3

Basic Scala

for (i <- Range.inclusive(1, 100)) {
  println(
    if (i % 3 == 0 && i % 5 == 0) "FizzBuzz"
    else if (i % 3 == 0) "Fizz"
    else if (i % 5 == 0) "Buzz"
    else i
  )
}
</> 3.1.scala

Snippet 3.1: the popular "FizzBuzz" programming challenge, implemented in Scala

This chapter is a quick tour of the Scala language. For now we will focus on the basics of Scala that are similar to what you might find in any mainstream programming language.

The goal of this chapter is to get familiar you enough that you can take the same sort of code you are used to writing in some other language and write it in Scala without difficulty. This chapter will not cover more Scala-specific programming styles or language features: those will be left for Chapter 5: Notable Scala Features.

For this chapter, we will write our code in the Ammonite Scala REPL:

$ amm
Loading...
Welcome to the Ammonite Repl 2.2.0 (Scala 2.13.2 Java 11.0.7)
@
</> 3.2.bash

3.1 Values

3.1.1 Primitives

Scala has the following sets of primitive types:

These types are identical to the primitive types in Java, and would be similar to those in C#, C++, or any other statically typed programming language. Each type supports the typical operations, e.g. booleans support boolean logic || &&, numbers support arithmetic + - * / and bitwise operations | &, and so on. All values support == to check for equality and != to check for inequality.

Numbers default to 32-bit Ints. Precedence for arithmetic operations follows other programming languages: * and / have higher precedence than + or -, and parentheses can be used for grouping.

@ 1 + 2 * 3
res0: Int = 7

@ (1 + 2) * 3
res1: Int = 9
</> 3.3.scala

Ints are signed and wrap-around on overflow, while 64-bit Longs suffixed with L have a bigger range and do not overflow as easily:

@ 2147483647
res2: Int = 2147483647

@ 2147483647 + 1
res3: Int = -2147483648
</> 3.4.scala

@ 2147483647L
res4: Long = 2147483647L

@ 2147483647L + 1L
res5: Long = 2147483648L
</> 3.5.scala

Apart from the basic operators, there are a lot of useful methods on java.lang.Integer and java.lang.Long:

@ java.lang.Integer.<tab>
BYTES                    decode                   numberOfTrailingZeros
signum                   MAX_VALUE                divideUnsigned
getInteger               parseUnsignedInt         toBinaryString
...

@ java.lang.Integer.toBinaryString(123)
res6: String = "1111011"

@ java.lang.Integer.numberOfTrailingZeros(24)
res7: Int = 3
</> 3.6.scala

64-bit Doubles are specified using the 1.0 syntax, and have a similar set of arithmetic operations. You can also use the 1.0F syntax to ask for 32-bit Floats:

@ 1.0 / 3.0
res8: Double = 0.3333333333333333

@ 1.0F / 3.0F
res9: Float = 0.33333334F
</> 3.7.scala

32-bit Floats take up half as much memory as 64-bit Doubles, but are more prone to rounding errors during arithmetic operations. java.lang.Float and java.lang.Double have a similar set of useful operations you can perform on Floats and Doubles.

3.1.2 Strings

Strings in Scala are arrays of 16-bit Chars:

@ "hello world"
res10: String = "hello world"
</> 3.8.scala

Strings can be sliced with .substring, constructed via concatenation using +, or via string interpolation by prefixing the literal with s"..." and interpolating the values with $ or ${...}:

@ "hello world".substring(0, 5)
res11: String = "hello"

@ "hello world".substring(5, 10)
res12: String = " worl"
</> 3.9.scala

@ "hello" + 1 + " " + "world" + 2
res13: String = "hello1 world2"

@ val x = 1; val y = 2

@ s"Hello $x World $y"
res15: String = "Hello 1 World 2"

@ s"Hello ${x + y} World ${x - y}"
res16: String = "Hello 3 World -1"
</> 3.10.scala

3.1.3 Local Values and Variables

You can name local values with the val keyword:

@ val x = 1

@ x + 2
res18: Int = 3
</> 3.11.scala

Note that vals are immutable: you cannot re-assign the val x to a different value after the fact. If you want a local variable that can be re-assigned, you must use the var keyword.

