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Chapter 1.5: Collections and References

About This Page

Questions Answered: How do I store information that consists of multiple related elements? How can I refer to such a collection of elements from multiple locations within a program?

Topics: The basics of using collections in general and buffers in particular. References.

What Will I Do? Program in the REPL and read.

Rough Estimate of Workload:? About an hour.

Points Available: A20.

Related Modules: None.

Introduction: Collections of Elements

Most computer programs manipulate many granules of information that are related to each other in some way. For instance, a program might need to keep track of multiple scientific measurements, multiple hotel experiences recorded by the user, multiple friends of the user, or multiple students enrolled in a course.

For such purposes, we use collections (kokoelma). A collection may contain, for instance, all the friends of a particular user, or a sequence of measurements.

A value stored in a collection is called an element (alkio) of the collection. An element could be a single measurement, a single experience, or a single person, for example.

Since the need for collections is so universal, programming languages provide a selection of collection types for the programmer to pick and choose from. Scala offers a plentiful selection, but initially we’ll use just one type of collection known as a buffer (puskuri).

Creating a Buffer

In Scala, a buffer is a collection that stores its elements in a specific order. You can add new elements to a buffer, remove elements that were previously added, or replace elements with new ones. Informally, you can think of a buffer as a sort of editable list of items stored in computer memory.

To get started, let’s try creating a buffer that contains four strings as its elements:

Buffer("the first element", "second", "third", "and a fourth")res0: scala.collection.mutable.Buffer[String] = ArrayBuffer(the first element, second, third, and a fourth)

As we create a buffer, we provide the buffer’s elements as parameters, enclosing them in round brackets. In this example, the elements are arbitrarily chosen strings. We could have chosen to use some other type as well; the elements could be numbers, for instance. Commas separate the parameter expressions.

The REPL reports that the result is of type Buffer[String]. In Scala, square brackets mark type parameters (tyyppiparametri) that further specify the data type. In our case, the type parameter is String, so what we have here isn’t just any buffer, but a buffer that contains strings. You can read Buffer[String] as “buffer of string”.

A description of the buffer and its elements appears in the REPL. As you can see, the buffer contains the strings in exactly the order in which we passed them to the command that created it.

The REPL further informs us that what we got is an ArrayBuffer: a buffer that is internally implemented using a particular construct (arrays). In Scala, buffers are ArrayBuffers by default, but that’s not something we need to concern ourselves with at this point.

Two more examples of buffer creation are shown below. We have a buffer with two String elements and another with five Doubles.

Buffer("ABC", "XYZ")res1: scala.collection.mutable.Buffer[String] = ArrayBuffer(ABC, XYZ)
Buffer(2.40, 3.11, 4.56, 10.29, 8.11)res2: scala.collection.mutable.Buffer[Double] = ArrayBuffer(2.4, 3.11, 4.56, 10.29, 8.11)

Note the buffers’ different type parameters, which Scala determines automatically from the elements you put in each buffer.

Side note about this ebook: The REPL reports the buffer’s type thoroughly, prefixing it with a package name as in scala.collection.mutable.Buffer[Double]. For your reading convenience, future examples in this ebook slightly simplify such outputs; you will see simply Buffer[Double] instead of the longer type description.

Accessing a Buffer’s Contents

The examples that follow will use the following example buffer and a numbers variable that refers to the buffer.

val numbers = Buffer(12, 2, 4, 7, 4, 4, 10, 3)numbers: Buffer[Int] = ArrayBuffer(12, 2, 4, 7, 4, 4, 10, 3)

This variable will be useful for accessing the collection after it’s been created, as you’ll see below.

Examining an element

Each of a buffer’s elements is stored at a particular location within the buffer; each such location is identified by its running number, called an index (indeksi). When we manipulate buffers, we often use indices to target specific elements.

Here’s how you can access the value stored at a particular index. First indicate the buffer that you wish to examine, then put the desired index in round brackets:

numbers(0)res3: Int = 12

Indices start from zero! The above expression accesses the buffer’s first element.

You can use expressions like the one above in lots of ways. For instance, you can access a buffer element and assign that value to a variable. In the example below, the buffer’s fourth element (that is, the value at index three) is assigned to the variable fourthNumber:

val fourthNumber = numbers(3)fourthNumber: Int = 7

Practice on buffers

The questions below should help you build an understanding of the basic buffer commands. Work out what the given code fragments do. Experiment in the REPL.

