1. Getting Started with Scala
28-Dec-18

2. Hello World
/** Everybody’s first program */ object HelloWorld { def main(args: Array[String]) { println("Hello, World!") } }
Save on a file named HelloWorld.scala
Compile with scalac HelloWorld.scala (or from your IDE)
Run with scala HelloWorld (or from your IDE)
Generate documentation with scaladoc HelloWorld.scala
The above code creates a single object
Everything in the object is like Java’s static
Scala doesn’t have static
Scala does have classes; we’ll get to those later

3. Comments // and /*...*/ comments are as in Java
/** Scaladoc comments are similar to Javadoc * comments. As in Javadoc, the first sentence * is a summary sentence, and additional * sentences give more detail. However, the * formatting convention is slightly different, * and wiki markup is used in preference to * HTML markup. Most of the same flags * etc.) are used. */ def doNothing = ()

4. Types The type hierarchy is a lattice, not a tree Any
That is, it has a “bottom” as well as a “top”
Any
AnyVal
Boolean, Char, Byte, Short, Int, Long, Float, Double
Scala has no primitives—these are objects
Unit (has only a single value, ()
Unit is returned by functions that “don't return anything” (e.g. println)
AnyRef (corresponds to Object in Java)
All Java reference types, for example, String
ScalaObject
All Scala reference types, including Array and List
Null (bottom of all AnyRef objects)
Nothing (bottom of Any)

5. Declaring variables and values
The syntax is var name: type = value // declares a variable val name: type = value // declares a value
Values are immutable: they can’t be changed
The : type can almost always be left off—the type is inferred from the value
The = value must (almost) always be supplied
This also works: var x: Double = 5
Rules for alphanumeric names are just like in Java
But there are other kinds of names 
Scope rules are just like in Java
Capitalization conventions are just like in Java
Arithmetic, comparison, and logical operators are just like in Java
Indentation is 2 spaces, not 4, but is otherwise the same as in Java

6. “Statements”
Scala’s “statements” should really be called “expressions,” because almost every statement has a value
For example, the value of a = 5 is 5
The value of many statements, for example the while loop, is ()
() is a value of type Unit
() is the only value of type Unit
() basically means “Nothing to see here. Move along.”
The value of a block, {…}, is the last value computed in the block
A statement is ended by the end of the line (not with a semicolon) unless it is obviously incomplete
For example, x = 3 * (2 * y + is obviously incomplete
Because Scala lets you leave out a lot of unnecessary punctuation, sometimes a line that you think is complete really isn’t complete (or vice versa)
You can end statements with semicolons, but that’s not good Scala practice

7. Familiar statement types
These are the same as in Java (but have a value):
variable = expression // also +=, *=, etc.
The value of the statement is the value of the variable
while (condition) { statements }
The value of the statement is ()
do { statements } while (condition)
The value is ()
if (condition) { statements }
The value is the value of the last statement executed
if (condition) { statements } else { statements }
Older descriptions of Scala say that the else is required. It isn’t--if omitted and the condition is false, the value of the if statement is ()

8. The for comprehension
Scala’s for is much more powerful than Java’s for
Consequently, it is used much more often than the other kinds of loops
We will just cover some simple cases here
for (i <- 1 to 10) { println(i) }
Prints the numbers 1 through 10
for (i <- 1 until 10) { println(i) }
Prints the numbers 1 through 9
for (x <- 0 until myArray.length) { println(myArray(x)) }
Prints all the values in myArray
for (x <- myArray) { println(x) }
for (x <- myArray if x % 2 == 0) { println(x) }
Prints all the even numbers in myArray

9. Arrays Arrays in Scala are parameterized types
Array[String] is an Array of Strings, where String is a type parameter
In Java we would call this a “generic type”
When no initial values are given, new is required, along with an explicit type:
val ary = new Array[Int](5)
When initial values are given, new is not allowed:
val ary2 = Array(3, 1, 4, 1, 6)
Arrays in Scala are just another kind of object; they have no special syntax
Scala’s Lists are more useful, and used more often, than Arrays
val list1 = List(3, 1, 4, 1, 6)
val list2 = List[Int]() // An empty list must have an explicit type

