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 (
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.
Notes: This chapter makes occasional use of sound, so speakers or headphones are recommended. They aren’t strictly necessary, though.
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?
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.
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
val(short for “value variable”), which is followed by...
factorial. The name is in turn followed by an equals sign and...
Here is how the REPL responds to your defining a variable:
val factorial = 1 * 2 * 3 * 4 * 5 * 6factorial: Int = 720
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.
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
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
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
val courseGrade = 9.5courseGrade: Double = 9.5
val name = "Anna"name: String = Anna val songIntro = "cccedddfeeddc---"songIntro: String = cccedddfeeddc---
Color or a
Pic, as in the following exercise.
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.
If a variable name comprises multiple words, we use upper-case letters to mark word borders. Here are a few examples:
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
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”,
val, properly termed reserved words (varattu sana), can’t be used as
Characters in upper case are distinct from those in lower case, so when you name your
myTest, be sure to always type it the same way. The name
mytest won’t access
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
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
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
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)
play, so the first command generates the exact same sounds as before.
Certainly, you’re not obliged to use variables. You can simply write the desired tempo into a string literal, as below.
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
val keyword. The value of a
val variable is “locked” into the variable and can’t be
replaced with another.
In Scala, we can use the word
var as an alternative to
val when we create a variable.
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
doubleThat were eventually
replaced with new ones. This was possible because we defined them with
var. If you
were to exchange the
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
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.
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
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!
Objection! Why would I ever use
var let me do all the same stuff and
var variables do present certain additional opportunities, but that isn’t just a good
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
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
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
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
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
variables than to
var variables. In fact, it’s been suggested that
vals that are variables in the true sense while
would be better called “assignables”. But we’ll leave this war of
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
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
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.
vars and stringed instruments
o1.play and MIDI sound synthesis
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
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
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
value. Using that
Double in arithmetic yields more
This interplay between
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.
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
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
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
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:
valgets assigned a value and continues to store that value thereafter. Favoring
vals makes programs easier to read and develop; you should primarily use
varcan 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,
valvariable; reserved word; DRY.
Finally, here’s the concept map from the previous chapter, expanded with a few key concepts from this one.
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.