@ x = 3
cmd41.sc:1: reassignment to val
val res26 = x = 3
              ^
Compilation Failed
</> 3.12.scala

@ var y = 1

@ y + 2
res20: Int = 3

@ y = 3

@ y + 2
res22: Int = 5
</> 3.13.scala

In general, you should try to use val where possible: most named values in your program likely do not need to be re-assigned, and using val helps prevent mistakes where you re-assign something accidentally. Use var only if you are sure you will need to re-assign something later.

Both vals and vars can be annotated with explicit types. These can serve as documentation for people reading your code, as well as a way to catch errors if you accidentally assign the wrong type of value to a variable

@ val x: Int = 1

@ var s: String = "Hello"
s: String = "Hello"

@ s = "World"
</> 3.14.scala

@ val z: Int = "Hello"
cmd33.sc:1: type mismatch;
 found   : String("Hello")
 required: Int
val z: Int = "Hello"
             ^
Compilation Failed
</> 3.15.scala

3.1.4 Tuples

Tuples are fixed-length collections of values, which may be of different types:

@ val t = (1, true, "hello")
t: (Int, Boolean, String) = (1, true, "hello")

@ t._1
res27: Int = 1

@ t._2
res28: Boolean = true

@ t._3
res29: String = "hello"
</> 3.16.scala

Above, we are storing a tuple into the local value t using the (a, b, c) syntax, and then using ._1, ._2 and ._3 to extract the values out of it. The fields in a tuple are immutable.

The type of the local value t can be annotated as a tuple type:

@ val t: (Int, Boolean, String) = (1, true, "hello")

You can also use the val (a, b, c) = t syntax to extract all the values at once, and assign them to meaningful names:

@ val (a, b, c) = t
a: Int = 1
b: Boolean = true
c: String = "hello"
</> 3.17.scala

@ a
res31: Int = 1

@ b
res32: Boolean = true

@ c
res33: String = "hello"
</> 3.18.scala

Tuples come in any size from 1 to 22 items long:

@ val t = (1, true, "hello", 'c', 0.2, 0.5f, 12345678912345L)
t: (Int, Boolean, String, Char, Double, Float, Long) = (
  1,
  true,
  "hello",
  'c',
  0.2,
  0.5F,
  12345678912345L
)
</> 3.19.scala

Most tuples should be relatively small. Large tuples can easily get confusing: while working with ._1 ._2 and ._3 is probably fine, when you end up working with ._11 ._13 it becomes easy to mix up the different fields. If you find yourself working with large tuples, consider defining a Class (3.4) or Case Class that we will see in Chapter 5: Notable Scala Features.

3.1.5 Arrays

Arrays are instantiated using the Array[T](a, b, c) syntax, and entries within each array are retrieved using a(n):

@ val a = Array[Int](1, 2, 3, 4)

@ a(0) // first entry, array indices start from 0
res36: Int = 1

@ a(3) // last entry
res37: Int = 4

@ val a2 = Array[String]("one", "two", "three", "four")
a2: Array[String] = Array("one", "two", "three", "four")

@ a2(1) // second entry
res39: String = "two"
</> 3.20.scala

The type parameter inside the square brackets [Int] or [String] determines the type of the array, while the parameters inside the parenthesis (1, 2, 3, 4) determine its initial contents. Note that looking up an Array by index is done via parentheses a(3) rather than square brackets a[3] as is common in many other programming languages.

You can omit the explicit type parameter and let the compiler infer the Array's type, or create an empty array of a specified type using new Array[T](length), and assign values to each index later:

@ val a = Array(1, 2, 3, 4)
a: Array[Int] = Array(1, 2, 3, 4)

@ val a2 = Array(
    "one", "two",
    "three", "four"
  )
a2: Array[String] = Array(
  "one", "two",
  "three", "four"
)
</> 3.21.scala

@ val a = new Array[Int](4)
a: Array[Int] = Array(0, 0, 0, 0)

@ a(0) = 1

@ a(2) = 100

@ a
res45: Array[Int] = Array(1, 0, 100, 0)
</> 3.22.scala

For Arrays created using new Array, all entries start off with the value 0 for numeric arrays, false for Boolean arrays, and null for Strings and other types. Arrays are mutable but fixed-length: you can change the value of each entry but cannot change the number of entries by adding or removing values. We will see how to create variable-length collections later in Chapter 4: Scala Collections.

