Layering: Building Functions out of Functions Functional Abstraction Reduces Complexity Based on Lawrence Snyder’s slides for UW-Seattle. This notice serves as permission for their use for teaching in public or private (not for profit) schools. © 2014
Today’s lesson Two threads of class merge again as we introduce functions in TouchDevelop and use them to build a pong game with bullseye ball! Function Syntax: FunctionName, Parameters, and Return Defining a function: writing a function Calling a function: using a function Arguments to parameters Two reasons for function: Abstraction + Re-use
Let’s review Functions in Lightbot 2.0: “ “ in Moonwalk Excercise … Recall that functions have two parts: Functions Procedures Methods Actions Function Definition A statement of how it works Function Call A request to have it preformed
Functions In TouchDevelop <name> ( <param list> ) ( <return list> ) { <body> } as in action MoveBallBy(x_dist: Number, y_dist: Number) ball->setX (ball->x + x_dist) ball->setY (ball->y + y_dist) end action pink ( ) returns (myColor:Color) myColor := Color(255, 200, 200); or
Functions: Result Void? Functions that do something, but do not return a value, have void as their <return type> Functions that return a value must say its type action MoveBallBy(x_dist: Number, y_dist: Number)returns void ball->setX (ball->x + x_dist) ball->setY (ball->y + y_dist) end action pink ( ) returns (myColor:Color) myColor := Color(255, 200, 200);
Functions: Parameters Parameters are the values used as input to the function; parameters are not required, but the parentheses are The type of each parameter must be given action MoveBallBy(x_dist: Number, y_dist: Number) ball->setX (ball->x + x_dist) ball->setY (ball->y + y_dist) end action pink ( ) returns (myColor:Color) myColor := Color(255, 200, 200);
Functions: Return A function returns its value with the return statement … The stuff following return is the result The returned item must be defined before the function ends action MoveBallBy(x_dist: Number, y_dist: Number) ball->setX (ball->x + x_dist) ball->setY (ball->y + y_dist) end action pink ( ) returns (myColor:Color) myColor := Color(255, 200, 200);
Refresher
Writing Functions Our function definitions are listed under Δcode Call Function Definition
Using Functions Once defined, functions can be called repeatedly … it’s the point of writing them! Function Call Function Definition
Review, Analogy
Arguments Become Parameters Notice that if the DEFINITION has n parameters, the CALL needs n arguments The parameters and arguments correspond
Arguments Become Parameters Notice that if the DEFINITION has n parameters, the CALL needs n arguments The parameters and arguments correspond Inside of the function, the parameter, e.g. xPos, is declared and initialized to the corresponding argument, e.g. 400. Then, the definition uses it, e.g. ◳a-> setpos(400, 300)
Parameters Automatically declared (& initialized) on call They remain in existence as long as the function remains unfinished When the function ends: The parameters vanish Will be recreated on the next call Choose meaningful parameter names xPos correlates to x position of the bullseye center Everytime you see xPos you know the meaning
Functions Tutorial, Scoreboard
Review
Function: why? Makes your code more readable Show your thinking/Hide the details Parameterized: Support re-use
Functional Abstraction Powers Layers Review What We Did The computation ONLY deals circles, but we don’t think of it that way … to us, it’s a ball Bunch of circles Bullseye as pong ball Create Reset Move Collide
More On Parameters … Return to the two slides on the topic of parameters … Parameters: Customize each function call to a specific situation – they are the input to the function Parameters are the names of the input values used inside of the procedure body Arguments are the values from outside to be used for each of the parameters
Arguments Become Parameters Notice that if the DEFINITION has n parameters, the CALL needs n arguments The parameters and arguments correspond Inside of the function, the parameter, e.g. xPos, is declared and initialized to the corresponding argument, e.g. 400. Then, the definition uses it, e.g. ◳a-> setpos(400, 300)
Parameters Automatically declared (& initialized) on call They remain in existence as long as the function remains unfinished When the function ends: The parameters vanish Will be recreated on the next call Choose meaningful parameter names xPos correlates to x position of the bullseye center Everytime you see xPos you know the meaning
Pop Quiz!
What Are Your Questions? We say that a function definition has 3 parts: name, parameters, body Name is critical: it names the “concept” created by the function Parameters are critical: they customize a function to many cases Body is critical: it defines how the function works Function uses (calls) have 2 parts: name, arguments (args) Name is critical: says what concept you will use Arguments are critical: says how this case handled
Functions Tutorial 2, Factorials