//here are three ways to implements the basic operation, each way is on the different level?in function definition.
/*
* defines the operations in prototype
* usage:
??var op = new Operator(10, 20);
??op.operate('add');
*/
/*
function Operator(x, y) {
?this.x = x;
?this.y = y;
}

Operator.prototype.add = function() {?
?return this.x + this.y;?
}

Operator.prototype.subtract = function() {
?return this.x - this.y;?
}

Operator.prototype.multiply = function() {
?return this.x * this.y;?
}

Operator.prototype.divide = function() {
?return this.x / this.y;?
}

Operator.prototype.operate = function(operateclass) {
?return this[operateclass](this.x, this.y);?
}
*/

?

/**
* defines the operations in every object instance
* usage:
??var op = new Operateor(10, 20);
??op.operate('add');
*/
function Operator(x, y) {
?this.x = x;
?this.y = y;
?this.add = add;
?this.subtract = subtract;
?this.multiply = multiply;
?this.divide = divide;
?this.operate = operate;
}

function add() {?
?return this.x + this.y;?
}

function subtract() {
?return this.x - this.y;?
}

function multiply() {
?return this.x * this.y;?
}

function divide() {
?return this.x / this.y;?
}

function operate(operateclass) {
?return this[operateclass](this.x, this.y);?
}

?

/*
* defines the operations as attributes of the Function object
* usage:
??Operator.operate('add', 10, 20);
*/
/*
function Operator() {
}

Operator.add = function(x, y) {
?return x + y;?
}

Operator.subtract = function(x, y) {
?return x - y;?
}

Operator.multiply = function(x, y) {
?return x * y;?
}

Operator.divide = function (x, y) {
?return x / y;?
}

Operator.operate = function(operateclass, x, y) {
?checkArguments(arguments);
?return Operator[operateclass](x, y);
}

function checkArguments(args) {
?var actual = args.length;
?var expected = args.callee.length;
?if (actual != expected) {
??throw new Error("the expected count of arguments is not equal to the actual!");
?}
?return true;?
}
*/