Assignment 6: Lists

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Algorithmic Thinking and Structured Programming (in Greenfoot)

Introduction

Lists are commonly used to store data that belongs together. Lists can comprise of almost anything.

Think of the lists that you use in every day life: Shopping lists, homework lists, lists of favorite websites.

In challenge 5.1 you wrote the a method to count all the eggs in the world: Dodo walks through the entire world and counts all the eggs she finds. However, there’s an even smarter way to look for and hatch eggs. For this, we will use a List. You could use a list to help a lazy Dodo hatch all the eggs in the world using the least amount of steps. Using a List gives you the opportunity to come up with smarter algorithms. By having a list of all of the Egg objects in the world, you would be also have their coordinates and their values at your fingertips (using getter methods).

In this assignment you will first learn how to use a List to store variables. You will then learn how to do specific things with a list, like walk through its elements, swap elements, or sort the elements.

Later you will learn how to use lists with object types such as Eggs. This will be helpful for Dodo’s race in the assignment that follows.

Learning objectives

After completing this assignment, you will be able to:

• declare and use List variables;

• use Java Library Documentation to look for and use existing Java methods;

• apply existing List methods, such as adding and removing elements from a list;

• use a for-each-loop for traversing the elements of a list;

• swap elements in a list.

• sort objects in a list.

• apply plans for calculating the average of a list of values;

• explain what null means and what it is used for;

• use a list to store both with primitive types (such as int, String)and object types (such as Egg).

Instructions

For this assignment you will need:

• scenario ’DodoScenario6’: to be downloaded from the course website².

Throughout the assignment you will also need to answer some questions. The following must be handed in:

• All ﬂowcharts: use pencil and paper, or go to https://www.draw.io/;

• Your code: the file MyDodo . jav contains all your code and must be handed in;

• The reﬂection sheet: complete and hand it in.

2http://course.cs.ru.nl/greenfoot/

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You must discuss all other answers with a programming partner. Jot down a short answer on (the assignment) paper.

There are three types of challenges:

Recommended. Students who need more practice or with limited programming experience should complete all of these.

Mandatory. Everyone must complete these.

Excelling. More inquisitive tasks, designed for students who completed 2 star tasks and are ready for a bigger challenge.

Students who skip 1-star challenges should complete all 3-star challenges.

A note in advance:

• In this assignment you may only make changes to the MyDodo class;

• You may use methods from the MyDodo or Dodo class, not from the Actor class;

• Teleportation is not permitted: if Mimi needs to get somewhere, she must walk there!

Theory

Theory 6.1: Object types and variables

We have learned that Java has primitive types such as int and boolean. Primitive types are built-in Java: you get primitive types as a gift.

In addition to primitive types, Java also has object types (sometimes called reference types). Object types are types that belong to a class. You can make a new object type by writing (or importing) a new class. Examples of object types are MyDodo and Egg.

Use of object types

You can use object types in the same way as you use primitive types. As a reminder, the following table summarizes how you have already used types:

Location Example Type

result int methodName( ) int

parameter void methodName( String text ) String

local variable int value = 4 int

instance variable private int myNrEggsFound = 0 int

Object variables

You have also already seen object types. For example, previously we gently skipped over the following:

World myWorld = getWorld ( ) ;

Here the variable with the name myWorld and type World is declared (made) and immediately initialised (given a value) with the result of getWorld ( ). The result type is World. You can see this by looking at the signature public World getWorld ( ). Because World is a class, its type is an object type (rather than a primitive type). A variable’s type determines the values it may contain. For example, a variable with type int (such as int nrOfEggs ) can only store integers. It can’t store a boolean or String. The same applies to object types. A variable whose type is an object type, such as Egg, can store Egg objects, but it

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can’t store a Fence object or an int value. Any attempt to assign an object to a variable with a different type will result in a compiler error. However, there is an exception to this rule, namely, we can store a BlueEggor a GoldenEgg object in an Egg variable. This is because each BlueEgg or GoldenEgg object is also an Egg (have a look back at the Theory 1.5 on ’Inheritance’). An example of a variable declaration and initialization with an object type is: Egg firstEgg = new BlueEgg ( );

In our figures, we draw variables storing objects using an arrow (Figure 1).

