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7 Built-in Control Structures

Sample run of chapter's interpreter examples

• 7.1 If expressions
• 7.2 While loops
• 7.3 For expressions
• 7.4 Exception handling with try expressions
• 7.5 Match expressions
• 7.6 Living without break and continue
• 7.7 Variable scope
• 7.8 Refactoring imperative-style code
• 7.9 Conclusion

7.1 If expressions

// In file built-in-control-structures/Misc.scala

  var filename = "default.txt"
  if !args.isEmpty then
    filename = args(0)


// In file built-in-control-structures/Misc.scala

  val filename =
    if !args.isEmpty then args(0)
    else "default.txt"


// In file built-in-control-structures/Misc.scala

  println(if (!args.isEmpty) args(0) else "default.txt")


  

7.2 While loops

// In file built-in-control-structures/Misc.scala

  def gcdLoop(x: Long, y: Long): Long =
    var a = x
    var b = y
    while a != 0 do
      val temp = a
      a = b % a
      b = temp
    b


  scala> def greet() = println("hi")
  def greet(): Unit

  scala> val iAmUnit = greet() == ()
  hi
  val iAmUnit: Boolean = true


// In file built-in-control-structures/Misc.scala

  import scala.io.StdIn.readLine
  while
    val line = readLine()
    println(s"Read: $line")
    line != ""
  do ()


  var line = ""         // This doesn't compile!
  while (line = scala.io.StdIn.readLine()) != "" do
    println(s"Read: $line")


// In file built-in-control-structures/Misc.scala

  def gcd(x: Long, y: Long): Long =
    if y == 0 then x else gcd(y, x % y)


  

7.3 For expressions

// In file built-in-control-structures/Files.scala

  val filesHere = (new java.io.File(".")).listFiles

  for file <- filesHere do
    println(file)


  scala> for i <- 1 to 4 do
       |  println(s"Iteration $i")
  Iteration 1
  Iteration 2
  Iteration 3
  Iteration 4


  scala> for i <- 1 until 4 do
       |  println(s"Iteration $i")
  Iteration 1
  Iteration 2
  Iteration 3


// In file built-in-control-structures/Files.scala

  // Not common in Scala...
  for i <- 0 to filesHere.length - 1 do
    println(filesHere(i))


// In file built-in-control-structures/Files.scala

  val filesHere = (new java.io.File(".")).listFiles

  for file <- filesHere if file.getName.endsWith(".scala") do
    println(file)


// In file built-in-control-structures/Files.scala

  for file <- filesHere do
    if file.getName.endsWith(".scala") then
      println(file)


// In file built-in-control-structures/Files.scala

  for
    file <- filesHere
    if file.isFile
    if file.getName.endsWith(".scala")
  do println(file)


// In file built-in-control-structures/Files.scala

  def fileLines(file: java.io.File) = 
    scala.io.Source.fromFile(file).getLines().toArray

  def grep(pattern: String) =
    for
      file <- filesHere
      if file.getName.endsWith(".scala")
      line <- fileLines(file)
      if line.trim.matches(pattern) 
    do println(s"file: {line.trim}")

  grep(".*gcd.*")


// In file built-in-control-structures/Files.scala

  def grep(pattern: String) =
    for
      file <- filesHere
      if file.getName.endsWith(".scala")
      line <- fileLines(file)
      trimmed = line.trim
      if trimmed.matches(pattern)  
    do println(s"file: trimmed")

  grep(".*gcd.*")


// In file built-in-control-structures/Files.scala

  def scalaFiles =
    for
      file <- filesHere
      if file.getName.endsWith(".scala")
    yield file


// In file built-in-control-structures/Files.scala

  val forLineLengths =
    for
      file <- filesHere
      if file.getName.endsWith(".scala")
      line <- fileLines(file)
      trimmed = line.trim
      if trimmed.matches(".*for.*")  
    yield trimmed.length


  

7.4 Exception handling with try expressions

// In file built-in-control-structures/Exceptions.scala

  throw new IllegalArgumentException


// In file built-in-control-structures/Exceptions.scala

  def half(n: Int) =
    if n % 2 == 0 then
      n / 2
    else
      throw new RuntimeException("n must be even")


