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 1.4: Storing Values in Variables

About This Page

Questions Answered: How can I define names for accessing the values I need in my program? How can I store information in the computer’s memory?

Topics: Variables (val and var). We’ll also revisit and expand on the topics of the previous chapter.

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

Rough Estimate of Workload:? An hour or a bit more.

Points Available: A35.

Related Projects: None.

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Notes: This chapter makes occasional use of sound, so speakers or headphones are recommended. They aren’t strictly necessary, though.

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Introduction: Intermediate Results

What arithmetic expressions would you write to compute the following three things?

  • the cube (third power) of the number six; that is, 63
  • the factorial of the number six; that is, 6!
  • the cube of the factorial of six; that is, 6!3

One answer is in the REPL interaction below.

6 * 6 * 6res0: Int = 216
1 * 2 * 3 * 4 * 5 * 6res1: Int = 720

That’s two down, no problem. But what about the third expression?

Argh:

1 * 2 * 3 * 4 * 5 * 6 * 1 * 2 * 3 * 4 * 5 * 6 * 1 * 2 * 3 * 4 * 5 * 6res2: Int = 373248000

That’s pretty unpleasant to write and pretty unpleasant to read. What’s more, the computer needs to carry out more multiplications than necessary as it executes our command (which doesn’t have any practical significance in this small example, though).

It would be nice to be able to write: “Compute the factorial, store that intermediate result, then cube it.” And we can.

Variables

Nearly all programs store values in the computer’s memory. By storing values, the computer can keep track of things that matter during a program run, such as intermediate results or, say, the ratings and prices entered by a user of the GoodStuff app.

Obviously, once something is stored, we need to be able to access it. The best way to do that is to define names that refer to the stored information.

For storing values, programmers use variables (muuttuja) ; a variable is a named location where you can store a single value. Commanding the computer to store a value in a variable is called assignment (sijoitus):

Assigning an intermediate result to a variable

Here is a better way to solve “cube of factorial of six”.

Let’s first define a variable and store the intermediate result in it. Here’s how. Again, recall that you can hover your mouse cursor over the green boxes to see what the explanations connect to.

val factorial = 1 * 2 * 3 * 4 * 5 * 6
We use the Scala keyword val (short for “value variable”), which is followed by...
... a name chosen by the programmer, such as factorial. The name is in turn followed by an equals sign and...
... the expression that is evaluated to produce the value that gets stored in the variable.

Here is how the REPL responds to your defining a variable:

val factorial = 1 * 2 * 3 * 4 * 5 * 6factorial: Int = 720
The REPL acknowledges the successful definition of a variable by displaying the name of the variable (rather than the usual resX) and...
... the variable’s data type and the value stored in the variable. In Scala, both variables and values have data types, and the data type of a variable must be compatible with the value stored therein.

Now we can use our variable to compute the cube of the factorial:

factorial * factorial * factorialres3: Int = 373248000

Notice that a variable’s name alone is an expression. Its value is the value that’s stored in the variable. Such a variable name, just like other expressions, can appear in more complex expressions. In our example, for instance, the variable name appears (three times) as a subexpression of an arithmetic expression.

Stages of assignment

The animation below details the execution of the code that we just discussed. Please watch the animation even if you feel that you understood the example! Pay particular attention to the order of execution steps. The order of these steps during a program run will be increasingly important as we encounter increasingly elaborate programs.

On the basis of the animation, determine which of the following claims are correct.

Watch out for familiar math!

In some ways, program code looks like familiar mathematical notation. But it’s not precisely the same thing, and this similarity has confounded many beginner programmers.

Consider the assignment command. It’s not an equation, and the two sides of the equals sign are not interchangeable. An assignment command instructs the computer to assign the value of the expression on the right to the memory location named on the left. If you try something like 1 * 2 * 3 * 4 * 5 * 6 = val factorial, you’ll only end up with an error message.

We Just Improved Code Quality

Here are the two programs that we just wrote:

1 * 2 * 3 * 4 * 5 * 6 * 1 * 2 * 3 * 4 * 5 * 6 * 1 * 2 * 3 * 4 * 5 * 6res4: Int = 373248000
val factorial = 1 * 2 * 3 * 4 * 5 * 6
factorial * factorial * factorialres5: Int = 373248000

The second version solves the same problem as the original one-liner but is easier for a human to read. Another improvement (in principle) is that the computer has a bit less work to do than in the first program. We have thus just had our first brush with two criteria of program quality: programming style and efficiency of execution.

