The latest instance of the course can be found at: O1: 2024
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 1.6: Using Subprograms
About This Page
Questions Answered: How do I launch a custom command (that’s been defined by someone else)? How do I pass parameters to the command and see what it accomplishes?
Topics: Subprograms. Effectful vs. effect-free subprograms. Calling
a subprogram: parameters and return values. Nested calls. The Unit
value.
What Will I Do? Program in the REPL and read.
Rough Estimate of Workload:? An hour, or perhaps an hour and a half. In addition to the assignments, there is some knotty (but useful) terminology to untangle.
Points Available: A20.
Related Projects: Subprograms (new).
Notes: This chapter makes occasional use of sound, so speakers or headphones are recommended. They aren’t strictly necessary, though.
Introduction
This and next two chapters are concerned with subprograms (aliohjelma).
A subprogram is an implementation of a particular piece of functionality. For instance, a subprogram may calculate a result, record some useful data in the computer’s memory, or print out a message. Or it may perform a specific combination of such tasks.
Programmers can create new subprograms, then use what they created. They can also use subprograms created by other programmers. A full program like GoodStuff or Pong contains many subprograms.
But you’ll have to wait a bit before we can tackle how those applications work. For now, let’s proceed as follows:
- In this chapter, you’ll see how to make use of custom subprograms that have been provided for you. We won’t care yet about how the subprograms that you use have been created.
- Week 1’s last chapters, 1.7 and 1.8, introduce the implementations various ready-made subprograms; that is, you’ll study how the given subprograms’ internal behavior has been defined and how those internals work as each subprogram is used. At the same time, you’ll get to practice creating subprograms of your own.
Subprograms are the last missing piece that we’ll need in Week 2 to discuss something called “object orientation”, which will help us put together entire programs.
Preparatory Steps
For this chapter, you need a new Eclipse project called Subprograms. It contains an assortment of subprograms for you to experiment with. The instructions below recap (from Chapter 1.2) how you can load the project into your workspace and import its contents in the REPL.
Load the project into Eclipse:
- Fetch the zip archive that contains the project onto your computer. You can find it at the top of this page under Related Projects.
- Existing Projects into Workspace and Next.
- Select archive file and indicate
the appropriate
zipfile. - Make sure that Subprograms is ticked and press Finish.
Start up the REPL like this:
- Choose or simply
press
Alt+F11. - Choose Subprograms.
- Choose or simply
press
Enter this command in the REPL.
import o1._
Example of a Custom Subprogram
Suppose we have some numbers — the results of a series of scientific measurements, say — stored in a buffer. Like so:
val results = Buffer(-2, 0, 10, -100, 50, 100, 5, -5, 2)results: Buffer[Int] = ArrayBuffer(-2, 0, 10, -100, 50, 100, 5, -5, 2)
Let’s further assume that the negative numbers in our buffer indicate failed measurements and we wish to discard them.
It’s possible to define a subprogram that takes care of removing all negative numbers
from any given buffer. Indeed, a subprogram just like that has been defined for you in
the Subprograms project. It’s called removeNegatives and works like this:
- When you activate the subprogram, you must provide a parameter: a reference to a buffer that contains integers.
- The subprogram modifies the buffer’s contents by removing any and all the negative numbers.
Calling a subprogram
Instructing the computer to execute a particular subprogram is known as calling
(kutsua) the subprogram (or invoking the subprogram; or applying it). It’s easy
to call removeNegatives:
removeNegatives(results)
This command that invokes removeNegatives doesn’t itself have a value in the same
sense as, say, the expression 1 + 1 does. (Well, at least the command doesn’t have a
meaningful value; more on that later.) That’s why the REPL doesn’t respond with any
output. However, we can take a look at the value of the variable results and verify
that the subprogram did indeed modify the buffer that the variable refers to:
resultsres0: Buffer[Int] = ArrayBuffer(0, 10, 50, 100, 5, 2)
println and other familiar subprograms
The preceding example must have felt a bit familiar. The way we invoked removeNegatives
just now is very similar to how you’ve already printlned and played: first, the name
of the command; then, a parenthesized parameter expression. This isn’t a coincidence.