// In file built-in-control-structures/Exceptions.scala

  import java.io.FileReader
  import java.io.FileNotFoundException
  import java.io.IOException

  try
    val f = new FileReader("input.txt")
    // Use and close file
  catch
    case ex: FileNotFoundException => // Handle missing file
    case ex: IOException => // Handle other I/O error


// In file built-in-control-structures/Exceptions.scala

  import java.io.FileReader

  val file = new FileReader("input.txt")
  try
    println(file.read()) // Use the file
  finally
    file.close()  // Be sure to close the file


// In file built-in-control-structures/Exceptions.scala

  import java.net.URL
  import java.net.MalformedURLException

  def urlFor(path: String) =
    try new URL(path)
    catch case e: MalformedURLException =>
      new URL("http://www.scala-lang.org")


// In file built-in-control-structures/Exceptions.scala

  def f(): Int = try return 1 finally return 2


// In file built-in-control-structures/Exceptions.scala

  def g(): Int = try 1 finally 2


  

7.5 Match expressions

// In file built-in-control-structures/Match.scala

  val firstArg = if !args.isEmpty then args(0) else ""

  firstArg match
    case "salt" => println("pepper")
    case "chips" => println("salsa")
    case "eggs" => println("bacon")
    case _ => println("huh?")


// In file built-in-control-structures/Match.scala

  val firstArg = if !args.isEmpty then args(0) else ""

  val friend =
    firstArg match
      case "salt" => "pepper"
      case "chips" => "salsa"
      case "eggs" => "bacon"
      case _ => "huh?"

  println(friend)


  

7.6 Living without break and continue

// In file built-in-control-structures/Break.java

  int i = 0;                // This is Java
  boolean foundIt = false;
  while (i < args.length) {
    if (args[i].startsWith("-")) {
      i = i + 1;
      continue;
    }
    if (args[i].endsWith(".scala")) {
      foundIt = true;
      break;
    }
    i = i + 1;
  }


// In file built-in-control-structures/Breakless1.scala

  var i = 0
  var foundIt = false

  while i < args.length && !foundIt do
    if !args(i).startsWith("-") then
      if args(i).endsWith(".scala") then
        foundIt = true
      else 
        i = i + 1
    else 
      i = i + 1


// In file built-in-control-structures/Breakless2.scala

  def searchFrom(i: Int): Int =
    if i >= args.length then -1
    else if args(i).startsWith("-") then searchFrom(i + 1) 
    else if args(i).endsWith(".scala") then i
    else searchFrom(i + 1)

  val i = searchFrom(0)


  import scala.util.control.Breaks.*
  import java.io.*

  val in = new BufferedReader(new InputStreamReader(System.in))

  breakable {
    while (true) {
      println("? ")
      if (in.readLine() == "") break
    }
  }


  

7.7 Variable scope

// In file built-in-control-structures/MultiTable.scala

  def printMultiTable() =

    var i = 1
    // only i in scope here

    while i <= 10 do

      var j = 1
      // both i and j in scope here

      while j <= 10 do

        val prod = (i * j).toString
        // i, j, and prod in scope here

        var k = prod.length
        // i, j, prod, and k in scope here

        while k < 4 do
          print(" ")
          k += 1

        print(prod)
        j += 1

      // i and j still in scope; prod and k out of scope

      println()
      i += 1

    // i still in scope; j, prod, and k out of scope


  while i <= 10 do

    var j = 1
    ...


  val a = 1
  val a = 2 // Does not compile
  println(a)


// In file built-in-control-structures/Misc.scala

  val a = 1
  if a == 1 then
    val a = 2 // Compiles just fine
    println(a)
  println(a)


  scala> val a = 1
  a: Int = 1

  scala> val a = 2
  a: Int = 2

  scala> println(a)
  2


  

7.8 Refactoring imperative-style code

// In file built-in-control-structures/MultiTable2.scala

  // Returns a row as a sequence
  def makeRowSeq(row: Int) =
    for col <- 1 to 10 yield
      val prod = (row * col).toString
      val padding = " " * (4 - prod.length)
      padding + prod

  // Returns a row as a string
  def makeRow(row: Int) = makeRowSeq(row).mkString

  // Returns table as a string with one row per line
  def multiTable() =

    val tableSeq = // a sequence of row strings
      for row <- 1 to 10
      yield makeRow(row)

    tableSeq.mkString("\n")


  

7.9 Conclusion