Transcript Getting Started with Scala

Getting Started with Scala
2-Apr-16
Hello World
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/** 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
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Everything in the object is like Java’s static
Scala doesn’t have static
Scala does have classes; we’ll get to those later
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Comments
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// 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
* (@author, etc.) are used. */
def doNothing = ()
Types
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The type hierarchy is a lattice, not a tree
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That is, it has a “bottom” as well as a “top”
Any
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AnyVal
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AnyRef (corresponds to Object in Java)
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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)
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)
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Declaring variables and values
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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
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Rules for alphanumeric names are just like in Java
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But there are other kinds of names 
Scope rules are almost the same as those 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
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“Statements”
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Scala’s “statements” should really be called “expressions,” because almost
every statement has a value
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For example, the value of a = 5 is 5
The value of many statements, for example the while loop, is ()
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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, or if the next line cannot begin a valid statement
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() is a value of type Unit
() is the only value of type Unit
() basically means “Nothing to see here. Move along.”
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
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Familiar statement types
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These are the same as in Java (but have a value):
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variable = expression // also +=, *=, etc.
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while (condition) { statements }
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The value is ()
if (condition) { statements }
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The value of the statement is ()
do { statements } while (condition)
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The value of the statement is the value of the variable
The value is the value of the last statement executed
if (condition) { statements } else { statements }
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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 ()
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The for comprehension
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Scala’s for is much more powerful than Java’s for
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We will just cover some simple cases here
for (i <- 1 to 10) { println(i) }
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Prints all the values in myArray
for (x <- myArray) { println(x) }
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Prints the numbers 1 through 9
for (x <- 0 until myArray.length) { println(myArray(x)) }
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Prints the numbers 1 through 10
for (i <- 1 until 10) { println(i) }
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Consequently, it is used much more often than the other kinds of loops
Prints all the values in myArray
for (x <- myArray
if x % 2 == 0) { println(x) }
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Prints all the even numbers in myArray
Arrays
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Arrays in Scala are parameterized types
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When no initial values are given, new is required, along with an
explicit type:
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Array[String] is an Array of Strings, where String is a type parameter
In Java we would call this a “generic type”
val ary = new Array[Int](5)
When initial values are given, new is not allowed:
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val ary2 = Array(3, 1, 4, 1, 6)
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Arrays syntax in Scala is just object syntax
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Scala’s Lists are more useful, and used more often, than Arrays
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val list1 = List(3, 1, 4, 1, 6)
val list2 = List[Int]() // An empty list must have an explicit type
Simple List operations
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By default, Lists, like Strings, are immutable
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Basic operations:
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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
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Operations on an immutable List return a new List
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!
Tuples
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Scala has tuples, up to size 22 (why 22? I have no idea.)
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Tuples are referenced starting from 1, using _1, _2, ...
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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)
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!
An aside...abbreviations
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Scala lets you omit a lot of “unnecessary” punctuation
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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,
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If you get mysterious syntax errors, try putting the punctuation
back in, both on this line and on the previous line
Maps
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scala> val m = Map("apple" -> "red", "banana" -> "yellow")
m: scala.collection.immutable.Map[java.lang.String,java.lang.String]
= Map((apple,red), (banana,yellow))
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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
Simple function definitions
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def isEven(n: Int) = {
val m = n % 2
m == 0
}
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def isEven(n: Int) = n % 2 == 0
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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
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The result is just a single expression, so no braces are needed
def countTo(n: Int) {
for (i <- 1 to 10) { println(i) }
}
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The result is the last value (in this case, a Boolean)
You can state the result type explicitly
In this example, half(7) will return 3.5 (!)
def half(n: Int): Int = return n / 2
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If you use a return statement, you must state the result type explicitly
Functions are first-class objects
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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):
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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
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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:
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myList.foreach(i => println(2 * i))
Functions as parameters
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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:
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(type1, type2, ..., typeN) => return_type
type => return_type // if only one parameter
() => return_type // if no parameters
Example:
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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
Higher-order methods on Lists
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map applies a one-parameter function to every element of a List, returning a
new List
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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
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scala> ll filter(n => n < 5)
res10: List[Int] = List(2, 3)
scala> ll filter (_ < 5) // abbreviated function where parameter is used once
res11: List[Int] = List(2, 3)
More higher-order methods
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def filterNot(p: (A) => Boolean): List[A]
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def count(p: (A) => Boolean): Int
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Tests whether a predicate holds for at least one of the elements of this list
def find(p: (A) => Boolean): Option[A]
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Tests whether a predicate holds for every element of this list
def exists(p: (A) => Boolean): Boolean
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Counts the number of elements in the list which satisfy a predicate
def forall(p: (A) => Boolean): Boolean
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Selects all elements of this list which do not satisfy a predicate
Finds the first element of the list satisfying a predicate, if any
def sortWith(lt: (A, A) => Boolean): List[A]
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Sorts this list according to a comparison function
Pattern matching
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Pattern matching on literal values:
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today match {
case "Saturday" => println("Party! Party! Party!")
case "Sunday" => println("Pray....")
case day => println(day + " is a workday. :( ")
}
Pattern matching on types:
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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! :( ")
}
The Option type
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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
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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!!!")
}
The require and assert methods
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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
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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
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takeCis700course
assert(languagesIKnow contains "Scala")
assert is often used as “executable documentation”
The ensuring method
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The ensuring method applies a predicate to a value and, if the
predicate result is true, returns the value, otherwise throws an
AssertionError
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Syntax: value ensuring(predicate)
scala> 12 ensuring(true)
res1: Int = 12
scala> 12 ensuring(_ > 10)
res2: Int = 12
scala> def twice(x: Int) = 2 * x ensuring(_ > 0)
twice: (x: Int)Int
scala> twice(3)
res3: Int = 6
scala> twice(-5)
java.lang.AssertionError: assertion failed (+ many lines)
ensuring can be useful to guarantee the result of a method
Dealing with exceptions
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Scala’s exception creation and throwing is like Java:
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class RottenEggException extends Exception
throw new RottenEggException
Catching a thrown exception uses pattern matching:
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try {
makeAnOmlet
} catch {
case ex: RottenEggException => println("#$%&#@")
case ex: Exception => println("What went wrong?")
}
File I/O
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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()
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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
val lines = Source.fromFile(file).getLines().toList
println(lines)
}
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}
Testing file I/O
List(first, second)
Use the source, Luke
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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
The End
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