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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
Microsoft Research © 2014 (With permission from Larry Snyder)
Today’s lessonTwo 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 ReturnDefining a function: writing a functionCalling a function: using a functionArguments to parameters
Two reasons for function:Abstraction + Re-use
Microsoft Research © 2014 (With permission from Larry Snyder)
Let’s reviewFunctions in Lightbot 2.0: “ “ in Moonwalk Excercise …
Recall that functions have two parts:
Function Definition• A statement of how
it worksFunction Call• A request to have it
preformed
Functions• Procedure
s• Methods• Actions
Microsoft Research © 2014 (With permission from Larry Snyder)
Functions In TouchDevelop<name> ( <param list> ) ( <return list> ) {
<body> }
as inaction 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);
end
or
Microsoft Research © 2014 (With permission from Larry Snyder)
Functions: ResultVoid?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);
end
Microsoft Research © 2014 (With permission from Larry Snyder)
Functions: ParametersParameters are the values used as input to the function; parameters are not required, but the parentheses are
The type of each parameter must be givenaction 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);
end
Microsoft Research © 2014 (With permission from Larry Snyder)
Functions: Return A function returns its value with the return statement … The stuff following return is the resultThe 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);
end
Microsoft Research © 2014 (With permission from Larry Snyder)
Refresher
Microsoft Research © 2014 (With permission from Larry Snyder)
Writing FunctionsOur function definitions are listed under Δcode
FunctionDefinition
FunctionCall
Microsoft Research © 2014 (With permission from Larry Snyder)
Using FunctionsOnce defined, functions can be called repeatedly … it’s the point of writing them!
Function Call
Function Call
Function Definition
Microsoft Research © 2014 (With permission from Larry Snyder)
Review, Analogy
Microsoft Research © 2014 (With permission from Larry Snyder)
Arguments Become ParametersNotice that if the DEFINITION has n parameters, the CALL needs n argumentsThe parameters and arguments correspond
Microsoft Research © 2014 (With permission from Larry Snyder)
Arguments Become ParametersNotice that if the DEFINITION has n parameters, the CALL needs n argumentsThe 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)
Microsoft Research © 2014 (With permission from Larry Snyder)
ParametersAutomatically declared (& initialized) on callThey remain in existence as long as the function remains unfinished
When the function ends: The parameters vanishWill be recreated on the next call
Choose meaningful parameter namesxPos correlates to x position of the bullseye centerEverytime you see xPos you know the meaning
Microsoft Research © 2014 (With permission from Larry Snyder)
Functions Tutorial, Scoreboard
Microsoft Research © 2014 (With permission from Larry Snyder)
Review
Microsoft Research © 2014 (With permission from Larry Snyder)
Function: why?Makes your code more readableShow your thinking/Hide the details
Parameterized: Support re-use
Microsoft Research © 2014 (With permission from Larry Snyder)
Functional Abstraction Powers LayersReview 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
Microsoft Research © 2014 (With permission from Larry Snyder)
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
Microsoft Research © 2014 (With permission from Larry Snyder)
Arguments Become ParametersNotice that if the DEFINITION has n parameters, the CALL needs n argumentsThe 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)
Microsoft Research © 2014 (With permission from Larry Snyder)
ParametersAutomatically declared (& initialized) on callThey remain in existence as long as the function remains unfinished
When the function ends: The parameters vanishWill be recreated on the next call
Choose meaningful parameter namesxPos correlates to x position of the bullseye centerEverytime you see xPos you know the meaning
Microsoft Research © 2014 (With permission from Larry Snyder)
Pop Quiz!
Microsoft Research © 2014 (With permission from Larry Snyder)
What Are Your Questions?We say that a function definition has 3 parts: name, parameters, bodyName is critical: it names the “concept” created by the functionParameters are critical: they customize a function to many casesBody 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 useArguments are critical: says how this case handled
Microsoft Research © 2014 (With permission from Larry Snyder)
Functions Tutorial 2, Factorials
