Scala for the Impatient

Control Structures and Functions

Understand conditional and block expressions

Conditional Expressions

• An if expression has a value:

  if (x > 0) 1 else -1

  Like x > 0 ? 1 : -1 in Java/C/C++.
• Type is the common supertype of the branches:

  if (x > 0) "positive" else -1 // Type is Any

• Scala Unit type is similar to void, but it has one value, denoted ().
• Missing else has value Unit:

  if (x > 0) 1

  is the same as

  if (x > 0) 1 else ()

Block Expressions

• Value of block is the value of the last expression:

  val distance = {
    val dx = x - x0
    val dy = y - y0
    sqrt(dx * dx + dy * dy)
  }

• If the last expression is an assignment, the block value is ():

  while (n > 0) {
    i += 1
    n = n / 2
  }

Use the “for each” and “for comprehension” control structures

For Loops

• No three-part loop for (i = 1; i <= n; i++).
• Use “for each” loop instead:

  for (i <- 1 to n)
  

  (Note: No val/var before i)
• Works for any collection:

  for (ch <- "Hello"

• Multiple “generators”:

  for (i <- 1 to 3; j <- 1 to 3) print((10 * i + j) + " ")

• Guards:

  for (i <- 1 to 3; j <- 1 to 3 if i != j) print((10 * i + j) + " ")

• Collecting results:

  for (i <- 1 to 10) yield i % 3

Define functions

Functions

• To define a function, specify name, parameters, body:

  def abs(x: Double) = if (x >= 0) x else -x

• Return type is inferred unless the function is recursive:

  def fac(n: Int): Int = if (n <= 0) 1 else n * fac(n - 1)

• If you omit the =, the function doesn't return a value:

  def box(s: String) { // Look carefully: no =
    val border = "-" * s.length + "--\n"
    println(border + "|" + s + "|\n" + border)
  }

• Caution—easy to accidentally omit the = symbol:

  def fac(n: Int) { // Probably an error
    var r = 1
    for (i <- 1 to n) r = r * i
    r
  }

Understand named, default, and variable function arguments

Named and Default Arguments

• “Named” argument is useful for greater clarity:

  regionMatches(ignoreCase = true, ...)

• Default arguments let you omit argument values:

  def decorate(str: String, left: String = "[", right: String = "]") =
    left + str + right

• Here, defaults are used for both left and right:

  decorate("Hello") // [Hello]

• Default is only used for right:

  decorate("Hello", ">>>[") // >>>[Hello]

• Default is only used for left, named parameter for right:

  decorate("Hello", right = "]<<<") // [Hello]<<<

Varargs

• Variable number of arguments indicated with * after type:

  def sum(args: Int*) = { // args is a Seq[Int]
    var result = 0
    for (arg <- args) result += arg
    result
  }

• Can call with any number of arguments:

  val s = sum(1, 4, 9, 16, 25)

• If you already have a Seq[Int], need decoration to pass it:

  val s = sum(1 to 5: _*)
  

• Needed in recursive calls:

  def recursiveSum(args: Int*): Int = {
    if (args.length == 0) 0
    else args.head + recursiveSum(args.tail : _*)
  }

Practice writing functions and using control structures

Lab

Vowels

1. Write a function that tests whether a character is a vowel (for now, a lowercase aeiou).

   def isVowel(ch: Char) = ...

2. Did you use an if statement? If so, write it without an if. (Hint: contains)
3. Write a function that, given a string, returns a string of all of its vowels. Use isVowel. Use a for loop.

   def vowels(s: String) = ...
   

4. Repeat with a for ... yield loop. (Hint: Guards)
5. Repeat with a recursive solution.
6. Repeat with a while loop.
7. Pick your favorite version. Add a parameter vowelChars with default "aeiuo" and a parameter ignoreCase with default true.
8. Call it to find all vowels in the German word "Übeltätergehör". (Yes, those things with dots are vowels in German.)