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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 previous chapter’s topics, especially strings.

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

Rough Estimate of Workload:? A bit over an hour.

Points Available: A40.

Related Modules: 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?

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 carries out more multiplications than necessary as it executes that command (which doesn’t really matter in practice in this little program, though).

It would be nice if we could 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 that a user has entered into the GoodStuff app.

Obviously, once something is stored, we need to be able to access it. We can do that by defining a name that refers to the stored information.

To store values, programmers store 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”.

To begin, let’s 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” or “value variable”). We follow it with...

... a name that we chose, such as factorial. The name is in turn followed by an equals sign and...

... an expression that is evaluated to produce the value that gets stored in the variable.

The REPL responds:

val factorial = 1 * 2 * 3 * 4 * 5 * 6factorial: Int = 720

The REPL acknowledges the successful variable definition by displaying the name you chose (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 the variable to compute the cube:

factorial * factorial * factorialres3: Int = 373248000

Notice that a variable’s name alone is an expression. The expression’s value is the value that’s stored in the variable. A variable name, like any other expression, can appear as part of a more complex expression. 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.

Question 1

Based on the animation, determine which of the following claims are correct.

As it runs the example, the computer computes the factorial of six only once.

As it runs the example, the computer computes the factorial of six three times.

As it runs the example, the computer stores a certain piece of code in a variable.

As it runs the example, the computer stores the expression 1 * 2 * 3 * 4 * 5 * 6 in a variable.

As it runs the example, the computer stores a value that it obtains by evaluating a certain expression.

When assigning to a variable, the expression on the right-hand side is evaluated first. Only then does the target variable indicated on the left receive a value.

Question 1

Assess these claims, too. You can try the suggested commands in the REPL.

The command val sum = 10 + 20 stores a number in a variable named sum.

You can swap around the parts of an assignment command. For instance, val sum = 10 + 20 and 10 + 20 = val sum do the same thing.

You can swap around the parts of an assignment command, as long as you always start with val. For instance, val sum = 10 + 20 and val 10 + 20 = sum do the same thing.

Alternatives to the equals sign

Since an assignment command is not a math equation, some programming languages mark assignment differently. In those other languages, one may instead use a left-pointing arrow ←, a character combination like :=, or something similar.

Still, in Scala and loads of other programming languages, assignment is marked with an equals sign. You can think about the equals sign as a left-pointing arrow, though, if that seems more intuitive to you.

It’s not the math notation that you know

In some ways, program code looks like familiar mathematical notation. That similarity has confused many beginner programmers, because the programs and school math don’t work quite the same way.

Consider the assignment command. It’s not an equation, and the two sides of the equals sign are not interchangeable. Instead, an assignment command instructs the computer to compute the value of the expression on the right and store that result in the memory location named on the left.

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 eight 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 careless mistakes.

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.

Question 1

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)?

Question 2

A variable name alone constitutes an expression:

another

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

Question 3

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 command, what value is stored in third?

Question 4

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?

Question 5

Consider the following command and its execution in the REPL.

val something = third * (myTest + 10)

Which of the following are correct?

The command defines a new variable.

The command instructs the computer to print out the value of a certain variable on a separate line.

The command instructs the computer to determine a value for a certain variable.

The command instructs the computer to store data in memory.

The intermediate result that is produced by evaluating the expression (myTest + 10) will also remain stored in a variable.

This line of code contains exactly one expression.

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.

Question 1

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

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)-- Error:
  |show(pictureOfCircle)
  |     ^^^^^^^^^^^^^^^
  |     Not found: pictureOfCircle

We forgot something! The error message complains that the nam 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 usually given names that begin with a lower-case letter. No technical necessity forces us to do so, but following this convention is good style.

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 — which are 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 a 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, 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 that guide at some point during the first weeks of O1, but it’s not necessary to do that just yet.

Variables and Strings

Embedding values in a string

Let’s use this integer variable:

val age = 20age: Int = 20

Suppose we want to produce a text that describes the information stored in that variable. For instance, the text might be of the form “The customer is X years old.”, where X is replaced by the value in the variable. (This is a common sort of thing to do. Countless application programs need to construct such textual reports of data stored in memory.)