At least in principle, we can spot a third improvement in code quality, too. Because we extracted the factorial into a separate command, our code now has less repetition of the same expression. Less repetition means that the program is easier to develop and modify: if you wanted to, say, tweak the program to cube the factorial of seven rather than six, you would find the second version more amenable, since you’d need to change the code in only one place rather than three. Not only does that mean less work for you, it also reduces the chance of making a careless mistake.

Of course, because this example program is so tiny, all these improvements to quality have little practical significance.

The principle of avoiding repetition goes by the acronym DRY (don’t repeat yourself); some people refer to breaches of this principle as WETWET (write everything twice write everything twice). When we write larger programs, it’s essential to keep our code DRY. At this introductory stage, however, it’s enough to sow a seed of thought: a programmer needs to consider not only whether a piece of code works but also whether it is of high quality.

Variable Names as Expressions and in Expressions

You can use different kinds of expressions as you define what should be assigned to a variable. The simplest kind of expression is the literal, and indeed you can store the value of a literal in a variable:

val myTest = 100myTest: Int = 100

Get to know variables by programming in the REPL and finding out the answers to the following easy questions.

You can use a variable name in an expression, so it makes sense that you can also use one you initialize another variable. First create a variable myTest as above, then issue this command:

val another = 1 + myTest

What is now the value of the second variable (which we gave the name another)?

A variable name alone constitutes an expression:

another

What is the value of this expression? (Assume that the variable has been defined as above.)

You can copy the value stored in a variable into another variable:

val third = another

Notice that here, too, assignment happens “from the right of the equals sign to the left”: the variable on the right-hand side must exist already (or you’ll be error-messaged); the variable mentioned on the left after the val keyword gets created by this command.

After executing the above commnd, what value is stored in third?

You can also use a variable as you pass parameters to a command. Here’s an example of a println whose parameter expression involves two variables:

println(myTest - third)

What value does this command print out?

Consider the following command and its execution in the REPL.

val something = third * (myTest + 10)

Which of the following are correct?

Variables of Various Types

All the variables we used above were of type Int and stored integer values, but we can define variables of different types, too. The easiest way to do so is simply to assign a differently typed value, such as a Double:

val courseGrade = 9.5courseGrade: Double = 9.5

Or a String:

val name = "Anna"name: String = Anna
val songIntro = "cccedddfeeddc---"songIntro: String = cccedddfeeddc---

Or a Color or a Pic, as in the following exercise.

Let’s define a couple of variables in the REPL and assign values to them:

import o1._import o1._
val sizeOfCircle = 300sizeOfCircle: Int = 300
val colorOfCircle = BluecolorOfCircle: o1.gui.Color = Blue

Now let’s try to display a circle with a diameter and color defined by the two variables that we just created:

show(pictureOfCircle)<console>:18: error: not found: value pictureOfCircle
      show(pictureOfCircle)
           ^

We forgot something! The error message informs us that the variable pictureOfCircle, which we tried to use, is not defined. Indeed it isn’t.

In the field below, write a command that defines a variable called pictureOfCircle such that the above show command is valid and displays a circle. Use the circle command from Chapter 1.3 and the two variables sizeOfCircle and colorOfCircle defined above. Please don’t enter the show command, just the variable definition.

How to Name a Variable

The programmer picks names (or identifiers; tunnus) for the variables in their program. As should already be apparent, variables in Scala are, by and large, given names that begin with a lower-case letter. No technical necessity forces us to do so, but this convention is part of good Scala programming style.

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If a variable name comprises multiple words, we use upper-case letters to mark word borders. Here are a few examples:

  • myLittleVariable
  • numberOfPlayers
  • xCoordinate

This naming convention is called “camelCase”. It’s widespread in Scala and common in other programming languages, although alternative conventions also exist.

Don’t use spaces in variable names. Numbers are fine, but a name can’t begin with a number. Beginners would do best to avoid special characters such as + or & entirely; some of these characters have particular meanings in Scala. Letters with diacritics (e.g., á or ü) are okay in principle, but since they cause occasional trouble in some programming environments, you may wish to steer clear of them. Scala’s “magic words”, such as val, properly termed reserved words (varattu sana), can’t be used as names.

Characters in upper case are distinct from those in lower case, so when you name your variable myTest, be sure to always type it the same way. The name mytest won’t access the variable.

It’s part of good programming practice to give variables names that describe their purpose. You will see many examples in this ebook. However, when experimenting on tiny pieces of code in the REPL, it’s fine to use short, generic names such as number, a, or myTest, even if they may somewhat obscure the variable’s purpose.