Question 1

Examine the following commands. What do they do with the buffer?

val testBuffer = Buffer(4, 10, 3, 10, 15, -2)
println(testBuffer(4))

Please enter the number that the last command prints out:

Question 2

Examine the following commands. What do they do with the buffer?

val testBuffer = Buffer(4, 10, 3, 10, 15, -2)
val oneOfTheNumbers = testBuffer(0)
val sum = oneOfTheNumbers + testBuffer(3)

Please enter the number that gets stored in sum:

Question 3

Examine the following commands. What do they do with the buffer?

val testBuffer = Buffer(4, 10, 3, 10, 15, -2)
val someIndex = 1
println(testBuffer(someIndex) + testBuffer(someIndex + 1))

Please enter the number that the last command prints out:

Question 4

Assume a buffer and a myNumbers variable have been defined like this:

val myNumbers = Buffer(10.5, 10.3, 9.8, 7.9, 10.2, 9.7)

Below, enter a Scala command that accesses the buffer’s fifth element (which is 10.2 in our example) and prints it out. (Print out just the number on a separate line, nothing else.)

Buffers are Mutable

Replacing an element

You can replace an element with a new value by adding an equals sign. In this example, the fourth element in the buffer is replaced by the number one:

numbers(3) = 1

That command does not in itself produce any value of interest. It just instructs the computer to modify the buffer’s contents and doesn’t evaluate to a value in itself. This is why the REPL stays silent. However, we can request the value of the variable and see that the fourth element has indeed changed in the computer’s memory:

numbersres4: Buffer[Int] = ArrayBuffer(12, 2, 4, 1, 4, 4, 10, 3)

That change affected only the buffer, not the variable that we used earlier to store the old value:

fourthNumberres5: Int = 7

Adding an element

The operator += adds a new element at the end of the buffer. This means that the buffer’s size goes up.

numbers += 11res6: Buffer[Int] = ArrayBuffer(12, 2, 4, 1, 4, 4, 10, 3, 11)

Using a buffer is similar to using a variable, with the addition that we need indices to target specific elements. In a sense, a buffer is like a numbered list of var variables.

Speaking of which: The variable that we used to refer to the buffer is a val. It being a val doesn’t prevent us from modifying the buffer’s contents; val is not an attribute of the buffer itself. If numbers the variable is defined as a val, then that variable cannot be reassigned so that it would later refer to some other buffer. Nevertheless, it is possible to change the contents of the buffer that numbers refers to.

Empty buffers

It’s often useful to create a buffer that contains no elements yet. For instance, when the GoodStuff application launches, there are initially no recorded experiences even though experiences may be added later.

The following doesn’t quite accomplish what we want:

val wonderIfThisWorks = Buffer()wonderIfThisWorks: Buffer[Nothing] = ArrayBuffer()

That command did make us a new empty buffer, but empty it will remain. The computer doesn’t know what we’re planning to store in the buffer, and the Scala toolkit determines the buffer’s type to be Buffer[Nothing]. Such a buffer won’t bring us any joy, since we can’t add anything to it.

We can, however, tell the computer what kind of buffer we want to create. Let’s try creating an empty buffer where we’ll be able to add strings. The type parameter — String in this case — goes in square brackets just like you already saw in REPL outputs:

val words = Buffer[String]()words: Buffer[String] = ArrayBuffer()

We can now add strings to this buffer:

words += "llama"res7: Buffer[String] = ArrayBuffer(llama)

Actually, you’re allowed to type in the type parameter whenever you create a buffer, even where it’s not necessary. This works, too:

val numbers = Buffer[Int](2, -1, 10)numbers: Buffer[Int] = ArrayBuffer(2, -1, 10)

We could have omitted the type parameter [Int], because the Scala toolkit figures out that we want a Buffer[Int] from the expressions 2, -1, and 10, which determine the buffer’s initial elements.

More practice

The questions below should help you build an understanding of the basic buffer commands. Work out what the given code fragments do. Experiment in the REPL.

Question 1

The following example adds elements to a buffer. What happens to the buffer at each step?

val words = Buffer[String]()
val word = "love"
words += "ents"
words += word
words += "elem" + words(0)
println(words(0) + " " + words(1) + " " + words(2))

Please enter the text printed out by the last line:

Question 2

Examine the following commands. What do they do to the buffer?

val testBuffer = Buffer(4, 10, 3, 10, 15, -2)
var index = 0
testBuffer(index) = 0
index = index + 1
testBuffer(index) = 0
index = index + 1
testBuffer(index) = 0
index = index + 1
testBuffer(index) = 0

After executing these commands, what is the sum of the buffer’s elements?

(If you try the code in the REPL, please enter the commands one by one.)

Question 3

Let’s assume the following lines of code have just been executed.

val myNumbers = Buffer(10.5, 10.3, 9.8, 7.9, 10.2, 9.7)
val numberOfElements = 6
val empty = Buffer[Double]()

Below are a few attempts to access an individual element of the buffer. Which of them result in an error message (and why)?

myNumbers(-1)

myNumbers(numberOfElements)

myNumbers(0)

empty(0)

Question 4

Say we’ve created a buffer as well as a variable myNumbers that refers to that buffer, exactly as in the previous question. In the field below, enter a Scala command that adds the number 9.9 as the last (seventh) element of the buffer.

References

Chapter 1.4 said that a variable stores just a single value. But didn’t we just have a buffer variable that contained multiple values?

Sort of, but not really.

