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 11.1: Arrays and a Faulty Train
About This Page
Questions Answered: How about some more practice on loops, code-reading, and debugging? The word “array” has appeared here and there — what does it mean?
Topics: See above.
What Will I Do? Start by reading a bit. Then spend most of the chapter reading and debugging a given program.
Rough Estimate of Workload:? Three or four hours? Remember to ask for help if you get stuck in the debugging assignment.
Points Available: B80.
Related Projects: Train (new).
Introduction: What’s an Array?
You’ve already seen the word “array” in O1:
val numbers = Buffer(1, 2, 3)numbers: Buffer[Int] = ArrayBuffer(1, 2, 3) val words = "one,two,three".split(",")words: Array[String] = Array(one, two, three)
ArrayBufferappears in the REPL output when we use buffers.
splitmethod (Chapter 4.5) divides a string and returns the pieces — not in a vector or a buffer but an array.
Arrays (taulukko) are collections of elements:
- Like buffers and vectors, arrays store elements at specific indices.
- You can replace an array element with another. In this respect, an array is similar to a buffer and different from a vector.
- However, the size of an array is fixed, just like a vector’s is. The size is set when the array is created and the number of indices in an array never changes.
A very familiar sort of construct, then. Let’s now look at a few examples of using an array before we discuss why you might want to use them.
Arrays in Scala
Arrays, like vectors and unlike buffers, are always available in Scala programs without
Creating an array looks familiar:
val myArray = Array("first", "second", "third", "fourth")myArray: Array[String] = Array(first, second, third, fourth) val anotherArray = Array.tabulate(5)( index => 2 * index )anotherArray: Array[Int] = Array(0, 2, 4, 6, 8)
Indices and methods also work like you’d expect:
myArray(2)res0: String = third myArray(3) = "last"myArraymyArray: Array[String] = Array("first", "second", "third", "last") myArray.sizeres1: Int = 4 myArray.indexOf("third")res2: Int = 2 myArray.mkString(":")res3: String = first:second:third:last myArray.map( _.length )res4: Array[Int] = Array(3, 4, 6, 4)
Appending an new element and thereby increasing the size of the array is impossible:
myArray += "one more?"<console>:13: error: type mismatch;
Creating an unitialized array with
Sometimes, it’s convenient to create a collection of a specific size whose contents are
initialized only later. For that, you can use a method that isn’t available
for vectors or buffers,
val myArray = Array.ofDim[Int](5)myArray: Array[Int] = Array(0, 0, 0, 0, 0)
ofDimneeds a type parameter. Here, we have elements of type
Creating an array so reserves the desired number of memory slots for the actual array elements, which we haven’t really set yet but whose (maximum) number we know.
You can read
Array.ofDim as “array of dimensions”. Like that name implies, the
method works for creating “multidimensional” (nested) arrays. They are similar to the
“multidimensional” vectors of Chapter 5.5:
val twoDimensional = Array.ofDim[Int](2, 3)twoDimensional: Array[Array[Int]] = Array(Array(0, 0, 0), Array(0, 0, 0)) val threeDimensional = Array.ofDim[Double](2, 2, 2)threeDimensional: Array[Array[Array[Double]]] = Array(Array(Array(0.0, 0.0), Array(0.0, 0.0)), Array(Array(0.0, 0.0), Array(0.0, 0.0)))
ofDim just creates the array, it doesn’t put any meaningful content in it. Still,
any array you create like this always contains something. The default value depends on
the element type:
- for numerical types (
Charincluded), it’s zero;
- for Booleans, it’s
- for all other types — including
String, for instance — it’s the “non-existent value”
null(which is a spawning bed for errors; Chapter 4.2).
You can and almost always should replace the default values with something more meaningful sooner or later.
Careful with those default values!