Multi-dimensional arrays, or arrays-of-arrays, are also supported:

@ val multi = Array(Array(1, 2), Array(3, 4))
multi: Array[Array[Int]] = Array(Array(1, 2), Array(3, 4))

@ multi(0)(0)
res47: Int = 1

@ multi(0)(1)
res48: Int = 2

@ multi(1)(0)
res49: Int = 3

@ multi(1)(1)
res50: Int = 4
</> 3.23.scala

Multi-dimensional arrays can be useful to represent grids, matrices, and similar values.

3.1.6 Options

Scala's Option[T] type allows you to represent a value that may or may not exist. An Option[T] can either be Some(v: T) indicating that a value is present, or None indicating that it is absent:

@ def hello(title: String, firstName: String, lastNameOpt: Option[String]) = {
    lastNameOpt match {
      case Some(lastName) => println(s"Hello $title. $lastName")
      case None => println(s"Hello $firstName")
    }
  }

@ hello("Mr", "Haoyi", None)
Hello Haoyi

@ hello("Mr", "Haoyi", Some("Li"))
Hello Mr. Li
</> 3.24.scala

The above example shows you how to construct Options using Some and None, as well as matching on them in the same way. Many APIs in Scala rely on Options rather than nulls for values that may or may not exist. In general, Options force you to handle both cases of present/absent, whereas when using nulls it is easy to forget whether or not a value is null-able, resulting in confusing NullPointerExceptions at runtime. We will go deeper into pattern matching in Chapter 5: Notable Scala Features.

Options contain some helper methods that make it easy to work with the optional value, such as getOrElse, which substitutes an alternate value if the Option is None:

@ Some("Li").getOrElse("<unknown>")
res54: String = "Li"

@ None.getOrElse("<unknown>")
res55: String = "<unknown>"
</> 3.25.scala

Options are very similar to a collection whose size is 0 or 1. You can loop over them like normal collections, or transform them with standard collection operations like .map.

@ def hello2(name: Option[String]) = {
    for (s <- name) println(s"Hello $s")
  }

@ hello2(None) // does nothing

@ hello2(Some("Haoyi"))
Hello Haoyi
</> 3.26.scala

@ def nameLength(name: Option[String]) = {
    name.map(_.length).getOrElse(-1)
  }

@ nameLength(Some("Haoyi"))
res60: Int = 5

@ nameLength(None)
res61: Int = -1
</> 3.27.scala

Above, we combine .map and .getOrElse to print out the length of the name if present, and otherwise print -1. We will learn more about collection operations in Chapter 4: Scala Collections.

3.2 Loops, Conditionals, Comprehensions

3.2.1 For-Loops

For-loops in Scala are similar to "foreach" loops in other languages: they directly loop over the elements in a collection, without needing to explicitly maintain and increment an index. If you want to loop over a range of indices, you can loop over a Range such as Range(0, 5):

@ var total = 0

@ val items = Array(1, 10, 100, 1000)

@ for (item <- items) total += item

@ total
res65: Int = 1111
</> 3.28.scala

@ var total = 0

@ for (i <- Range(0, 5)) {
    println("Looping " + i)
    total = total + i
  }
Looping 0
Looping 1
Looping 2
Looping 3
Looping 4

@ total
res68: Int = 10
</> 3.29.scala

You can loop over nested Arrays by placing multiple <-s in the header of the loop:

@ val multi = Array(Array(1, 2, 3), Array(4, 5, 6))

@ for (arr <- multi; i <- arr) println(i)
1
2
3
4
5
6
</> 3.30.scala

Loops can have guards using an if syntax:

@ for (arr <- multi; i <- arr; if i % 2 == 0) println(i)
2
4
6
</> 3.31.scala

3.2.2 If-Else

if-else conditionals are similar to those in any other programming language. One thing to note is that in Scala if-else can also be used as an expression, similar to the a ? b : c ternary expressions in other languages. Scala does not have a separate ternary expression syntax, and so the if-else can be directly used as the right-hand-side of the total += below.

