This course has already ended.

Luet oppimateriaalin englanninkielistä versiota. Mainitsit kuitenkin taustakyselyssä osaavasi suomea. Siksi suosittelemme, että käytät suomenkielistä versiota, joka on testatumpi ja hieman laajempi ja muutenkin mukava.

Suomenkielinen materiaali kyllä esittelee englanninkielisetkin termit.

Kieli vaihtuu A+:n sivujen yläreunan painikkeesta. Tai tästä: Vaihda suomeksi.

Chapter 8.1: Robots

About This Page

Questions Answered: Can I apply what I’ve learned on a larger program?

Topics: A particular simulator app. There are no new topics per se, but we will discuss a program that is more complex than any of the previous ones.

What Will I Do? Program.

Rough Estimate of Workload:? Two hours?

Points Available: B50.

Related Modules: Robots (new).




Our virtual robots live in a grid. The robots take turns to act. Robot actions largely involve moving and turning.

Much of Week 8 revolves around a programming assignment in which you construct a robot simulator. In this simulator, a “robot world” is essentially a grid where “robots” of very little brain conduct their virtual existence.

The assignment has been broken down in nine parts. The first two are in this chapter; the rest are in Chapters 8.2 and 8.3.

Before we get started in earnest, let’s form an overview of the program that you’ll be working on.

The Robots Module

The Robots module contains two packages. The classes in o1.robots constitute the robot simulator’s internal model; o1.robots.gui provides a user interface. The user interface is ready for use as given and we won’t discuss it further here.

We’ll build the simulator on the same Grid class that you used in Chapters 7.4 and 7.5.

The following table briefly describes each of the simulator’s main components. Below the table, you’ll find a diagram of the components’ relationships.

Package Component Description Status
o1 class GridPos Represents pairs of coordinates. (Familiar from Chapter 6.3 and Week 7.) ready
o1 abstract class Grid Represents grids in general. (Familiar from Week 7.) ready
o1 sealed abstract class CompassDir Represents the main compass directions. There are exactly four instances of this class, which stand for north, east, south, and west. (Familiar from Chapter 6.3.) ready
o1.robots class RobotBody Represents the robots’ physical form. Each instance of RobotBody has properties such as location and facing. A robot can be either intact or broken. partially implemented
o1.robots abstract class RobotBrain Represents the general properties of the robots’ “artificial intelligence”. partially implemented
o1.robots classes Spinbot, Nosebot, Staggerbot, Lovebot, and Psychobot Subclasses of RobotBrain. Each of these subclasses represents a different sort of robot behavior. Spinbot partially implemented; others missing
o1.robots trait Square Represents a single square of a robot world in general terms. ready
o1.robots class Floor Extends Square. Represents floor squares that the robots move on. partially implemented
o1.robots singleton object Wall Extends Square. Represents a wall. (A single object is enough for this purpose, since all walls are identical.) partially implemented
o1.robots class RobotWorld A subclass of Grid. Represents robot-inhabited grids that are composed of Square objects. A RobotWorld object also tracks which robot’s turn it is to act next. partially implemented

Part 0 of 9: Answer Some Questions

Before you go on

Try to get a general sense of the Robots program by reading the description above and the Scaladocs. Browse the source code, too.

For this question, assume that the program has already been implemented and works as specified. Also assume that we’ve created a robot world and a variable testWorld that refers to it. Moreover, we’ve created two robots and added them to the world. Neither of the two robots has yet had a turn to act. We now execute the following lines of code:

testWorld.addRobot(GridPos(1, 1), North)  // let's say this square is previously empty

Which of the robots has the next turn to act?

Read the claims below and select all the correct ones.

Clarification: below, the expression “each object knows” means “each object has stored in its instance variables or can trivially determine with a simple method call”. This is not a trick question. Its only purpose is to encourage you to study the Scaladocs and learn about the given program.

Part 1 of 9: Fundamental Repairs to RobotWorld

You’ve been given a whole bunch of code, much of which works in principle, but the program is not ready to run. IntelliJ spews a stream of errors.