10. Simple List operations
By default, Lists, like Strings, are immutable
Operations on an immutable List return a new List
Basic operations:
list.head (or list head) returns the first element in the list
list.tail (or list tail) returns a list with the first element removed
list(i) returns the ith element (starting from 0) of the list
list(i) = value is illegal (immutable, remember?)
value :: list returns a list with value appended to the front
list1 ::: list2 appends (“concatenates”) the two lists
list.contains(value) (or list contains value) tests whether value is in list
An operation on a List may return a List of a different type
scala> "abc" :: List(1, 2, 3) res22: List[Any] = List(abc, 1, 2, 3)
There are over 150 built-in operations on Lists--use the API!

11. Tuples Scala has tuples, up to size 22 (why 22? I have no idea.)
scala> val t = Tuple3(3, "abc", 5.5) t: (Int, java.lang.String, Double) = (3,abc,5.5)
scala> val tt = (3, "abc", 5.5) tt: (Int, java.lang.String, Double) = (3,abc,5.5)
Tuples are referenced starting from 1, using _1, _2, ...
scala> t._1 res28: Int = 3
t _1 also works (the dot is optional)
Tuples, like Lists, are immutable
Tuples are a great way to return more than one value from a method!

12. An aside...abbreviations
Scala lets you omit a lot of “unnecessary” punctuation
For example, if (name startsWith "Dr.") { ... } is more readable (and easier to type) than if (name.startsWith("Dr.")) { ... }
Readability matters!
Therefore, you should experiment with leaving out punctuation anywhere you think it might be okay
However,
If you get mysterious syntax errors, try putting the punctuation back in, both on this line and on the previous line

13. Maps
scala> val m = Map("apple" -> "red", "banana" -> "yellow") m: scala.collection.immutable.Map[java.lang.String,java.lang.String] = Map((apple,red), (banana,yellow))
Notice that a Map is really just a list of Tuples
The -> is provided as a more readable syntax
scala> m("banana") res2: java.lang.String = yellow
scala> m contains "apple" res3: Boolean = true
scala> m("cherry") java.util.NoSuchElementException: key not found: cherry

14. Simple function definitions
def isEven(n: Int) = { val m = n % m == 0 }
The result is the last value (in this case, a Boolean)
def isEven(n: Int) = n % 2 == 0
The result is just a single expression, so no braces are needed
def countTo(n: Int) { for (i <- 1 to 10) { println(i) } }
It’s good style to omit the = when the result is ()
If you omit the =, the result will be ()
def half(n: Int): Double = n / 2
You can state the result type explicitly
In this example, half(7) will return 3.5 (!)
def half(n: Int): Int = return n / 2
If you use a return statement, you must state the result type explicitly

15. Functions are first-class objects
Functions are values (like integers, etc.) and can be assigned to variables, passed to and returned from functions, and so on
Wherever you see the => symbol, it’s a literal function
Example (assigning a literal function to the variable foo):
scala> val foo = (x: Int) => if (x % 2 == 0) x / 2 else 3 * x + 1 foo: (Int) => Int = <function1> scala> foo(7) res28: Int = 22
The basic syntax of a function literal is parameter_list => function_body
In this example, foreach is a function that takes a function as a parameter:
myList.foreach(i => println(2 * i))

16. Functions as parameters
To define a function, you must specify the types of each of its parameters
Therefore, to have a function parameter, you must know how to write its type:
(type1, type2, ..., typeN) => return_type
type => return_type // if only one parameter
Example:
scala> def doTwice(f: Int => Int, n: Int) = f(f(n)) doTwice: (f: (Int) => Int,n: Int)Int scala> def collatz(n: Int) = if (n % 2 == 0) n / 2 else 3 * n + 1 collatz: (n: Int)Int scala> doTwice(collatz, 7) res2: Int = 11 scala> doTwice(a => 101 * a, 3) res4: Int = 30603