@ var total = 0

@ for (i <- Range(0, 10)) {
    if (i % 2 == 0) total += i
    else total += 2
  }

@ total
res74: Int = 30
</> 3.32.scala

@ var total = 0

@ for (i <- Range(0, 10)) {
    total += (if (i % 2 == 0) i else 2)
  }

@ total
res77: Int = 30
</> 3.33.scala

3.2.3 Fizzbuzz

Now that we know the basics of Scala syntax, let's consider the common "Fizzbuzz" programming challenge:

Write a short program that prints each number from 1 to 100 on a new line.

For each multiple of 3, print "Fizz" instead of the number.

For each multiple of 5, print "Buzz" instead of the number.

For numbers which are multiples of both 3 and 5, print "FizzBuzz" instead of the number.

We can accomplish this as follows:

@ for (i <- Range.inclusive(1, 100)) {
    if (i % 3 == 0 && i % 5 == 0) println("FizzBuzz")
    else if (i % 3 == 0) println("Fizz")
    else if (i % 5 == 0) println("Buzz")
    else println(i)
  }
1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
...
</> 3.34.scala

Since if-else is an expression, we can also write it as:

@ for (i <- Range.inclusive(1, 100)) {
    println(
      if (i % 3 == 0 && i % 5 == 0) "FizzBuzz"
      else if (i % 3 == 0) "Fizz"
      else if (i % 5 == 0) "Buzz"
      else i
    )
  }
</> 3.35.scala

3.2.4 Comprehensions

Apart from using for to define loops that perform some action, you can also use for together with yield to transform a collection into a new collection:

@ val a = Array(1, 2, 3, 4)

@ val a2 = for (i <- a) yield i * i
a2: Array[Int] = Array(1, 4, 9, 16)

@ val a3 = for (i <- a) yield "hello " + i
a3: Array[String] = Array("hello 1", "hello 2", "hello 3", "hello 4")
</> 3.36.scala

Similar to loops, you can filter which items end up in the final collection using an if guard inside the parentheses:

@ val a4 = for (i <- a if i % 2 == 0) yield "hello " + i
a4: Array[String] = Array("hello 2", "hello 4")
</> 3.37.scala

Comprehensions can also take multiple input arrays, a and b below. This flattens them out into one final output Array, similar to using a nested for-loop:

@ val a = Array(1, 2); val b = Array("hello", "world")

@ val flattened = for (i <- a; s <- b) yield s + i
flattened: Array[String] = Array("hello1", "world1", "hello2", "world2")
</> 3.38.scala

You can also replace the parentheses () with curly brackets {} if you wish to spread out the nested loops over multiple lines, for easier reading. Note that the order of <-s within the nested comprehension matters, just like how the order of nested loops affects the order in which the loop actions will take place:

@ val flattened = for{
    i <- a
    s <- b
  } yield s + i
flattened: Array[String] = Array("hello1", "world1", "hello2", "world2")

@ val flattened2 = for{
    s <- b
    i <- a
  } yield s + i
flattened2: Array[String] = Array("hello1", "hello2", "world1", "world2")
</> 3.39.scala

We can use comprehensions to write a version of FizzBuzz that doesn't print its results immediately to the console, but returns them as a Seq (short for "sequence"):

@ val fizzbuzz = for (i <- Range.inclusive(1, 100)) yield {
    if (i % 3 == 0 && i % 5 == 0) "FizzBuzz"
    else if (i % 3 == 0) "Fizz"
    else if (i % 5 == 0) "Buzz"
    else i.toString
  }
fizzbuzz: IndexedSeq[String] = Vector(
  "1",
  "2",
  "Fizz",
  "4",
  "Buzz",
...
</> 3.40.scala

We can then use the fizzbuzz collection however we like: storing it in a variable, passing it into methods, or processing it in other ways. We will cover what you can do with these collections later, in Chapter 4: Scala Collections.

3.3 Methods and Functions

3.3.1 Methods

You can define methods using the def keyword:

@ def printHello(times: Int) = {
    println("hello " + times)
  }

@ printHello(1)
hello 1

@ printHello(times = 2) // argument name provided explicitly
hello 2
</> 3.41.scala

Passing in the wrong type of argument, or missing required arguments, is a compiler error. However, if the argument has a default value, then passing it is optional.