Figure 1: A variable referring to an object

Here, the variable firstEgg stores a reference to the egg object. In fact, object variables always store references to objects, and not the objects themselves.

Theory 6.2: Calling a method from another class

You have seen that a world consists of objects, and each object is created (instantiated) from a specific class. This class determines what an object can do in terms of methods. When defining a new method, you often want to re-use existing methods. When calling such an existing method, there are two possible situations:

1. The method belongs to an object’s own class (or a class that is inherited). In this case the object calls the method directly (as you have been doing all along).

2. The method belongs to a different class. Such a method cannot be called directly. Instead, you first need to have an object from that class, which you can use to invoke the desired method.

General explanation

Consider an object from that class called obj, and a method from obj’s class called meth. Assuming that the method has no parameters, use obj . meth ( ); to invoke the method. You can read the instruction as follows: obj is asked to execute his method meth.

Example of calling a method from a different class

Each Egg has a value. A Dodo and an Egg don’t belong to the same class. For the Dodo to get the value of some egg, she must do two things:

1. Dodo must get the egg: Egg thisEgg = getEgg ( );

Here, the variable named thisEgg is used to store the Egg object returned by the method getEgg ( ).

Note that the latter method belongs to the class Dodo, and therefore can be called directly.

2. Dodo must ask the value of thisEgg as follows: int valueOfThisEgg = thisEgg . getValue ( );

The value of the egg is stored in the variable valueOfThisEgg .

Note

Tip: After typing the Egg variable’s name followed by ’.’ (in the example above: ’thisEgg . ’), use ’Ctrl+Space’

to see a list of all the methods available for that object (Figure 2).

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Figure 2: To see a list of all available methods, use: ’Ctrl+space’

Theory 6.3: Lists

The object variables that we used in the previous assignments could hold exactly one object. How- ever, sometimes it can be useful to store a whole series (or row) of objects, rather than just one object.

Obviously you could make a separate variable for each object, but if you have lots of objects that’s not practical. What you need is a way to group objects. There are several ways to do this in Java, the easiest is using a List.

A List is a series of variables. These variables are the elements of a list. They are stored in sequence, and therefore you can number them. And that is exactly the way in which you can select an element.

Namely, you can select (or find) an element by using its position or index. We start counting at 0. This is done as follows: the first element has index (is at position) 0, the second element has index 1, ... , the tenth element has index 9. So, you can select the 10th element out of a list by asking for the element with index 9.

Elements in lists

All elements in a list are of the same type. You can’t store Eggs and Dodos in the same list: it is either a list of Eggs or a list of Dodos.

Operations on lists

The number of elements in a list is called its size. You can add elements to a list. You can remove elements out of a list. You can add or remove anywhere in a list: the front, back, or somewhere in the middle (using the index to indicate exactly where). A list with no elements is empty.

Lists in Java

The Java class for lists is called List. Before using lists you must import a special library. This sounds more complicated than it is: just add the following line of code to the top of the class document where you want to use lists: import java . util . List ; .

The Java List library has lots of useful methods which you can use:

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Method call Effect

size( ) Returns the number of elements in the list (its length).

get( int index ) Returns the element at position ’index’.

add( int index , E element ) Adds ’element’ to the list at position ’index’, increasing the length of the list.

remove(E element ) Removes ’element’ from the list, decreasing the length of the list.

remove( int index ) Removes an element from list at position ’index’.

isEmpty( ) Returns whether or not the list is empty. Because you can’t remove elements from an empty list, this is a very useful check before trying to remove any elements.

contains( Object o) Returns whether or not object ’o’ is in the list.

