var student = {
            "name" : "Guo",
            "sex" : "",
            "age" : "",
            "num ": 01,
            "scores" : [
                    {
                        "subject" : "English",
                        "score" : 50,
                        "comment" : ""
                    },
                    {
                        "subject" : "Computer",
                        "score" : "",
                        "comment" : "absent"
                    }
                ]

        };
        var exclude = ["sex", "comment"];

        // method 1 to validate obj
        validateObj1 = function(obj, excluded){
            var value;
            for(var key in obj){
                value = obj[key];
                if($.isArray(value)){
                    obj = validateArray1(obj, key, excluded);
                }else if(($.inArray(key, excluded) == -1) && ($.isBlank(value))){
                    obj[key] = "N/A";
                }
            }

            return obj;

        }

        validateArray1 = function(obj, key, excluded){
            var subValue;
            for(var i = 0, length = obj[key].length; i < length; i++){
                for(var subKey in obj[key][i]){
                    subValue = obj[key][i][subKey];
                    if(($.inArray(subKey, excluded) == -1) && ($.isBlank(subValue))){
                        obj[key][i][subKey] = "N/A";
                    }
                }
            }

            return obj;
        }

        // method 2 to validate obj
        validateObj2 = function(obj, excluded){
            $.each(obj ,function(key, value){
                if($.isArray(value)){
                    obj = validateArray2(obj, key, excluded);
                }else if(isInvalid(key, value, excluded)){
                    obj[key] = "N/A";
                }
            });

            return obj;
        }

        validateArray2 = function(obj, key, excluded){
            for(var i = 0, length = obj[key].length; i < length; i++){
                $.each(obj[key][i] ,function(subKey, subValue){
                    if(isInvalid(subKey, subValue, excluded)){
                        obj[key][i][subKey] = "N/A";
                    }
                });
            }

            return obj;
        }

        isInvalid = function(key, value, excluded){
            return (($.inArray(key, excluded) == -1) && ($.isBlank(value))) ? true : false;
        }

        $.isBlank = function(obj){
            return(!obj || $.trim(obj) === "");
        };