@ printHello("1") // wrong type of argument
cmd128.sc:1: type mismatch;
 found   : String("1")
 required: Int
val res128 = printHello("1")
                         ^
Compilation Failed
</> 3.42.scala

@ def printHello2(times: Int = 0) = {
    println("hello " + times)
  }

@ printHello2(1)
hello 1

@ printHello2()
hello 0
</> 3.43.scala

3.3.1.1 Returning Values from Methods

Apart from performing actions like printing, methods can also return values. The last expression within the curly brace {} block is treated as the return value of a Scala method.

@ def hello(i: Int = 0) = {
    "hello " + i
  }

@ hello(1)
res96: String = "hello 1"
</> 3.44.scala

You can call the method and print out or perform other computation on the returned value:

@ println(hello())
hello 0

@ val helloHello = hello(123) + " " + hello(456)
helloHello: String = "hello 123 hello 456"

@ helloHello.reverse
res99: String = "654 olleh 321 olleh"
</> 3.45.scala

3.3.2 Function Values

You can define function values using the => syntax. Functions values are similar to methods, in that you call them with arguments and they can perform some action or return some value. Unlike methods, functions themselves are values: you can pass them around, store them in variables, and call them later.

@ var g: Int => Int = i => i + 1

@ g(10)
res101: Int = 11

@ g = i => i * 2

@ g(10)
res103: Int = 20
</> 3.46.scala

Note that unlike methods, function values cannot have optional arguments (i.e. with default values) and cannot take type parameters via the [T] syntax. When a method is converted into a function value, any optional arguments must be explicitly included, and type parameters fixed to concrete types. Function values are also anonymous, which makes stack traces involving them less convenient to read than those using methods.

In general, you should prefer using methods unless you really need the flexibility to pass as parameters or store them in variables. But if you need that flexibility, function values are a great tool to have.

3.3.2.1 Methods taking Functions

One common use case of function values is to pass them into methods that take function parameters. Such methods are often called "higher order methods". Below, we have a class Box with a method printMsg that prints its contents (an Int), and a separate method update that takes a function of type Int => Int that can be used to update x. You can then pass a function literal into update in order to change the value of x:

@ class Box(var x: Int) {
    def update(f: Int => Int) = x = f(x)
    def printMsg(msg: String) = {
      println(msg + x)
    }
  }
</> 3.47.scala

@ val b = new Box(1)

@ b.printMsg("Hello")
Hello1

@ b.update(i => i + 5)

@ b.printMsg("Hello")
Hello6
</> 3.48.scala

Simple functions literals like i => i + 5 can also be written via the shorthand _ + 5, with the underscore _ standing in for the function parameter.

@ b.update(_ + 5)

@ b.printMsg("Hello")
Hello11
</> 3.49.scala

This placeholder syntax for function literals also works for multi-argument functions, e.g. (x, y) => x + y can be written as _ + _.

Any method that takes a function as an argument can also be given a method reference, as long as the method's signature matches that of the function type, here Int => Int:

@ def increment(i: Int) = i + 1

@ val b = new Box(123)

@ b.update(increment) // Providing a method reference

@ b.update(x => increment(x)) // Explicitly writing out the function literal

@ b.update{x => increment(x)} // Methods taking a single function can be called with {}s

@ b.update(increment(_)) // You can also use the `_` placeholder syntax

@ b.printMsg("result: ")
result: 127
</> 3.50.scala

3.3.2.2 Multiple Parameter Lists

Methods can be defined to take multiple parameter lists. This is useful for writing higher-order methods that can be used like control structures, such as the myLoop method below:

@ def myLoop(start: Int, end: Int)
            (callback: Int => Unit) = {
    for (i <- Range(start, end)) {
      callback(i)
    }
  }
</> 3.51.scala

@ myLoop(start = 5, end = 10) { i =>
    println(s"i has value ${i}")
  }
i has value 5
i has value 6
i has value 7
i has value 8
i has value 9
</> 3.52.scala

The ability to pass function literals to methods is used to great effect in the standard library, to concisely perform transformations on collections. We will see more of that in Chapter 4: Scala Collections.

