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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. Myös suomenkielisessä materiaalissa käytetään ohjelmaprojektien koodissa englanninkielisiä nimiä kurssin alkupään johdantoesimerkkejä lukuunottamatta.

Voit vaihtaa kieltä A+:n valikon yläreunassa olevasta painikkeesta. Tai tästä: Vaihda suomeksi.

Chapter 7.5: City Simulator

About This Page

Questions Answered: How about some more practice? How about an example of modeling real-world phenomena in a program?

Topics: The main themes are (still) inheritance and collection methods. Algorithm-writing practice. The optional assignment is an example of applying inheritance while refactoring code.

What Will I Do? Study given code and program.

Rough Estimate of Workload:? Three or four hours?

Points Available: C60.

Related Projects: CitySim (new). AuctionHouse2 (new) in the optional assignment.


Citizens in a Simulator


Our simulator is based on a grid that represents a city map. Each location on the grid is an address that a citizen (or family) can move into. Blue and red represent different demographics. Addresses marked in white are currently vacant.

One of the most powerful things about programming is that you can create dynamic models of phenomena and processes in the real world. In this assignment, we’ll form a computational model of a social phenomenon. You’ll take control of an application that simulates people’s movements on a city map; in particular, this simulation will explore how demographics may impact on the social layout of a city.

In this context, a demographic is any subset of the city’s population that the citizens may perceive to be relevant as they assess their own neighborhood. For instance, we can split the citizens in demographics on the basis of their financial status, political views, ethnic background, or age. In this chapter, our simulator will have only two demographics, red and blue. In Chapter 9.2, we’ll extend the simulator to support more demographics.

At the start of each simulation run, we place red and blue citizens at random addresses on the city map. The simulation then advances in steps: at each step, the citizens assess how satisfied they are with their current neighborhood and may decide to move to a vacant address instead of remaining where they are. A citizen is satisfied in case a sufficiently large proportion of their neighbors belongs in the same demographic.

This model is based on the work of Thomas Schelling, a winner of the Nobel Prize in Economics. Naturally, it is a simplification of the real world; models are.

Let me remind you of the particular characteristics of all of these behavior systems[.] It is that people are impinging on other people and adapting to other people. What people do affects what other people do.

—Thomas Schelling


The CitySim project contains two packages. The simulation model itself is in package, whose main contents are:

  • Simulator (partial implementation given). You can ask a simulator object to launch a new simulation run (startNew) or advance the most recently launched simulation by one step (moveResidents). The simulator delegates some of its work to another object that represents the map of the city and is of the type:
  • CityMap (ready as given). A city map is a grid, and CityMap inherits Grid much like GameBoard did in the Chapter 7.4’s Viinaharava assignment. Each of the elements in this grid is of the type:
  • Demographic (missing entirely). This simple sealed trait serves as a supertype for an Occupied class and a singleton Vacant, which represent occupied and vacant addresses, respectively.

The classes in define the user interface, which you can safely ignore apart from the fact that SimulatorApp is an app object: it creates an instance of Simulator and calls its methods when the user presses the buttons in the GUI and slides the sliders.

Here’s a diagram of these components:


Task description

Add the trait Demographic and its subordinate concepts Occupied and Vacant.

Add the methods findDemographic, dissatisfiedResidents, and moveResidents in class Simulator. (The residents method is also missing but we’ll leave that for later.)

Run the program, experiment with different settings in the UI, and consider how they affect the results.

Instructions and hints

On Demographic trait:

  • This is an exceedingly simple trait: it doesn’t have any methods at all.
  • Occupied and Vacant just need an extends, and that’s about it.
  • Seal the trait with sealed (Chapter 7.2). Once you do, any Demographic is guaranteed to be either an Occupied object or Vacant.
  • The outcome is a set of types that resemble the Option class and its derivatives Some and None. And indeed we could have used Option here instead. However, Demographic and its subtypes communicate our model better.

On the findDemographic method:

  • Note that the cityMap instance variable in Simulator gives you a reference to currently active map, which is a CityMap object that contains Demographic objects in a grid pattern.
  • This method is pretty simple to implement once you find the right tools in the documentation of Simulator and/or CityMap.

On the dissatisfiedResidents method:

  • A citizen’s satisfaction depends on whether the percentage of similar citizens among the citizen’s neighbors is high enough. The desired percentage, which the end user sets with a slider in the GUI, is available to you as a number between 0 and 100 in the variable similarityDesired.
  • Every address does not have the same number of neighbors. Note the details in the Scaladocs.
  • Try to split the method’s overall task in subtasks. For instance, one subtask could be to examine whether the neighborhood of an address is unsatisfactory.
  • Consider writing auxiliary functions for the subtasks. You can define them either as private methods or as local functions within dissatisfiedResidents.
  • Again, make use of what the CityMap object gives you. Keep in mind that a CityMap is a sort of Grid and has all the methods it inherits from its superclass.
  • If you use division, remember that any and all decimals are discarded when you divide an Int by an Int.

