Functions (Alice In Action, Ch 3) 17 July 2013 Slides Credit: Joel - - PDF document

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Functions (Alice In Action, Ch 3)

Slides Credit: Joel Adams, Alice in Action

CS 101 Lecture 17

17 July 2013

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Objectives

• Create functions — messages that return a

value to their sender

• Learn Alice techniques: the vehicle property

for synchronizing the movements of two objects

• Understand the concept of variable scope.

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Alice Tip: Using the Vehicle Property

• Vehicle

– The thing on which an object “rides” – The default vehicle for an object is the world

• Vehicle property is used to synchronize movements
• Using vehicle property in program with girl and horse

– Scene 4 initially shows the nativeGirl on a horse – Add set()method for nativeGirl’s vehicle property – Set the value of the vehicle to the entire horse – Include a move()method below the set()method – Send playScene4()to my_first_method() and test

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Alice Tip: Using the Vehicle Property (continued)

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Alice Tip: Using the Vehicle Property (continued)

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Functions

• Messages used to retrieve information
• May be defined by the user
• Methods and Functions are essentially the same.

– Both are a way give a name to a block of code so it can be reused (eg. flapWings) and generalized (singVerse). – Methods generally do something, but do not return any information – Functions return information.

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Example: Retrieving an Attribute from an Object

• User story involves program with twin wizards

– Objective: enhance program by using titles with names – Jim uses The Enchanter, and Tim uses The Magus

• Creating the title property

– Define property variable myTitle with type String – Assign title values to each wizard’s myTitle variable

• Simplest way to access name and title

– Pass myTitle concatenated with myName to say() – Problem: extra time needed to build the full name – Alternative: use a function to produce the full name

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Example: Retrieving an Attribute from an Object (continued)

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Alice in Action with Java 9

Example: Retrieving an Attribute from an Object (continued)

• Defining a function to produce a full name

– Select wizard object and then click functions pane – Click the create new function button – Declare function called getFullName as String type – Create variable called fullName – Set value of fullName to concatenated name and tile – Include fullName in the return statement

• Defining a simpler function for the second wizard

– Return myTitle concatenated with myName

• Send both functions to my_first_method

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Example: Retrieving an Attribute from an Object (continued)

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Example: Retrieving an Attribute from an Object (continued)

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Example: Retrieving an Attribute from an Object (continued)

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Functions with Parameters

• Functions can use parameters to store arguments
• Adapting skaterGirl program to use functions

– Create Number type called calculateHypotenuse() – Add parameters Number, leg1, and leg2 – Create local variable named hypot – Set value of hypot to mathematical expression – Incorporate new function into computeHypotenuse()

• Comparing functions to methods

– Functions return a value and appear in parameter list – Methods do not return a value and appear as statements

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Functions with Parameters (continued)

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Alice in Action with Java 15

Functions with Parameters (continued)

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Summary

• Variable: named container for storing and retrieving

values

• Types of variables: method variables, parameters,
• bject variables
• Data types: Number, Boolean, Object, String,…
• Parameter: container for an argument
• Argument: data passed to a method or function

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Summary (continued)

• A method does not return a value to its sender
• A method is sent from a place where a statement can

appear

• A function does return a value to its sender
• A function is sent from a place where a value can

appear

• Object vehicle: thing on which an object “rides”
• Variable scope: Where a variable can be accessed.

Student To Do’s

• Readings:

– Alice in Action, Chapter 3 (This week) – Alice in Action, Chapter 4 (Next Week)

• This week’s Alice HW (Due Thursday night):
• Problem 2.5 (loopDeeLoop)
• Problem 2.9 (add scenes 1 and 3 to the wizard/troll

animation).

• Next week’s Alice HW (to be assigned – if you want

to get a head start):

• 3.3 (walking in square)
• 3.9 (shrink the trolls)
• Quiz 2 on Thursday

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