Return to chapter index

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)
    filename = args(0)


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

  val filename =
    if (!args.isEmpty) 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) {
      val temp = a
      a = b % a
      b = temp
    }
    b
  }


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

  var line = ""
  do {
    line = readLine()
    println("Read: " + line)
  } while (line != "")


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

  scala> () == greet()
  hi
  res0: Boolean = true


  var line = ""
  while ((line = readLine()) != "") // This doesn't work!
    println("Read: " + line)


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

  def gcd(x: Long, y: Long): Long =
    if (y == 0) 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)
    println(file)


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


  scala> for (i <- 1 until 4)
       |  println("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)
    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"))
    println(file)


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

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


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

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


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

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

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

  grep(".*gcd.*")


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

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


  for clauses yield body


  for (file <- filesHere if file.getName.endsWith(".scala")) {
    yield file  // Syntax error!
  }


// 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

  val half =
    if (n % 2 == 0)
      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 {
    // 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.length > 0) 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) 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) {
    if (!args(i).startsWith("-")) {
      if (args(i).endsWith(".scala"))
        foundIt = true
    }
    i = i + 1
  }


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

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

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

      while (j <= 10) {

        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) {
          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) {

    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;
  {
    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


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

  val a = 1;
  {
    val a = 2;
    {
      println(a)
    }
  }


  

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