Even though we didn’t attend to the fact at the time, println is a subprogram just
like removeNegatives is. It’s just that println is a general-purpose subprogram
defined to be a part of Scala whereas removeNegatives is an example subprogram invented
for the purposes of this one ebook chapter. Similarly, play and show from Chapter 1.3
are subprograms, too.
Now let’s take a look at a rather different sort of subprogram.
A Subprogram that Returns a Value
The same project contains a subprogram named average, which has a simple purpose:
you pass two numbers to it as parameters, and it computes their average. This
subprogram returns (palauttaa) a value:
average(5.0, 11.0)res2: Double = 8.0
average is an expression; the expression’s
value is the average of two numbers, which is computed by the
subprogram. This value is called the subprogram’s return value
(palautusarvo). The REPL reports the value of the expression
as per usual.Here are a few more examples of calling this subprogram. As shown in the examples, you can use a subprogram-calling expression as a component of a longer expression:
val first = average(5, 1) + 2.1first: Double = 5.1 val second = average(10.9, first) + average(-5, -10) - 1second: Double = -0.5 1 + average(second + 1, 1)res1: Double = 1.75
Program State and How to Affect It
You’ll get to see many more examples of subprograms in just a moment, but first, let’s stop to consider the attributes of the subprograms that you’ve already seen.
This chapter has introduced two very different subprograms:
- Calling the
removeNegativessubprogram instructs the computer to take care of a specific task. Such a call brings about changes in the state (tila) that the program has stored in the computer’s memory. We term such state-modifying subprograms effectful (vaikutuksellinen). An effectful subprogram doesn’t necessarily return any interesting value. - Calling the
averagesubprogram doesn’t bring about any lasting changes in the data stored in memory. We term such subprograms effect-free (vaikutukseton). In order to be useful to us, an effect-free subprogram must produce a meaningful return value, asaveragedoes.
We have also used the subprogram println to output text. When a println command is
executed, it has an effect on the world in the sense that the output visible onscreen
changes in an observable way. This is why we’ll place println in the category of effectful
subprograms along with removeNegatives. The subprograms play and show from Chapter 1.3
are similarly effectful.
That chapter also introduced circle and rectangle, which are subprograms too. These
subprograms, like average, are effect-free. Just as average merely computes and returns
a numerical value based on of the parameters that you give to it, circle and rectangle
merely compute and return a picture based on the parameters they receive.
For any subprogram, you can ask: when you call the subprogram, does anything observably change? (We won’t count time passing or receiving a return value as a change.) The answer tells you whether the subprogram is effectful or effect-free. This classification already highlights the fact that different subprograms can do some very different things. As we go on, you’ll see that this question has a profound significance in programming. One may even profit from programming in a way that relies exclusively on effect-free subprograms (Chapter 10.2). But let’s not get ahead of ourselves; right now, we can use the classification to illuminate a few basic terms and concepts.
An Important Term: Function
In a number of programming languages, it’s customary to call all subprograms functions (funktio), whether they are effectful or effect-free. Scala is one such language. In the rest of this ebook, the word “function” will show up often; we’ll use it to mean a subprogram.
The word sounds very familiar, and there are reasons why that is so. However:
Watch out for math! (again)
Many beginner programmers have tripped over the concept of function because of the mismatch between what programmers and mathematicians mean by the word.
It’s true that effect-free functions resemble the functions from school mathematics. Given a mathematical function f(x), just determining its value for a particular x doesn’t change any stored data or output anything onscreen. Similarly, an effect-free function just takes in some parameter values and produces a result (a return value), and that’s it.
However, when you call a Scala function, it may generate output or otherwise affect program state. We must remember that what our functions can be effectful!
Moreover, the functions that we use in programming, even effect-free ones, differ from familiar mathematical functions in that we don’t merely define a relation between inputs and outputs like f(x) = x + 1. Our functions also implement the step-by-step processes (algorithms) that compute the return values. The next chapter will illustrate this in detail.