O1’s style guide has a bit more to say about naming. We recommend that you take a look at the guide at some point during the first weeks of O1, but it’s not necessary to do that just yet.

Variables and Strings

More about string concatenation

If you use the plus operator to combine a string with a number, you get a string that contains the characters that describe the numerical value. Like this:

"kit" + 10res6: String = kit10
val report = "grade: " + courseGradereport: String = grade: 9.5
println(name + ", " + report)Anna, grade: 9.5
As you see, the value is of type String and...
... contains the digits as written characters, not as numerical values.

Music at different speeds

Return your thoughts to o1.play, the command introduced in Chapter 1.3, and the song Ukko Nooa (“Uncle Noah”). Let’s use variables to form a slightly longer string that covers the entire song. In Ukko Nooa, the bit at the beginning repeats at the end. This effect is easy to achieve:

import o1._import o1._
val beginning = "cccedddfeeddc---"intro: String = cccedddfeeddc---
val middlePart = "eeeeg-f-ddddf-e-"middlePart: String = eeeeg-f-ddddf-e-
val wholeSong = beginning + middlePart + beginningwholeSong: String = cccedddfeeddc---eeeeg-f-ddddf-e-cccedddfeeddc---
play(wholeSong)

Now let’s see how we can play this song at two different tempos (speeds). Let’s begin by storing the tempos that we intend to use in descriptively named variables:

val normalTempo = 120normalTempo: Int = 120
val slowTempo = 60slowTempo: Int = 60

The play command can play melodies at different speeds. If you want something other than the default tempo, you need to give play a string that contains the melody followed by a slash followed by the tempo. Such as this one:

wholeSong + "/" + normalTempores7: String = cccedddfeeddc---eeeeg-f-ddddf-e-cccedddfeeddc---/120

Here are a couple more examples for you to try:

play(wholeSong + "/" + normalTempo)play(wholeSong + "/" + slowTempo)
It just so happens that 120 is the default tempo used by play, so the first command generates the exact same sounds as before.
A tempo of 60 means a more leisurely pace as befits Uncle Noah’s advanced age.

Certainly, you’re not obliged to use variables. You can simply write the desired tempo into a string literal, as below.

play("cdefg/140")

Let’s play Ukko Nooa really fast: at double the normal tempo. And let’s make the computer figure out how much that double speed is.

play(wholeSong + "/" + normalTempo + normalTempo)

But it doesn’t work! Find out what’s wrong and select all the correct claims among the following options. You can find the solution to each item by experimenting in the REPL and paying attention to both the actual values you get and their data types. You can also try printing strings instead of playing them.

Changing the Value of a Variable

Programs routinely keep track of changing information. For instance, a game might have a character whose coordinates change as the player moves the character, or the favorite hotel of a GoodStuff user might change as the user records new experiences.

One way to deal with such needs is to store of the changing information in a variable and replace the value of the variable with another as needed.

Sounds pretty good, but it turns out that we can’t do that using variables defined with the val keyword. The value of a val variable is “locked” into the variable and can’t be replaced with another.

var variables

In Scala, we can use the word var as an alternative to val when we create a variable. A var variable is assigned a value just like a val variable is. The only, but very significant, difference between the two is that a var lets us replace the stored value with a new one later on.

Take a close look at the following animation.

In this example, the values of both variables number and doubleThat were eventually replaced with new ones. This was possible because we defined them with var. If you were to exchange the vars for vals, the above code would fail to work and produce the message “error: reassignment to val”.

A potentially confusing feature of the REPL

The REPL lets you define a variable that has the same name as one you had previously defined: simply write a new var or val definition. If you do this, you might get the impression that you can change the value of a val variable. But in reality, what you’re doing is discarding the old variable and making an entirely new one in its stead (perhaps even with a different data type).

This is a feature specific to the REPL. In Scala programs outside the REPL, you cannot enter consecutive commands to create namesake variables in this way. So forget about this, at least until you’re fluent with variables.

Another example

When you replace the value of a var, you can make use of the variable’s old value as you specify the new one:

Watch out for math! (again)

Notice and remember: In mathematics, a variable is a symbol that corresponds to a value. In programming (of the sort that we do here), a variable is a named location in memory capable of storing a single value.

In practice, the difference is particularly significant when it comes to var variables. A program is not a group of equations! The same program can very well contain, say, the instructions number = 10 and number = 5. Even number = number + 10 is valid, despite looking very suspicious through the familiar lens of math. And the order in which a sequence of commands assigns values to variables can make a lot of difference!