On the moveResidents method:

  • Make use of the two other methods you implemented.
  • You may find Random.shuffle useful again (see Chapter 7.4), as well as other methods in class Random (see Chapter 3.6).

Additional hint for moveResidents

Show the hintHide the hint

Here’s an outline of a solution:

  1. Form a buffer that contains the addresses of all vacant homes.
  2. Form a collection that contains the locations of all the unsatisfied residents, in random order.
  3. Repeat for each unsatisfied resident: Pick a random address from the buffer of vacant homes. Move the resident to that address on the CityMap. In the buffer, replace that destination address with the vacated address.

Use the app

Run the simulator on the default settings. Press Single Step repeatedly to advance the simulation. Try Run, too. Note that the satisfaction threshold is set at 70%, meaning that the citizens are very easily dissatisfied with their neighborhood.

Try higher and lower values for the threshold.

It seems obvious that if the citizens demand a great number of neighbors similar to themselves, they end up living among their own demographic. Something that’s not equally obvious is how demanding the citizens need to be for the phenomenon to occur. Explore and find out.

Could we use a similar model to explain the “echo chambers” on social media?

What real-world factors are ignored by this model?

What would happen if one demographic cares about what their neighbors are like but the other is always or almost always satisfied? Or what if the citizens didn’t just set a minimum but also a maximum for the degree of similarity between themselves and their neighbors?

Submission form

A+ presents the exercise submission form here.

In case this assignment piqued your interest

The book Networks, Crowds, and Markets: Reasoning About a Highly Connected World, which is available as a free online edition, will tell you more about computational modeling of social, economic, and medical phenomena, among other things. The Schelling model we just used features in Chapter Four of the book.

Reimplementing Auctions with Inheritance

The rest of this chapter consists of an assignment that revisits our earlier auction-themed programs. The programming assignment itself is optional, but we highly recommend that you at least read what it’s about.

In Chapter 5.1, you presumably wrote FixedPriceSale and may have also written DutchAuction and EnglishAuction. These classes represents items put up for sale in a variety of ways. Then, in Chapter 5.5, we designed AuctionHouse to represent auction houses where all the items are sold in the traditional “English” style.

You can use your own earlier solutions as a basis for the upcoming assignment. If you didn’t do some or all of them, feel free to use the example solutions (FixedPriceSale, DutchAuction, EnglishAuction).

A new class hierarchy

Here’s how the existing classes relate to each other:


In other words: an AuctionHouse contains EnglishAuctions. The classes FixedPriceSale and DutchAuction are unconnected to the others.

In this assignment, you’ll refactor the classes. The purpose of refactoring is to improve program quality: you’ll modify FixedPriceSale-, EnglishAuction, and DutchAuction so that it’s easier to use them all in combination. At the same time, you’ll eliminate a great deal of redundant code. In this exercise, inheritance (Chapter 7.3) will be your main refactoring tool.

The goal is a hierarchy of classes that looks like this:


At the heart of our plan is the abstract class ItemForSale, which will serve as a generic superclass for items being sold in all sorts of ways. We’ll be able to use this superclass to write a more generic AuctionHouse class. We’ll also introduce an InstantPurchase class to capture what fixed-price items and Dutch-style auctions have in common.

Implement the refactoring

Implement ItemForSale, EnglishAuction, InstantPurchase, FixedPriceSale, DutchAuction, and AuctionHouse so that they match the documentation provided in project AuctionHouse2.

Instructions and hints

  • We recommend implementing the classes in the order listed above.

  • As you read the Scaladocs, be sure to note which classes and methods are abstract and which methods each class inherits from its superclass(es).

  • Just like in Chapter 7.3: if a superclass already defines a concrete instance variable, don’t repeat the val in the subclass. For instance, the description variable is defined in the superclass ItemForSale, so don’t redefine it as a val in the subclasses, even though the subclasses do need a description as a constructor parameter.

  • You can use the given test app to try some of the key methods. You’ll notice that the app object o1.auctionhouse.gui.TestApp generates a bunch of error messages to begin with, but they’ll vanish once you make the requested changes.

  • In the AuctionHouse class, you’ll need to replace EnglishAuction with a more general type, but that’s the only change needed there.

Submission form

A+ presents the exercise submission form here.

Once you finish the assignment, pause for a moment to admire the results: inheritance turned the disconnected and redundant classes into a beautiful conceptual model. The definition of each concept (class) includes only what is necessary to distinguish it from related concepts.


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!

Weeks 1 to 13 of the ebook, including the assignments and weekly bulletins, have been written in Finnish and translated into English by Juha Sorva.

Weeks 14 to 20 are by Otto Seppälä. That part of the ebook isn’t available during the fall term, but we’ll publish it when it’s time.

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 have done 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 behind O1Library’s tools 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+ has been created by Aalto’s LeTech research group and is largely developed by students. The current lead developer is Jaakko Kantojärvi; many other students of computer science and information networks are also active on the project.

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

Additional credits for this page

The assignment on Schelling’s model of emergent social segregation has been adapted from a programming exercise by Frank McCown.

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