Knowing which methods are available to you, and understanding how to use them can save you a lot of time. Why reinvent the wheel when these methods have been written and tested for you!

Have a look at the complete Java library List. To get an overview of all the methods available to you, you can do one of the following two steps:

• Go to https://docs.oracle.com/javase/7/docs/api/java/util/List.html.

• In the Greenfoot menu choose ’Help’, and then ’Java Library Documentation’. Then search for ’List’.

A list can contain objects. But it’s important to realize that lists are objects too! A cookbook has a list of recipes and each recipe has a list of ingredients. So, a list can contain a list of other objects.

Declaring a list: an example

The same rules which apply to any other objects also apply to list objects. If you want to use a list, you first declare a variable in which you can store a reference to a list object. For example, a list of compliments for Mimi can be declared as follows:

List<String> dodosCompliments;

where dodosCompliments is the variable’s name and String is the type of the objects in the list. Notice that a special notation is use to indicate the type of the elements. Formally, this is called a generic type or a parameterized types, because it accepts an additional type (surrounded by < . . . >) as parameter.

Generic types require that such a type parameter is an object type, and not a primitive type. In the previous example this is okay because String is indeed an object type. List<int>, however, would be illegal because int is primitive. In case you need a list of integers, you should use the object type Integerinstead of int. As a matter of fact, Java provides object types for all its primitive types.

The MyDodo class has an example of lists:

public List<Egg> getListOfEggsInWorld( )

This method returns a list of all the eggs in the world. This can be useful when programming Mimi to find the eggs in the world which she has to hatch.

Using a list: an example

To use this list of eggs, a variable to store the list itself must first be declared:

List<Egg> eggsInTheWorld = getListOfEggsInWorld ( ) ; In the code above:

• A variable with the name eggsInTheWorld is declared;

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• The type of eggsInTheWorld is List<Egg> (a list of eggs);

• The variable eggsInTheWorld is set to the result of getListOfEggsInWorld (the eggsInTheWorld is a list of all the eggs in the world at that moment).

Theory 6.4: For-each Plan

To traverse a list (i.e. to go through each of the elements one-by-one), in Java a for-each-loop is used.

void methodDoSomethingWithList ( ) {

for ( ElemType elemVar : listOfElems ) { doSomethingWithElement ( elemVar ) ; }

}

Here a list named listOfElems is traversed. For each element in the list, the code in the loop is executed. So for a list with 5 elements, the code in the loop is executed 5 times. During each loop the variable elemVar attains the next object in the list. The object’s type is ElemType. In the method call doSomethingWithElement, something is done with that particular object. The corresponding ﬂowchart is as follows:

Figure 3: Flowchart for a for-each-loop

Example:

The following code traverses a list of Egg elements and increments their value by 1:

public void incrementValueOfEachEggInWorld ( ) {

List<Egg> eggList = getListOfEggsInWorld ( ) ; //get the list of eggs for ( Egg egg : eggList ) { // get an egg from the list

int currentEggValue = egg . getValue ( ) ; // get the value of the current egg

currentEggValue ++; // and increment its value

egg. setValue ( currentEggValue + 1 ) ; // store new value in egg }

}

In this example, the method getListOfEggsInWorld ( ) returns a list of eggs in the world. The list variable eggList is declared and initialized, it stores the list of eggs. In the for-each-loop, one-by-one, we take a look at each Egg in the eggList. For this egg the methods setValue and getValueare both called.

So, at the end of the for-each-loop, each of the eggs in the world will have an increased valued.

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Note:A for−each is not suitable for actions which change the list during the loop, like swapping elements. If the intention is to change the list during the loop, use a while.