Different kinds of functions, a summary
Here is a table of the kinds of functions we have just discussed. Some bits of text have been underlined; you can mouse over them to view additional explanations.
| Does the function affect state? | Does it return a value? | The term we use in O1 | Alternative terms |
|---|---|---|---|
| Never | Yes | Effect-free function | Function with no side effects |
| No | Empty function | ||
| At least sometimes | Yes | Effectful
function
|
Function with side effects, procedure |
| No |
Function Parade
This section will take you through a whole cavalcade of example functions. This should demonstrate the variety of things that functions can do while letting you practice reading program code that invokes functions.
We’ll use some of Scala’s built-in functions as well as a number of example functions created for O1.
Generic library functions
The REPL interaction below features several Scala functions that instruct the computer to do math.
Some of the functions are probably familiar to you from calculator apps, such as abs
(absolute value), pow (power), sqrt (square root), and sin (sine). All of these functions
are located in the package scala.math, so we need to import them either individually or,
as below, all at once.
import scala.math._import scala.math._
Now we can easily call the functions:
abs(-50)res2: Int = 50 pow(10, 3)res3: Double = 1000.0 sqrt(25)res4: Double = 5.0 sin(1)res5: Double = 0.8414709848078965
The functions max and min return the greater and lesser of their two parameters,
respectively:
max(2, 10)res6: Int = 10 min(2, 10)res7: Int = 2
All those functions are effect-free, just as average was in our ealier example. They
only return values and don’t modify any data we’ve previously stored, or print or play or
draw anything.
Try calling at least some of the functions in the REPL. You can also experiment with
the following: other trigonometric functions (cos, atan, etc.), cbrt (cubic root),
floor (rounds down), ceil (rounds up), round (rounds to closest), hypot
(hypotenuse of two given legs), and log and log10 (logarithms). For whole list of
functions in the package, see Scala’s documentation.
In Chapter 1.1, we already touched on the concept of library (kirjasto): a selection of components that programmers can use in different projects. Scala’s mathematics package is an example of a library, and we can call functions such as the above library functions.
Now please don’t go off trying to memorize all these library functions, let alone all the
thousands of others out there, even though no doubt you’ll be needing some of them later
on. You can check the names later from the Scala documentation or this ebook. When it
happens, and it does happen, that a function is so frequently useful that you need it
repeatedly, you’ll end up remembering it without trying. (All that being said, here’s a
hint: you’ll find min and max repeatedly useful in the forthcoming chapters.)
Student question: Do I waste memory if I use an underscore in an import?
No. An import statement is there to put on record what some words
mean within a particular piece of code. For instance, importing
scala.math._ indicates that the names min, max, sqrt etc.
refer to functions in that package.
Yes, it’s true that the computer must hold the tools that you use
in memory, functions included. But in terms of a program’s runtime
memory usage, what matters is the tools you actually use, not the
import commands that express your intention to use them.
Practice on example functions
A function can make use of external resources such as files or networked computers. The
following example function (from package o1) finds out which movie is in a particular
position in the all-time Top 250 according to the votes cast on the IMDb web site.
imdbMovie(3)res8: String = The Godfather: Part II imdbMovie(1)res9: String = The Shawshank Redemption
The example function works even if you don’t have a network connection, because it’s been
designed to take its information from the folder top_movies that comes with the
Subprograms project. (This also means that the movie info isn’t quite up to date.)
One more example
It’s perfectly possible to create a function that interacts with the user when it’s called.
Try out one such function, called playDoodads. This function challenges you to a game
that lets the computer show off. It expects a single string parameter that represents the
player’s name; you can use your own first name, for instance.