RobotWorld has been only partially implemented. Fix it:

  1. The error messages indicate that RobotWorld doesn’t have the variables width and height as expected. It should inherit these methods from Grid. Add an extends clause that makes RobotWorld a subclass of Grid.
    • You may wish to look at GameBoard in the Viinaharava game for inspiration.
    • Read the documentation carefully. Make sure you pass in the right numbers as constructor parameters to the superclass.
    • Once you’re done with this step, try launching the program via o1.robots.gui.RobotApp. You should see a robot world that’s completely dark.
    • Be sure to give Grid the type parameter it needs. (If you receive error messages that feature Nothing as the type of the grid’s squares, you probably overlooked this.)
  2. Repair the RobotWorld method initialSquare, which now fills the entire world with walls. The method is private and not detailed in the Scaladocs, but there’s a comment in the given code that explains what it should do: put walls on the edges and floors in the middle.
    • That is, you’ll need to check the coordinates to determine whether you should create a Floor or use a reference to the Wall singleton.
  3. Relaunch the application. You should be able to create empty robot worlds, but clicking the floor squares won’t let you add robots or walls.

Part 2 of 9: Adding Content

  1. Implement the RobotWorld method addWall.
    • Hint: use update in class Grid.
  2. Run the program again and try right-clicking floors to add walls.
  3. Implement addRobot in the same class.
    • The method needs to handle several interrelated subtasks. Make sure you attend to each of the things mentioned in the Scaladoc.
    • Reveal additional hints below if you feel you want them.
  4. Try adding Spinbots in the GUI. They should appear in the robot world, but they don’t do anything yet. You can break them and repair them, though.

Hint: subtasks in addRobot

Show the hintHide the hint

The method should: 1) create a robot; 2) add it to the end of the robot list; 3) add it in the appropriate square within the robot world; and 4) return a reference to the added robot.

Tools for each subtask: 1) create a new RobotBody object; 2) update the list in this.robots; 3) pick the right square and call that square’s addRobot method; and 4) return a reference to the RobotBody object that you created.

Hint: picking the appropriate square in addRobot

Show the hintHide the hint

A RobotWorld is a Grid.

Grids have an elementAt method for accessing a single element (square) of the grid.

Use that method to pick the appropriate square. Then call the square’s robot-adding method.

Submit your solution to Parts 1 and 2. The assignment continues in upcoming chapters.

A+ presents the exercise submission form here.


Please note that this section must be completed individually. Even if you worked on this chapter with a pair, each of you should submit the form separately.


Thousands of students have given feedback that has contributed to this ebook’s design. Thank you!

The ebook’s chapters, programming assignments, and weekly bulletins have been written in Finnish and translated into English by Juha Sorva.

The appendices (glossary, Scala reference, FAQ, etc.) are by Juha Sorva unless otherwise specified on the page.

The automatic assessment of the assignments has been developed by: (in alphabetical order) Riku Autio, Nikolas Drosdek, Joonatan Honkamaa, Jaakko Kantojärvi, Niklas Kröger, Teemu Lehtinen, Strasdosky Otewa, Timi Seppälä, Teemu Sirkiä, and Aleksi Vartiainen.

The illustrations at the top of each chapter, and the similar drawings elsewhere in the ebook, are the work of Christina Lassheikki.

The animations that detail the execution Scala programs have been designed by Juha Sorva and Teemu Sirkiä. Teemu Sirkiä and Riku Autio did the technical implementation, relying on Teemu’s Jsvee and Kelmu toolkits.

The other diagrams and interactive presentations in the ebook are by Juha Sorva.

The O1Library software has been developed by Aleksi Lukkarinen and Juha Sorva. Several of its key components are built upon Aleksi’s SMCL library.

The pedagogy of using O1Library for simple graphical programming (such as Pic) is inspired by the textbooks How to Design Programs by Flatt, Felleisen, Findler, and Krishnamurthi and Picturing Programs by Stephen Bloch.

The course platform A+ was originally created at Aalto’s LeTech research group as a student project. The open-source project is now shepherded by the Computer Science department’s edu-tech team and hosted by the department’s IT services. Markku Riekkinen is the current lead developer; dozens of Aalto students and others have also contributed.

The A+ Courses plugin, which supports A+ and O1 in IntelliJ IDEA, is another open-source project. It was created by Nikolai Denissov, Olli Kiljunen, Nikolas Drosdek, Styliani Tsovou, Jaakko Närhi, and Paweł Stróżański with input from Juha Sorva, Otto Seppälä, Arto Hellas, and others.

For O1’s current teaching staff, please see Chapter 1.1.

Additional credits appear at the ends of some chapters.

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