Here’s one way to do it:

s"The customer is $age years old."res6: String = The customer is 20 years old.

Note the letter s just before the leading quotation mark. It indicates that we’re embedding values within a string literal. This is known as string interpolation.

Within the string itself, you can include a variable name preceded by a dollar sign. It will be replaced by the variable’s value. This works as long as you don’t forget the s at the beginning. (What happens if you do forget? Take a guess and try it in the REPL to see if you were right.)

The value is of type String and contains the digits as written characters, not as numerical values.

See below for additional examples of string interpolation.

val number = 10number: Int = 10
s"The number twice with spaces in the middle: $number   $number"res7: String = The number twice with spaces in the middle: 10   10

You may embed multiple expressions in a single string. The code above embedded the same expression twice; the code below embeds two different expressions.

s"$number is slightly less than ${number + 1}."res8: String = 10 is slightly less than 11.

You may also embed a more complex expression than a variable name. This calls for curly brackets that delimit the expression from the rest of the string, as shown for the arithmetic expression just above. (What happens if you forget the curly brackets? Take a guess and try it in the REPL so see if you were right.)

The same works for other data types than Ints as well. Doubles, for instance:

val grade = 9.5grade: Double = 9.5
val report = s"grade: $grade"report: String = grade: 9.5
s"$grade is the grade you got"res9: String = 9.5 is the grade you got

You can also embed strings within a longer string:

val name = "Anna"name: String = Anna
println(s"$name, $report")Anna, grade: 9.5

That last command does the same as this next one, which puts strings together with the familiar plus operator:

println(name + ", " + report)Anna, grade: 9.5

Question 1

Let’s assume that we have an Int variable called population. In the field below, write a String expression whose value is of the form "The city has X inhabitants.", where X is replaced by the value stored in population.

(Please don’t write a print command or define any new variables. Just write the expression that forms the string. Don’t include any unnecessary curly brackets. You can try the expression in the REPL if you first define population and give it some value.)

Question 2

Let’s assume that we have two Int variables called city1 and city2. In the field below, write a String expression whose value is of the form "The cities have X and Y inhabitants for a total of Z.", where X and Y are replaced by the values of city1 and city2, respectively, and Z is replaced by the sum of those values.

When embedding the sum in the string, recall what was said above about curly brackets.

(Again, please don’t enter a print command or define any additional variables.)

The plus operator on Strings

You’ve seen how to use the plus operator to combine strings one after the other. You may also combine a string to a different type of value with a plus. The result is the same as what you get from string interpolation:

"The customer is " + age + " years old."res10: String = The customer is 20 years old.
"The number twice with spaces in the middle: " + number + "   " + numberres11: String = The number twice with spaces in the middle: 10   10
val report = "grade:" + gradereport: String = grade: 9.5

In real-world Scala programs, both the plus operator and string interpolation (s-and-dollar) are widely used. You should know both notations, and we will be using both in this ebook too.

However, Scala’s plus operator has one limitation that you should know about. Compare these two expressions:

s"$grade is the grade you got"res12: String = 9.5 is the grade you got
grade + " is the grade you got"-- Deprecation Warning:
  |grade + " is the grade you got"
  |^^^^^^^
  |... Adding a number and a String is deprecated. Use the string interpolation `s"$num$str"`

The first command uses string interpolation and works fine. It’s easy to assume that the second command would work equally well. But instead, the second command produces a warning message: the Scala toolkit warns us not to do that.

The plus operation that combines a string with a number works like this: “combine the string on the left of the plus sign with the number on the right”. The other way around — with the number on the left — is not okay.

Don’t use the plus operator when you have a number on the left like that. You can use string interpolation instead, as the warning message suggests.

Alternative approaches

(This is not important at this time but may interest some readers. Don’t feel bad about skipping this bit.)

There are still other ways of constructing strings. For example, all the following expressions evaluate to the same result:

s"$grade is the grade you got"res13: String = 9.5 is the grade you got
"" + grade + " is the grade you got"res14: String = 9.5 is the grade you got
grade.toString + " is the grade you got"res15: String = 9.5 is the grade you got

"" stands for the empty string (tyhjä merkkijono) that contains zero characters but is nevertheless a string. If you combine the empty string with a number, you get that number’s digits in a string (which you can further combine with other values). We’ll use the empty string more in Chapter 4.1.