Nested Function Calls
When you call a function, you write expressions to indicate which parameters you wish to pass in. On the other hand, a function call is itself an expression. So it makes sense that you can nest a function call within another:
When you would end up with lots of nested function calls, you may wish to reformat your code to make it easier to read. Variables are an excellent tool for this. For instance, the last line of code in the preceding animation could be replaced by these commands:
val fifthPower = pow(2, 5)fifthPower: Double = 32.0 val smaller = min(fifthPower, 100 - sqrt(100))smaller: Double = 32.0 println(abs(-5.5) + smaller)37.5
Or these:
val fifthPower = pow(2, 5)fifthPower: Double = 32.0 val difference = 100 - sqrt(100)difference: Double = 90.0 val smaller = min(fifthPower, difference)smaller: Double = 32.0 val finalResult = abs(-5.5) + smallerfinalResult: Double = 37.5 println(finalResult)37.5
When it comes to such matters of style, use your common sense. Consider, on a case-by-case basis, what you think is best in terms of clarity and brevity. In any case, you’ll need to get used to seeing code written in all these different styles, since programmers (and learners of programming) read a lot of code written by others.
Evaluation order in nested expressions
The Unit value
Scala (and many other languages) feature a special value known as Unit. This value
is used as the return value of functions that don’t return a value that represents
meaningful information. Such functions include println and play from before and
removeNegatives from this chapter. Technically, these functions also return a value,
though: a Unit value.
When we receive a return value of Unit from a function, there’s practically nothing
that we can do with it. You can think of it as meaning: “The function was executed but
nothing of significance was produced as a return value.” The removeNegatives function,
for instance, does modify the given buffer’s state but doesn’t produce an “output” in the
sense of a return value. For our practical purposes in O1, it’s okay think and say that
Unit means “no return value” and that functions such as println and removeNegatives
“don’t return a value” or “return nothing”.
The Unit value isn’t necessarily conspicuous in the program code that you’ll write, but
it isn’t just a curiosity either. If for no other reason, it’s good to be aware of Unit
because you’ll see it in error messages. For instance, 1 + println("mammoth") begets a
message to the effect that you “can’t add one to Unit”. Moreover, the word shows up frequently
in the documentation of Scala programs to indicate that a particular program component has no
significant return value.
Unit in O1’s animations
Unit return values haven’t appeared in the animations you’ve seen so far in this ebook.
But they will be shown in future animations, such as the tiny one below.
As the animation shows, println returns a value, too, even if it’s one with no content:
Returning vs. Printing
Beginner programmers sometimes find it hard to discern just what the difference is between returning a value and printing a value. The REPL environment, useful as it is otherwise, may contribute to the confusion. So let’s take a moment to underline the difference:
- Returning values is something that functions do in general.
A function can communicate a value as a “response” to the party
that calls the function.
- A return value doesn’t necessarily get displayed anywhere.
- It’s up to the code that calls a function
to decide what it does with the return value
that it receives in response; the calling code
may, for instance, use the returned value as
part of a calculation, or print it onscreen,
or pass it as a parameter to
play, or simply ignore it. - Just returning a value from a function isn’t an effect on program state in the sense discussed earlier in this chapter.
- By printing a value, we mean displaying text in the computer’s
text console. In Scala, we accomplish this with the
printlncommand.- The text to be printed can be determined by
any expression, as in
println("Hi!")andprintln(1 + 1). - That expression can also be a function call,
as in
println(average(10, 20)), in which case it’s the function’s return value that gets printed. - Printing text onscreen is one of the things that we consider to be an effect on state.
- The text to be printed can be determined by
any expression, as in
Returning and printing in the REPL
The REPL evaluates whichever expression you type in. It then outputs a report of the
value of the expression without you explicitly telling it to. If the expression specifies
a function call, this means that the function gets called and a report of its return value
gets reported by the REPL. So, if you enter the expression max(3 + min(1, 4), 3),
you’ll see a printout that describes the return value of the max function. However,
min’s return value of 1, which was used while evaluating the entire expression, does not
get reported.
If you use the REPL to call a function that returns Unit, the REPL will simply discard
the contentless return value.
Returning and printing within the same function
You’ve already seen that println not only prints something, it also returns a value,
albeit only the Unit value. Nothing prevents programmers from defining functions that
both print text onscreen and return a value.
On Functions and Abstractions
Functions as tools
Some goals are easily reached because there is a tool available for them. For instance, if our goal is to compute the sine or square root of a particular number, we can simply call a library function to do that.
