Methods 9-Mar-17

Overview In this presentation we will discuss these 4 topics: Main method vs. Jeroo methods Choosing behaviors to turn into methods Writing a Jeroo method Preconditions and postconditions

Jeroo has many built-in methods Actions Hop, turn, … Sensor methods isNet? isFlower? … The programmer can define additional methods to extend the behavior of every Jeroo.

Behaviors and methods Behavior = an action that an object can take or a task it can perform in response to a request from an external source Method = Collection of statements written in a programming language to describe a specific behavior. 1st define and name a behavior 2nd write code for the method

Which parts of the program should be methods? choose a behavior Any complex, well-defined step Any sequence of steps that more than 1 Jeroo will perform Any sequence of steps that occur several times. A good behavior to turn into a method has a very clear definition and is used more than once in the program.

Writing the method A Jeroo method contains the code describing what any Jeroo needs to do to carry out a particular behavior. Choose a name that is meaningful Empty parenthesis required on the header line (first line) Indent code inside the method Sub method_name ( ) End Sub ‘== method_name== Body of method Containing statements that describe how any arbitrary Jeroo can carry out this particular behavior.

Important differences Main method Only one main method Must be called Main Always comes first Instantiate Jeroos Jeroo methods Can be many other methods Choose your own names Must be called by the main method Can’t instantiate in the method

Example of a Jeroo method Comments clearly explain what the method does ‘ *********************************************************** ‘ Turn Around: makes a Jeroo turn 180 degrees Sub turnAround( ) turn(LEFT) End Sub ‘ === end turnAround === Good , descriptive name for the method The code is indented Notice that no Jeroo name is specified in the steps Everything in yellow is good! 

Important difference Main method All steps must specify which Jeroo does each action: objectname.method() Example in Main for a Jeroo named Betty Betty.pick() Betty.hop(2) Betty.give(ahead) Jeroo methods Define a behavior available to ALL Jeroos so do not specify which Jeroo performs the steps inside the method Example in a Jeroo method for any Jeroo pick() hop(2) give(ahead)

Using a Jeroo method After you write your code Use it just like any other method Sub plantCorner( ) plant() hop() turn(right) End Sub ‘ === plant Rectangle === Sub main( ) Dim Ali as Jeroo = New Jeroo(5,5,8) Ali.plantCorner( ) End Sub ‘ === main ===

One method can call another The plantThree method is used twice in the plantRectangle method Sub plantThree( ) plant( ) hop ( ) End Sub ‘ === plantThree === Sub plantRectangle( ) plantThree( ) moveDown( ) End Sub ‘ === plantRectangle === B This code is missing most of its comments! Defined above A Must also be defined somewhere

Preconditions and Postconditions A complete definition for a behavior includes: Preconditions Assumptions of what is true before the method is called Postconditions What will be true after the method is finished In Jeroo, the only things that change are: island cells (flower planted, net gone…) Jeroo attributes (location, number of flowers …)

Things to consider when writing a precondition What must be true for this method to work? Ask: does the Jeroo need A certain number of flowers? To be facing a particular direction? To be in a certain location? Ask: are the contents of certain island cells important? ‘ ******************************************** ‘ Plant 3 flowers in a row starting at current location ‘ PRECONDITIONS: ‘ 1. There are 2 clear spaces directly in front ‘ 2. Jeroo has at least 3 flowers Sub plantThree( ) …

Things to consider when writing a postcondition Describe what is true after the method finishes Ask: how will the method change the number of flowers? the direction the Jeroo is facing? the Jeroo’s location? Ask: have the contents of certain island cells changed? ‘ ‘ POSTCONDITIONS: ‘ 1. Three flowers have been planted, starting at the ‘ beginning location and proceeding straight ahead ‘ 2. The jeroo is standing on the last flower, ‘ facing its original direction ‘ ******************************************** Sub plantThree( ) …

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