Why val?

Objection! Why would I ever use val? Doesn’t var let me do all the same stuff and more?

var variables do present certain additional opportunities, but that isn’t just a good thing.

Programmers constantly reason about how their code works as they write it and as they try to locate errors in it. This reasoning can be much easier if the programmer knows that certain things in the code are unchangable. As a simple example, the word val tells the programmer that the variable’s value will never, ever change no matter what else happens during the program run. Such matters will grow in significance as you encounter larger and more complex programs; no doubt you will notice the benefits of vals already during this introductory course.

In small REPL experiments, it doesn’t much matter which kind of variables you use, but here is a rule of thumb for all future programming tasks outside the REPL:

Make every variable a val, unless you have a good reason right now to make it a var.

Don’t make your variables vars “just in case I need to change the value”. That is a poor programming practice. If it turns out later that a val really isn’t appropriate for what you’re trying to achieve, you can modify your program to use a var instead.

How is a val even a “variable”?

One may think that, in a sense, only vars are proper variables, since their value can vary. However, a val can quite reasonably also be called a variable. For one thing, it can receive different values during different program runs (e.g., from user input). For another thing, it’s possible for the same val definition to be executed multiple times during a program run, so that each execution creates a separate val with a different value. You’ll see examples of both things later on.

The mathematical concept of variable is also closer to val variables than to var variables. In fact, it’s been suggested that it’s only vals that are variables in the true sense while vars would be better called “assignables”. But we’ll leave this war of words be.

Functional programming

Functional programming (funktionaalinen ohjelmointi) is one of the major varieties, or paradigms, of programming. In its purest form, functional programming uses only vals. We’ll discuss that in Chapter 10.2.

Student question: In terms of memory use or efficiency, does it make a difference if I pick val or var?

Taken in isolation, there’s no difference between the two in this respect. The amount of memory reserved for a variable depends only on the variable’s data type; we’ll discuss that in Chapter 5.2.

In practice, the matter is more complicated. Among other things, The choice between var and val affects the optimizations that compilers (Chapter 5.2) apply as they translate the Scala code written by a programmer into a more readily executable form. Also noteworthy is the fact that vals help us write programs whose parts can be run efficiently in parallel by multiple computers or processor cores. Parallel execution is not a theme that we’ll be exploring further in O1, however.

More vars and stringed instruments

What is the output of this piece of code?

var example = 2
example = example * example
example = example * example
println(example * example)

Enter your answer here:

Here is another example that features strings:

var word = "camel"
word = word + "opard"
word = "ant"
word = "gr" + word
word = "fra" + word

What is the value of the variable word after the last line has been executed?

In addition to letting you adjust the tempo, o1.play lets you choose among a variety of virtual instruments. You do that by inserting the number of the instrument in square brackets within the parameter string — right at the beginning, perhaps. (The number must be an integer between 1 and 128.)

Let’s take out our recorder flutes — instrument number 75.

import o1._import o1._
play("[75]>cccedddfeeddc---")

In this context, the square brackets don’t have anything to do with Scala programming more generally. They are just characters within a string (inside the quotation marks). O1’s play command interprets them as an instrument tag. (In the next chapter, 1.5, we will find another use for square brackets in Scala programs.)

Inspect the following piece of code carefully. Consider what happens as the commands are issued, one at a time. Track the value of each variable mentally.

var instrument = 13
val melody = "<<<h.>c#.d.e.f#.d.f#-.e#.c#.e#-.e.c.e-.<h.>c#.d.e.f#.d.f#.h.a.f#.d.f#.a-- "
var trollDance = "["+ instrument + "]" + melody + "/138"
play(trollDance)
instrument = 72
play(trollDance)

Which of the following describes what happens when the last line of code is executed? Explore the phenomenon in the REPL as needed. In addition to, or instead of, playing the strings, you can try printing them out.

Suppose our goal is to add one more line to the six so that the program, when run line by line, first plays the troll dance on instrument 13 (the marimba) and then on instrument 72 (the clarinet).

Here’s the required line of code:

trollDance = "["+ instrument + "]" + melody + "/138"

We need to insert that line among the other six. What would its line number be in the working program? Please enter a single integer between 1 and 7.

o1.play and MIDI sound synthesis

The play command supports instruments that are defined in the General MIDI standard, where MIDI is short for Musical Instrument Digital Interface. MIDI synthesizes sound on a variety of virtual instruments; the quality of the output varies greatly. O1’s play command is an easy-to-use, string-based interface to some of the basic MIDI features.