If we can’t find such a ready-made solution to our exact problem, we’ll form our own by
combining the tools that are available to us. If, for instance, we wish to find out the
volume of a sphere whose radius is 6371 km, we
can use arithmetic operators and the exponentiation function pow:
import scala.math.powimport scala.math.pow 4 * 3.14159 * pow(6371, 3) / 3res10: Double = 1.0832060019000126E12
Above, we evaluated a specific expression that contains specific numbers that gave us a specific result. But if we want to handle a variety of different cases — say, compute the volumes of various spheres of various sizes — it’s more practical to build a custom tool for that purpose. Perhaps our tool could work like this:
volumeOfSphere(6371)res11: Double = 1.0832060019000126E12 volumeOfSphere(1)res12: Double = 4.188786666666666
Benefits of custom functions
You just saw one reason why we might wish to create our own custom functions: when we have implemented a function that solves a particular, perhaps complex, task, we can easily reuse the solution by calling the function. Moreover, writing functions helps programmers avoid reinventing the sphere: a function, once written, can be distributed for other programmers to use.
Functions also have a role in organizing program code into named components that are clearly delimited. This makes the code easier to understand and modify.
One more benefit arises from how a function hides within itself the implementation of
a particular solution, whose details are unnecessary for the function’s user to know.
As long as a programmer has access to a volumeOfSphere function, they can use it even
without knowing the formula for calculating volumes. In this chapter, you yourself
have called various functions even though you couldn’t have implemented the functions
yourself or even understood their implementation given just what we’ve covered so far.
It’s been enough that you’re familiar with a few aspects of each function: what kinds
of parameters it expects, what effects (if any) it brings about, and what it returns.
In other words, functions are a form of abstraction (abstraktio) in programming.
Abstractions in programming
abstract: something that concentrates in itself the essential qualitiesof anything more extensive or more general; essence—a definition of “abstract” at Infoplease.com
By abstracting, we can choose to ignore details and specific cases. For instance, the
function volumeOfSphere represents the general, abstract algorithm for computing
the volume of any sphere.
Besides functions, you’ve already encountered other programming abstractions as well.
A variable is an abstraction: in the expression number + 1, the name of the variable
is there to indicate that we wish to sum two values without concerning ourselves (at
that juncture) with what specific value might be stored in the variable number.
Function parameters promote abstraction. Consider the function sin, which takes a
parameter (as in sin(1)). That function’s programmer didn’t cater to just one use
case of the function but to any and all possible parameter values. Consequently, the
function is more abstract and more generally useful than a variable sineOfOne.
Programming revolves around the design, implementation, and use of abstractions. This is a theme that we’ll return to repeatedly (in Chapters 2.1 and 3.2, among others). Creating your own custom functions is something you’ll get to practice right away in the next chapter.
Summary of Key Points
- A subprogram is an implementation of a specific piece of functionality. In many contexts, including O1, subprograms are known as functions.
- A function can have an effect on program state or return a value (such as the result of a computation) or both.
- Telling the computer to execute a function is known as calling or invoking the function. As you call a function, you may pass parameters to it.
- Programming languages come with libraries that contain functions and other program components. Scala’s library contains a number of mathematical functions, among many other things.
- Functions are one of several forms of abstraction in programming. Abstractions enable people to work on complex wholes and help eliminate repetitive labor.
Unitis a special value that means, roughly, “no meaningful value”.- Links to the glossary: function / subprogram; function call,
parameter expression, return value; effect-free function,
effectful function;
Unit; library; abstraction.
Here’s the concept map from the previous chapter, newly adorned with functions:
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 developed by: (in alphabetical order) Riku Autio, Nikolas Drosdek, Joonatan Honkamaa, Jaakko Kantojärvi, Niklas Kröger, Teemu Lehtinen, Strasdosky Otewa, 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 behind O1Library’s tools 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+ 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 Neal Hefti, Mike Oldfield, and Dave Stewart. Thank you and sorry.
