [ 76, 82, 81 ], [ 91, 94, 89 ] ];
    alert(grades[2][2]); // 显示 89

二、对于大规模数据

    Array.matrix = function(numrows, numcols, initial) {
        var arr = [];
        for ( var i = 0; i < numrows; ++i) {
            var columns = [];
            for ( var j = 0; j < numcols; ++j) {
                columns[j] = initial;
            }
            arr[i] = columns;
        }
        return arr;
    }
    var nums = Array.matrix(5, 5, 0);
    alert(nums[1][1]); // 显示 0