You can find a numbered list of instruments on the midi.org web site.

In O1, we use MIDI sound for fun: for learning, not for serious audio quality. We use strings to represent notes, not sound as such. The digital representation of sound and recorded audio are topics for Programming Studio 1.

o1.play and dots

The melody that we just played had a string representation with period-dots here and there. The play command interprets each note followed by a dot as a crisp-sounding staccato, in which the note has a shorter duration and is followed by a short pause.

var and data types

The data type of a variable determines which values you can store in the variable. A variable’s type never changes, not even if the variable is a var. For instance, if you have a variable of type String, you can assign only strings to it, as demonstrated below.

var title = "Ms."title: String = Ms.
title = "M.Sc."title: String = "M.Sc."
title = 12345Int(12345) <: String?
false
<console>:8: error: type mismatch;
found   : Int(12345)
required: String
title = 12345
          ^

Interpreting error messages is a skill that you’ll develop as you gain experience. The above message means something like:

“Is the integer 12345 some kind of string? Nope. Error: Data types are incompatible; on the line title = 12345, the integer 12345 where a string should be.”

Combining numerical types

Sometimes it seems as if we can break the rule of type compatibility. One such scenario arises when we assign an Int value to a variable of type Double, as at the end of this interaction:

var someNumber = 123.45someNumber: Double = 123.45
val evenFigure = 100evenFigure: Int = 100
evenFigure * evenFigureres8: Int = 10000
someNumber = evenFiguresomeNumber: Double = 100.0
someNumber * evenFigureres9: Double = 10000.0

The last assignment command, too, was valid: the Int value that we accessed through evenFigure “serves as a Double”. But as you can tell from the last few lines of the example, it’s not the Int that gets stored in someNumber but the corresponding Double value. Using that Double in arithmetic yields more Doubles.

This interplay between Ints and Doubles is convenient when we need a program component that should work on Doubles but that should also work similarly on integers. Which is quite common.

Once upon a time, there were two var variables named hansel and gretel, who had the same data type. The following code was then executed:

hansel = gretel
gretel = hansel

Which of the following claims best describes what happens to the values of the two variables? Assume that the variables have been created and initialized to some values and the two lines of code are then executed one at a time. Program in the REPL as needed to explore the phenomenon.

In the code below, a couple of expressions have been replaced with question marks, producing a little puzzle. Read the code and reflect on what it does to the values of the two variables defined on the first two lines.

var first = ???
var second = ???
val helperVariable = first
first = second
second = helperVariable
println(first + ", " + second)

Now suppose that we know that the last line outputs "Tegan, Sara". What must have been the initial value of the variable second?

You’ve seen above that we can assign the value of an Int expression to a variable of type Double. When we do, the variable stores a decimal-number equivalent of the integer. Does this also work in reverse; that is, can you use a decimal number where an integer is called for? Try it in the REPL.

res Variables in the REPL

You’ll be familiar already with the way the REPL replies with a res prefix when you feed it an expression. In fact, what the REPL does here is create new val variables whose names begin with res. You can use these variables just as you use variables that you defined explicitly yourself:

1 + 1res10: Int = 2
res10 * 10res11: Int = 20
val total = res10 + res11total: Int = 22

You can make use of this fact as you experiment in the REPL. In this ebook, we don’t use these res-prefixed variables, however. One of the reasons is that we wish to focus on teaching you programming techniques that work outside of the REPL, too. The numbered res variables are peculiar to the REPL environment.

Summary of Key Points

  • A variable is a named storage location for a single value. You can use variables for storing information in the computer’s memory.
    • For instance, in the GoodStuff program, variables store information about each experience (price, rating) and the favorite experience of the user.
  • You can access the stored value through the variable’s name. A variable’s name constitutes an expression and can therefore also appear as a part of a compound expression.
  • Scala has two kinds of variables: val and var.
    • A val gets assigned a value and continues to store that value thereafter. Favoring vals makes programs easier to read and develop; you should primarily use vals.
    • A var can be assigned a new value, which replaces the old one. vars enable us to mutate program state directly with assignment commands; they should be used considerately where needed.
  • Variables with sensible names improve readability. Variables may also affect the efficiency and modifiability of a program.
  • Links to the glossary: variable, assign; expression, value, to evaluate; var variable, val variable; reserved word; DRY.

Finally, here’s the concept map from the previous chapter, expanded with a few key concepts from this one.

Feedback

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.

Credits

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 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 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 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 for this page

This chapter does injustice to music by Edvard Grieg. Thank you and sorry.

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