Note:The code above can be written shorter, and without a help-variable:

public void incrementValueOfEachEggInWorld ( ) {

List<Egg> eggList = getListOfEggsInWorld ( ) ; //get the list of eggs for ( Egg egg : eggList ) { // get an egg from the list

egg. setValue ( egg . getValue ( )+1 ) ; // get, increment and store the value of the current egg }

}

Theory 6.5: Average loop plan

Finding the average of a series of numbers is a common task in programming. To calculate the average, we need the number of items and their sum. Assuming we have these two values we calculate the average by dividing the sum by the number of items: average = sum / nrOfItems. For this plan, follow the next steps:

1. Declare the variables needed;

(a) The number of items.

(b) The sum of the items thus far (a running total).

2. Traverse all items using a for−each-loop;

3. For each item, add its value to the running total (current sum) and increase the number of items by one;

4. Check that the number of items is larger than 0 (to guard for a ’divide-by-zero’), if not report an error;

5. Calculate the average by dividing the sum by the number of items. Note that the average may be a decimal value (requiring a cast to double, see Theory 5.3);

6. Return the average.

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Figure 4: Flowchart for finding the average of several values in a list

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Challenges

Please read Theory 6.1: Object types and variables.

Please read Theory 6.2: Calling a method from another class.

Please read Theory 6.3: Lists.

Please read Theory 6.4: For-each Plan.

Challenge 6.1: Walking through a list

In this challenge we will practice using a list of integers. We will now use the list created by createListOfNumbers to experiment with.

a) Find the List<Integer> createListOfNumbers ( ) method in MyDodo.

b) The method has a helper method asList. A list can be created just by indicating the element values. Add comments to the code explaining what this method does.

c) Have a look at the method practiceWithLists .

i. Add comments explaining what the first line does.

ii. The second line is supposed to print the first element to the console. What’s wrong? Fix the code and add comments.

iii. Declare a variable to hold a list of integers. Initialize it with the list returned by createListOfNumbers . Tip: use List<Integer> listOfNumbers = createListOfNumbers ( );

iv. Print the first element to the console.

v. Add code to print the number of elements (size) to the console.

d) Write a new method void printNumberList ( List<Integer> listOfNumbers ) which prints all the numbers in a given list. Tips:

• Use a for-each-loop to step through each element in the list (use the For-each Plan described in Theory 6.4);

• Print the value of each element to the console (tip: to print a value without a new line, use System. out . print instead of println);

• The following ﬂowchart visualizes this algorithm:

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e) Write a method which prints the sum of all the elements of a list to the console. Call the method in your practiceWithLists method.

The following ﬂowchart visualizes the algorithm of the Sum Plan (described in Theory 5.1) using a list:

Challenge 6.2: Java methods for lists

We will now have a look at List in the Java library. You will learn how to read the Java library documentation so that you can use predefined methods.

a) Go to the Java Library for lists:

https://docs.oracle.com/javase/8/docs/api/java/util/List.html. Scroll down until you see the following summary of methods:

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b) As you can see, there is a method called add ( int index , E element ) which adds the element Eat the position index.

In your practiceWithLists method, add the value ’8’ in the third position in your list. Then call your printNumberList to test your code.

c) Match the Java Library methods to the description in the following table:

Method call Effect

removes an element from a list at a particular index returns the third element in the list

adds an element to the front of the list adds an element to the end of the list

returns how many element a list has (i.e. how long it is) removes the last element in the list

checks if the list is empty

checks if the list exists (does the listOfNumbers variable point to a list?) d) Add the following code to the practiceWithLists method:

i. Add the value 6 to the end of the list.

ii. Call your printNumberList to print the list.

iii. Remove the fourth value in the list.

iv. Print the index of the ’8’.

v. Test your method by printing the list after each modification.

Challenge 6.3: Print coordinates and values of all eggs in a list

New to Dodo’s world are surprise eggs. The class SurpriseEgg is a subclass of Egg (just like BlueEgg and GoldenEgg). Blue eggs are worth one point, golden eggs are worth five points, and how many points a surprise egg is worth, well, that’s a surprise! In the constructor of SurpriseEgg its value is set randomly.

This is done using the Greenfoot method getRandomNumber . So, every surprise egg has a (different) random value.