As we take a look at this question, you’ll get to know the concept of reference (viittaus). Eventually, it will turn out that this concept has a significance in programming that extends well beyond buffers.

Did you notice that test is the name of a variable, not the name of a buffer? (Buffers don’t have names.) This fact will be important momentarily.

References have a concrete impact on how programs work. Take a look at another animation.

Practice on references

Think through the following code and answer the questions. If you find this assignment hard, you may want to draw a diagram of the buffers, variables, and references involved. You can also enter the code in the REPL. If you struggle to make progress, review the material on buffers and references above or ask for help.

Question 1

var person = Buffer("Sauli")
var another = Buffer("Sauli")
var presidentOfFinland = another

How many buffers does this code create?

Question 2

What about variables that store a reference to some buffer? How many of those does the code create?

Question 3

Let’s continue by executing the following lines next.

presidentOfFinland += "Väinämö"
another += "Niinistö"

How many buffers exist now?

Question 4

How many elements are now stored in the buffer that the variable presidentOfFinland refers to?

Question 5

How many elements are now stored in the buffer that the variable another refers to?

Question 6

Next, let’s execute this command:

another = Buffer("Jenni", "Haukio")

How many elements are there now in the buffer that the variable presidentOfFinland refers to?

Question 7

How many elements are there now in the buffer that the variable another refers to?

Question 8

And finally:

another = person
person = Buffer("Osku")

How many of the variables now refer to a buffer that contains the names "Jenni" and "Haukio"?

Question 9

How many of the variables now refer to a buffer that contains only the name "Sauli" and nothing else?

On References and This Chapter

There is a myriad of things that you can do with buffers and other kinds of collections. We have hardly scratched the surface. You’ll see many examples of collections in the chapters that follow; as we already suggested, the experience categories in the GoodStuff application will be one such example. We’ll be storing many different kinds of values in collections, not just individual numbers or strings.

Right now, what matters most is that you know how to create buffers and how to access and modify their individual elements. And that you know that you manipulate buffers via references.

You can expect the concept of reference to come up time and again. We’ll use references to point at many other things than buffers, too.

Beginners often find references a hard concept to grasp. One of the reasons is language. When we humans talk or write about programming, we often take “shortcuts”. It’s rare for us to say that “the variable numbers contains a reference to a buffer”; instead, we often say “the variable numbers contains a buffer” or something similarly imprecise. We may also speak of a “numbers buffer” even though the variable really contains only a reference and numbers is the name of a variable, not of a buffer. (As you’ve seen, multiple differently named variables can store a reference to the same buffer.) Speaking simply and imprecisely is natural and convenient, and we don’t need to strictly avoid it, but if we are to write programs that work, we do need to understand what the words really mean.

Psst! As a matter of fact...

... to be precise, the values of many standard Scala data types — including Strings — are “at the ends of references” just like you’ve seen buffers to be. We didn’t bring this up before, and the animated diagrams in this ebook generally don’t display references when it comes to certain common data types. But here’s one animation that is more “truthful” (detailed) than the earlier animations with strings, even if that truthfulness makes this animation a bit more cumbersome:

Generally, it’s more convenient to think in simplified terms. For example, we may think of the string “cat” itself as being stored in a variable (as in earlier animations) rather than the variable referring to another memory location that contains the string (as above). We’ll continue use the simplified representation in later animations.

The above simplification is “safe” for the String data type and won’t lead us to reason erroneously about program behavior because — unlike a buffer — a string value never changes internally. Even when we combine a string with another, like in the preceding example, we don’t modify the existing strings but create a new string. That’s something we’ll return to in Chapters 5.2 and 11.2.

Summary of Key Points

• A programmer can store multiple “granules” of information — elements — in a single collection.

• One kind of collection is the buffer.

  • Each of a buffer’s elements is associated with a running number called an index.

  • You can replace the elements of a buffer with others. You can also add new elements.

  • In a sense, a buffer is similar to a bunch of var variables grouped together.

• You manipulate buffers via references. A variable can store a reference that indicates where in the computer’s memory the actual buffer is.

  • Multiple variables may refer to the same buffer.

  • After you make a change to a buffer’s contents via one variable, you can also observe the change via any other variables that refer to the same buffer.

• References are similarly used for accessing data other than collections, as you’ll see later.

• Links to the glossary: reference; collection, buffer, element, index; type parameter; package.

An updated concept map:

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Credits

Thousands of students have given feedback and so 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, Kaisa Ek, Joonatan Honkamaa, Antti Immonen, Jaakko Kantojärvi, Niklas Kröger, Kalle Laitinen, Teemu Lehtinen, Mikael Lenander, Ilona Ma, Jaakko Nakaza, Strasdosky Otewa, Timi Seppälä, Teemu Sirkiä, Anna Valldeoriola Cardó, 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 has been designed and implemented by various students in collaboration with O1’s teachers.

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

Additional credits appear at the ends of some chapters.

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