3.4 Classes and Traits

You can define classes using the class keyword, and instantiate them using new. By default, all arguments passed into the class constructor are available in all of the class' methods: the (x: Int) above defines both the private fields as well as the class' constructor. x is thus accessible in the printMsg function, but cannot be accessed outside the class:

@ class Foo(x: Int) {
    def printMsg(msg: String) = {
      println(msg + x)
    }
  }
</> 3.53.scala

@ val f = new Foo(1)

@ f.printMsg("hello")
hello1

@ f.x
cmd120.sc:1: value x is not a member of Foo
Compilation Failed
</> 3.54.scala

To make x publicly accessible you can make it a val, and to make it mutable you can make it a var:

@ class Bar(val x: Int) {
    def printMsg(msg: String) = {
      println(msg + x)
    }
  }
</> 3.55.scala

@ val b = new Bar(1)

@ b.x
res122: Int = 1
</> 3.56.scala

@ class Qux(var x: Int) {
    def printMsg(msg: String) = {
      x += 1
      println(msg + x)
    }
  }
</> 3.57.scala

@ val q = new Qux(1)

@ q.printMsg("hello")
hello2

@ q.printMsg("hello")
hello3
</> 3.58.scala

You can also use vals or vars in the body of a class to store data. These get computed once when the class is instantiated:

@ class Baz(x: Int) {
    val bangs = "!" * x
    def printMsg(msg: String) = {
      println(msg + bangs)
    }
  }
</> 3.59.scala

@ val z = new Baz(3)

@ z.printMsg("hello")
hello!!!
</> 3.60.scala

3.4.1 Traits

traits are similar to interfaces in traditional object-oriented languages: a set of methods that multiple classes can inherit. Instances of these classes can then be used interchangeably.

@ trait Point{ def hypotenuse: Double }

@ class Point2D(x: Double, y: Double) extends Point{
    def hypotenuse = math.sqrt(x * x + y * y)
  }

@ class Point3D(x: Double, y: Double, z: Double) extends Point{
    def hypotenuse = math.sqrt(x * x + y * y + z * z)
  }

@ val points: Array[Point] = Array(new Point2D(1, 2), new Point3D(4, 5, 6))

@ for (p <- points) println(p.hypotenuse)
2.23606797749979
8.774964387392123
</> 3.61.scala

Above, we have defined a Point trait with a single method def hypotenuse: Double. The subclasses Point2D and Point3D both have different sets of parameters, but they both implement def hypotenuse. Thus we can put both Point2Ds and Point3Ds into our points: Array[Point] and treat them all uniformly as objects with a def hypotenuse method, regardless of what their actual class is.

3.5 Conclusion

In this chapter, we have gone through a lightning tour of the core Scala language. While the exact syntax may be new to you, the concepts should be mostly familiar: primitives, arrays, loops, conditionals, methods, and classes are part of almost every programming language. Next we will look at the core of the Scala standard library: the Scala Collections.

@ flexibleFizzBuzz(s => {} /* do nothing */)

@ flexibleFizzBuzz(s => println(s))
1
2
Fizz
4
Buzz
...
</> 3.62.scala

@ var i = 0

@ val output = new Array[String](100)

@ flexibleFizzBuzz{s =>
    output(i) = s
    i += 1
  }

@ output
res125: Array[String] = Array(
  "1",
  "2",
  "Fizz",
  "4",
  "Buzz",
...
</> 3.63.scala

class Msg(val id: Int, val parent: Option[Int], val txt: String)
def printMessages(messages: Array[Msg]): Unit = ...
</> 3.64.scala

TestPrintMessages.scprintMessages(Array(
  new Msg(0, None, "Hello"),
  new Msg(1, Some(0), "World"),
  new Msg(2, None, "I am Cow"),
  new Msg(3, Some(2), "Hear me moo"),
  new Msg(4, Some(2), "Here I stand"),
  new Msg(5, Some(2), "I am Cow"),
  new Msg(6, Some(5), "Here me moo, moo")
))</> 3.65.scala

expected.txt#0 Hello
    #1 World
#2 I am Cow
    #3 Hear me moo
    #4 Here I stand
    #5 I am Cow
        #6 Here me moo, moo</> 3.66.output

TestContextManagers.scwithFileWriter("File.txt") { writer =>
  writer.write("Hello\n"); writer.write("World!")
}
val result = withFileReader("File.txt") { reader =>
  reader.readLine() + "\n" + reader.readLine()
}
assert(result == "Hello\nWorld!")</> 3.67.scala