There is a method for making a list of surprise eggs in the class SurpriseEgg . The first parameter indicates the number of eggs you want to have in the list, the second is the world.

public static List<SurpriseEgg> generateListOfSurpriseEggs( int size , World world ) ; For example, the following code makes a list of 10 surprise eggs:

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In the following challenge you will write several methods to print data stored in lists.

a) Create a list of surprise eggs:

i. Come up with an appropriate name for this method.

ii. Use the generateListOfSurpriseEggs method to make a list of 10 surprise eggs, just like in the figure above.

b) Print the coordinates of the first element in the list:

i. Write a sub-method to print the coordinates of an egg: void printCoordinatesOfEgg ( Egg egg ).

This can be very useful in other parts of the program (as you will see later) or while debugging.

Instead of restricting this method to surprise eggs only, we can make it slightly more general by defining it for Egg objects. In that case, it can also be used for blue and golden eggs.

• Pass the egg to be printed as a parameter, and add this method to MyDodo. Tip: With egg. getX ( ) and egg . getY ( ) you can get the coordinates of an egg (with the name egg);

Challenge 6.4: Print value of most valuable egg in a list

Write a method which returns the value of the most valuable (surprise) egg in the world.

a) Fill the world with 10 surprise eggs.

b) Come up with an algorithm to find (and return) the egg in the list that has the highest value. Tip:

Use the Min/Max plan as explained in Theory 5.2.

c) Implement this algorithm.

d) Test your code, by calling it in your practicePrintingLists method. Tip: For testing purposes, first print the values of all the eggs in the surprise eggs list.

e) Is the result correct? Execute the code several more times and check if the correct value is found each time. Fix any errors before you continue onto the next challenge.

Challenge 6.5: Print coordinates of most valuable egg in a list

Your mission is:

”Print the coordinates of the most valuable egg in a list.”

It may be possible for two eggs to have the maximum value. Come up with an appropriate strategy to deal with this case. Add appropriate (JavaDoc) comments to your code

Please read Theory 6.5: Average loop plan.

Challenge 6.6: Print average value of list

Your mission is:

”Get the average of the egg-values in a list.”

Tips:

• The Average Loop Plan is described in Theory 6.5;

• Theory 5.3 describes how to cast an int value to a (decimal) double value.

• Also test your method for an empty list (generate a list of 0 eggs using:

generateListOfSurpriseEggs (0, getWorld ( ) ). Does your program deal with this situation ap- propriately?

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Challenge 6.7: Reversing a list

Write a method that turns the list of surprise eggs around. That is to say, reorganizes the list so that the last element is put in the front, followed by the second to last, etc.

a) Come up with a name for your method.

b) Generate a new list of 10 surprise eggs, and print their values.

c) Come up with an algorithm to reorder the elements in the list accordingly. Tips:

i. You already learned how to switch two values in Theory 4.4, the ’The Swap Plan’.

ii. Have a look at the different methods available to you in the Java Library Documentation (such as add, get, set, remove). Choose which one(s) you want to use in your algorithm.

iii. First implement a small part of the algorithm and test this:

• Write a method

swapElements( List<Integer> listOfNumbers , int indexFirstElement , int indexSecondElement ) which swaps two elements in a list of integers. The method should use the List methods

getand set.

• Start by swapping the first and last element.

• Print the list again.

• Test this. Also check if, in the end, the list still has just as many elements (size).

iv. Think about the rest of the algorithm, without implementing it:

• Place 5 items in front of you. Organise them from smallest to largest. Mark their indexes (see the figure above). Then use your algorithm to sort them (thus by only swapping) from largest to smallest.

• After the first swap, which are the next two elements that must be swapped? Name the elements using their indexes.

• How many swaps do you need all together?

• Test your algorithm with 6 (an even number of) items.

d) Now, implement the algorithm to turn the whole list around. Tips:

• Use a while−loop (and not a for-each-loop).