The toString command tells the computer to produce the string that corresponds to the number. This useful command will come up again in Chapters 2.5 and 5.2.

Music at different speeds

Return your thoughts to the play command that was 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:

val beginning = "cccedddfeeddc---"beginning: 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 + "/" + normalTempores16: 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")

Question 1

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.

The way the code is written, the tempo (that is, the part of the parameter string that follows the slash) is not 240 as desired. Instead, it consists of characters that describe another number that is much too large.

The code doesn’t work because the string is formed by appending the characters 120 to the end and then appending those three characters again.

println("2 + 2" + 3 + 3) outputs 2 + 233 because of the same phenomenon: the threes don’t get summed; the digits get concatenated instead.

Question 2

Here are some more claims about that example for you to assess.

One way to put the double speed of 240 at the end is to add round brackets around the expression normalTempo + normalTempo in the given code.

One way to put the double speed of 240 at the end is to embed it into the string like this: s"$wholeSong/$normalTempo + $normalTempo"

One way to put the double speed of 240 at the end is to embed it into the string like this: s"$wholeSong/${normalTempo + normalTempo}"

One way to put the double speed of 240 at the end is to embed it into the string like this: s"$wholeSong/($normalTempo + $normalTempo)"

One way to put the double speed of 240 at the end is to embed it into the string like this: "$wholeSong/($normalTempo + $normalTempo)"

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 11.2.

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 5.4.

In practice, the matter is more complicated. Among other things, the choice between var and val affects the optimizations that compilers 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

Question 1

What is the output of this piece of code?

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

Try to work out the answer mentally. (You may also try the code in the REPL. If you do, enter each line separately.)

Enter your answer here:

Question 2

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?

(Again, enter the lines one at a time if you try this in the REPL.)

Question 3

In addition to letting you adjust the tempo, 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.

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.

The last command plays the troll dance on the same instrument (13) as before, because the value of the variable trollDance is the same each time.

The last command plays the troll dance on a different instrument (72), because the variable instrument was assigned a new value. This also results in a change in which string trollDance refers to.

Question 4

Let’s add one more line to the above program. Our goal is 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 is 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.

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.

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 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 = 12345-- Error:
  |title = 12345
  |        ^^^^^
  |        Found:    (12345 : Int)
  |        Required: String

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

“I found 12345 here on the right of the equals sign; that’s an integer. But such a context (where we assign to a string variable) calls for a String instead.”

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 * evenFigureres17: Int = 10000
someNumber = evenFiguresomeNumber: Double = 100.0
someNumber * evenFigureres18: Double = 10000.0

That 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, 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.

Question 1

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.

Each variable retains its original value, because the commands are invalid in Scala.

The two variables exchange values.

The variables will have identical values.

None of the above. It’s not possible to know what happens without knowing the two original variables’ shared data type.

None of the above. It’s not possible to know what happens without knowing the two original variables’ values.

Question 2

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?

Question 3

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.

No, it doesn’t work like that.

Yes, it works.

res Variables in the REPL

You’ll be familiar already with the way the REPL replies with a val 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 + 1val res19: Int = 2
res19 * 10val res20: Int = 20
val total = res19 + res20val total: Int = 22

This ebook’s REPL examples usually don’t show these ubiquitous val outputs, but you see them in IntelliJ. Each of these three vals is there to say that a variable got defined; the first two have been automatically named by the REPL.

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 to store 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; string interpolation.

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.

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Written comment or question:

You aren’t required to give written feedback. Nevertheless, please do ask something, give feedback, or reflect on your learning! (However, the right place to ask urgent questions about programs that you’re currently working on isn’t this form but the lab sessions or Piazza. We can’t guarantee that anyone will even see anything you type here before the weekly deadline.)

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, Joonatan Honkamaa, Antti Immonen, Jaakko Kantojärvi, Niklas Kröger, Kalle Laitinen, Teemu Lehtinen, 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 for this page

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

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