Suppose you have a class
Footballer and you create this array:
val finnishTeam = Array.ofDim[Footballer](11)
It’s a common beginner’s error to forget that the above command does not actually create
even a single instance of the
Footballer class, even though it ostensibly creates “an
array of footballers”. What you get is an array that contains eleven copies of the
reference. If you want to create
Footballers and store them in the array, you’ll need
to do that separately:
finnishTeam(0) = new Footballer("Tinja-Riikka Korpela") finnishTeam(1) = // etc.
Unless there is an actual object reference at the appropriate index, commands like these
will result in a
NullPointerException at runtime:
finnishTeam(9).score() val keeper = finnishTeam(0) println(keeper.name)
Doesn’t Sound Too Useful?
Since an array’s size never changes, the
Array type supports fewer operations than the
Buffer`type; any piece of functionality that you might wish to implement using arrays
you can also implement using buffers. (There reverse is also true.) If you want an
immutable, numerically indexed collection, you can use a vector. Uninitialized arrays are
infested with `nulls.
Given all that, why bother with yet another collection type? Here are a few reasons:
- Commonness: Arrays are part of a programmer’s general knowledge. They are a basic data structure that is used for implementing other collection types. Arrays are available in many programming languages; in quite a few of languages, they are the most common type of collection.
- Efficiency: Arrays sometimes make a program more efficient, which is due to differences in the implementations of the various collection types. As far as Scala arrays are concerned, this is one of the more likely reasons why you might occasionally opt for an array, but — once again — we leave efficiency concerns for later courses to deal with. The Scala website has a table that compares the efficiency characteristics of different collections.
- Natural usage scenarios: An array is an intuitive choice
when you need a collection whose contents can change but whose size
Gridclass of Chapter 7.4, for instance, has been implemented using a “two-dimensional” array. You can also consider an array if you need a collection whose size is capped at a preset limit.
- Incidence in libraries: The
Arraytype crops up in some software libraries, including Scala’s core API. The aforementioned
splitmethod is an example.
Debugging a Train
This assignment puts you in a position that is familiar to many professional programmers: you need to untangle a mess of code that someone else wrote.
The Train project contains classes that represent train cars and the seats and cabins
in those cars. Think of the classes as (poor) parts of an imaginary software system
where customers can reserve places on a train. The code is written in a heavily
imperative style: it is built on arrays,
and effects on mutable state.
The code isn’t even close to exemplary. The worst thing is that it doesn’t work right.
Your task is to write an app object that tests the given classes and to use it to locate the errors in the program. You should also fix the errors, but that’s the easy part.
See the Scaladocs for how the classes should work.
Instructions and hints
Please don’t get stuck on this assignment. Use the lab sessions and the online forums for hints. There’s a lot more to do in Week 11 after this!
First, test the code thoroughly. Call the methods on different values and in different order. Work out what works and what doesn’t work. Then debug the code: find and fix what causes the problems.
Write an app object for testing. You may also wish to use Eclipse’s debugger.
There are eight bugs. Each of them is in a distinct location in the program code.
Various methods work peculiarly or crash the program if you pass in “obviously silly” values (e.g., if you set a negative number of cabins or if you add a
nullto the train instead of an actual train car). In this assignment, that does not count as a bug. Concentrate your efforts on finding behaviors that clearly contradict the specification even on valid parameters.
SittingCaris the most complex of the classes. Inspect the other classes first. Look at
SittingCarlast, when you’ll have developed a better understanding of the given code and a better workflow for testing.
Some of the classes have companion objects in the same file. Those objects are there just to store constants.
You might feel the urge to improve the programming style or efficiency of the given code. You’re not required to do that, though.
So-called desk checking (pöytätestaus) can help, as suggested by a former O1 student:
In its own way, this was a tough assignment, but in the end I solved it pretty easily once I printed out the code, picked up my pencil, and took the whole stack of papers with me for reading in the sauna. :)
Sauna is probably inessential in this method.
A+ presents the exercise submission form here.
Summary of Key Points
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 automatic assessment of the assignments has been programmed by Riku Autio, Jaakko Kantojärvi, Teemu Lehtinen, 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 pedagogy of using tools from O1Library (such as
Pic) for simple graphical programming
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 appear at the ends of some chapters.