• Use a variable called eggIndex to keep track of where you are in the list. Don’t forget to increment this variable in the while loop.

e) Compile and test your code by printing the values of each element in the list.

f) Does your program work as you expect it to? If not, think of why it doesn’t work properly. To help debugging, print the list of values throughout the code to figure out what happens. Fix your program.

g) Does your program also work if there are 11 eggs in your list? And also if there are 0?

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Challenge 6.8: Sorting a list: Selection sort

A well known sorting algorithm is called selection sort. This is how it’s done:

a) Determine where (at which index) the unsorted list starts (initially this is at the front of the list, 0).

b) Traverse the unsorted part of the list and find the smallest element.

c) Swap that smallest element with the element in the front of the unsorted list.

d) Update the index where the unsorted list starts.

e) Repeat the last three steps until the entire list has been sorted.

Write a method which sorts a list of numbers using selection sort. Tips:

• Write a sub-method which returns the position of the smallest value in a part of the list (for the unsorted part of the list).

• Use a while loop

• Make use of your swapElements method that you wrote in Challenge 6.7.

Answer the following questions assuming you would have a list of 5 numbers:

a) How often are two values compared?

b) For a list which is twice as big (having 10 numbers), how often are two values compared?

c) For a list which is 100 times as big, how often are two values compared?

d) Can you write a general formula to calculate the number of comparisons needed for a list with n elements?

Challenge 6.9: Calculate distance

Help Mimi figure out how many steps it takes to get from where she is to any place she wishes to go to.

Write a method int determineDistance ( int x_coord , int y_coord ) which calculates the number of steps (or moves) from the current location to another. Note:

• You don’t have to use Pythagoras’s theorem to calculate the distance, just count the number of moves she will need to take.

• If you need to turn a negative number into a positive number, the method Math . abs ( number ) will return the absolute value of any given number.

Challenge 6.10: Goto the most distant egg in the world

Help Mimi to determine which egg requires the most steps to get to, and walk to that egg. Break your algorithm down into several logical modules. Program new methods, re-use existing methods, and test each individually. Tip: check the coordinates of an egg by ’inspecting’ it.

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Challenge 6.11: Re-factoring

An important part of programming is creating code that is made up of logical modules. Amongst other things, this makes re-using and testing so much easier.

Have a look back at the code you wrote in the previous Challenge 6.10 for ”Goto the most distant egg in the world”. Are there any fragments of code in your method that could have been written as a separate sub-method?

If you have not written a separate method to determine the distance from Mimi’s position to another location, do that now. Test your sub-method. Replace the code in your method for going to the most distant egg (Challenge 6.10) by your sub-method determineDistance .

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Reﬂection

In this assignment you practiced using list to store values. You also learned about the for-each-loop to traverse lists and how to use a list to store (Egg) objects. One of the most important steps in becoming good at anything is to evaluate and reﬂect on what you did and how it went:

Result

I know my solution works because ...

I am proud of my solution because ...

I could improve my solution by ...

Method

My approach was good because ...

What I could do better next time is ...

Fill the following table with smileys

indicating how things went. I can do it

I did it a bit but didn’t fully get it I didn’t get it at all

I can use lists to store either primitive types or object types.

I can use a for-each-loop to walk through a list.

I can use the Java Library Documentation to look for and use existing Java methods.

I can swap elements in a list.

I can sort objects in a list.

I can calculating the average of a list of values.

I can break a complex algorithm down into subtasks and implement and test these separately.

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Saving and Handing in

You have just finished the assignment. Save your work! You will need this for future assignments. In the Greenfoot menu at the top of the screen, select ’Scenario’ and then ’Save’. You now have all the scenario components in one folder. The folder has the name you chose when you selected ’Save As ...’.

Handing in

Hand in the following:

• Your code: The java file MyDodo . jav;

• Flowcharts: paste (photo’s of) your ﬂowcharts in a Word document;