17. Higher-order methods on Lists
map applies a one-parameter function to every element of a List, returning a new List
scala> def double(n: Int) = 2 * n double: (n: Int)Int
scala> val ll = List(2, 3, 5, 7, 11) ll: List[Int] = List(2, 3, 5, 7, 11)
scala> ll map double res5: List[Int] = List(4, 6, 10, 14, 22)
scala> ll map (n => 3 * n) res6: List[Int] = List(6, 9, 15, 21, 33)
scala> ll map (n => n > 5) res8: List[Boolean] = List(false, false, false, true, true)
filter applies a one-parameter test to every element of a List, returning a List of those elements that pass the test
scala> ll filter(n => n < 5) res10: List[Int] = List(2, 3)
scala> ll filter (_ < 5) // abbreviated function where one parameter is used once res11: List[Int] = List(2, 3)

18. More higher-order methods
def filterNot(p: (A) => Boolean): List[A]
Selects all elements of this list which do not satisfy a predicate
def count(p: (A) => Boolean): Int
Counts the number of elements in the list which satisfy a predicate
def forall(p: (A) => Boolean): Boolean
Tests whether a predicate holds for every element of this list
def exists(p: (A) => Boolean): Boolean
Tests whether a predicate holds for at least one of the elements of this list
def find(p: (A) => Boolean): Option[A]
Finds the first element of the list satisfying a predicate, if any
def sortWith(lt: (A, A) => Boolean): List[A]
Sorts this list according to a comparison function

19. Pattern matching Pattern matching on literal values:
today match { case "Saturday" => println("Party! Party! Party!") case "Sunday" => println("Pray....") case day => println(day + " is a workday. :( ") }
Pattern matching on types:
something match { case x: Int => println("I'm the integer " + x) case x: String => println("I'm the String \"" + x + "\"") println("My length is " + x.length) case _ => println("I don't know what I am! :( ") }

20. The Option type
Scala has null because it interoperates with Java; it shouldn’t be used any other time
Instead, use an Option type, with values Some(value) and None
def max(list: List[Int]) = { if (list.length > 0) { val biggest = (list(0) /: list) { (a, b) => if (a > b) a else b } Some(biggest) } else { None }
max(myList) match { case Some(x) => println("The largest number is " + x) case None => println("There are no numbers here!!!") }

21. The require and assert methods
require and assert are methods that throw an exception when their argument is false
require is used to document something that must be true in order for the code to work
def sqrt(x: Double) = { require(x >= 0); ... }
require is often used at the beginning of a method
assert is used to document something that you “know” to be true
takeCis700course assert(languagesIKnow contains "Scala")
assert is often used at the end of a block of code, to say what that code has accomplished

22. Dealing with exceptions
Scala’s exception creation and throwing is like Java:
class RottenEggException extends Exception
throw new RottenEggException
Catching a thrown exception uses pattern matching:
try { makeAnOmlet } catch { case ex: RottenEggException => case ex: Exception => println("What went wrong?") }

23. File I/O Use correct case for file names (only Windows ignores case)
object TestIO { def main(args: Array[String]) { println("Testing file I/O") import java.io._ import scala.io.Source val file = new File("testio.txt") val writer = new PrintWriter(file) writer write "first\r\n" writer write "second" writer.close() val lines = Source.fromFile(file).getLines().toList println(lines) } }
Testing file I/O List(first, second)
Use correct case for file names (only Windows ignores case)
Use forward slashes, /, in paths, which work on any platform, not backslashes, \, which work only on Windows
Windows, Mac, and Linux have different “end-of-line codes (\r\n is Windows), and this causes problems

24. Use the source, Luke
Books and tutorials are good for learning the syntax of Scala, but they aren’t much help learning the API
Unfortunately, Scala’s API documentation isn’t very complete
The good news is, it’s easy to get to the source code--and in most cases, the source code is easier to read than you might think

25. The End