on 25-Aug-2014 (Mon)

Tags

#odersky-programming-in-scala-2ed #scala

Question

What does to stand for in this example?

for (i <- 0 to 2)
  print(greetStrings(i))

Answer

a method call (0).to(2)
if a method takes only one parameter, you can call it without a dot or parentheses.

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#odersky-programming-in-scala-2ed #scala

Question

What does + really stand for in 2 + 3?

Answer

it is a method call: (2).+(3)
method is named "+"

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#odersky-programming-in-scala-2ed #scala

Question

What will greetStrings(0) = "Hello" be transformed into?

Answer

greetStrings.update(0, "Hello")

Tags

#odersky-programming-in-scala-2ed #scala

Question

What method are you calling here and where is it defined?
val numNames = Array("zero", "one", "two")

Answer

a factory method apply which takes variable number of argumetns and is defined on Array companion object
the code gets transformed to
val numNames2 = Array.apply("zero", "one", "two")

Tags

#odersky-programming-in-scala-2ed #scala

Question

What does the cons expression, e.g. 1::twoThree, gets translated to (in terms of method calls)?
What rule is used?

Answer

twoThree.::(1)
if a method's name used in operator notation ends with colon, it is called on the right operand

Tags

#odersky-programming-in-scala-2ed #scala

Question

When you add a key-value pair to a map: treasureMap += (2 -> "Dig.") , what is 1 -> "Dig." really?

Answer

It is a method call (1).->("Dig") which returns a tuple containing key and value

Tags

#odersky-programming-in-scala-2ed #scala

Question

What is the class that has companion object called?

Answer

companion class of that object

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#odersky-programming-in-scala-2ed #scala

Question

Where a companion class and its companion objects have to be defined?

Answer

in the same source file

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#odersky-programming-in-scala-2ed #scala

Question

What is special about access control between companion object and companion class?

Answer

they can access each other's private members

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#odersky-programming-in-scala-2ed #scala

Question

How to express s.indexOf(’o’, 5) in operator notation?

Answer

s indexOf ('o', 5)

Tags

#odersky-programming-in-scala-2ed #scala

Question

if you define a method unary_* and then try to call it on object in prefix notation *p, how will Scala parse it and why?

Answer

*.p because * is not one of the four names allowed for prefix methods

Tags

#odersky-programming-in-scala-2ed #scala

Question

How does scala decide on precedence when methods are used in operator notation, like 2 + 2 * 7 ? What is Scala's default rule here?

Answer

Scala decides precedence based on the first character of the methods used in operator notation (this rule has exceptions). If the method name starts with a *, for example, it will have a higher precedence than a method that starts with a +.

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#odersky-programming-in-scala-2ed #scala

Question

Why won't this compile?

class Rational(n: Int, d: Int) { // This won’t compile
  require(d != 0)
  override def toString = n +"/"+ d
  def add(that: Rational): Rational =
  new Rational(n * that.d + that.n * d, d * that.d)
}

Answer

Because n and d can only be accessed in methods for this object - they seem to be captured like in closure for toString. They are not fields / members though

Tags

#odersky-programming-in-scala-2ed #scala

Question

How to call Thread.yield() in Scala, where yield is a reserved word?

Answer

Thread.`yield`()
string enclosed in backticks becomes an identifier, no matter how arbitrary it is.

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#odersky-programming-in-scala-2ed #scala

Question

How to write Unit as a literal?

Answer

()

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#odersky-programming-in-scala-2ed #scala

Question

Why doesn't it work?

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

Answer

the result of assignment expression is always Unit, so the above check Unit != "" is always true

Tags

#odersky-programming-in-scala-2ed #scala

Question

How to change it to exclude upper bound 4?

for (i <- 1 to 4)
  println("Iteration "+ i)

Answer

Use until method instead of to method:

for (i <- 1 until 4)
  println("Iteration "+ i)

Tags

#odersky-programming-in-scala-2ed #scala

Question

How to move the condition to the generator in the loop?

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

Answer

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

Tags

#odersky-programming-in-scala-2ed #scala

Question

How to introduce a val like variable mid-stream into for clauses?

Answer

Simply x=<whatever>, like val x=<whatever> elsewhere in the code, but without val keyword.

Tags

#odersky-programming-in-scala-2ed #scala

Question

What will calling g() return?

def g(): Int = try { 1 } finally { 2 }

Answer

1

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#odersky-programming-in-scala-2ed #scala

Question

How to fix this compilation error?
scala> val f = _ + _
<console>:4: error: missing parameter type for expanded
function ((x$1, x$2) => x$1.$plus(x$2))
   val f = _ + _

Answer

Add types, e.g.:

val f = (_: Int) + (_: Int)

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#odersky-programming-in-scala-2ed #scala

Question

val b = sum(1, _: Int, 3)
What is b? How is it created in the example above?

Answer

a function (or a "function value") of one argument, integer (function value to be specific). it is created by partial application of sum function

Tags

#odersky-programming-in-scala-2ed #scala

Question

How can you simplify this class?

class ArrayElement(conts: Array[String]) extends Element {
  val contents: Array[String] = conts
}

Answer

Create a parametric field:

class ArrayElement(
  val contents: Array[String]
) extends Element

Tags

#odersky-programming-in-scala-2ed #scala

Question

What is this (Array(s)) after superclass name?

class LineElement(s: String) extends ArrayElement(Array(s)) {
  // ...
}

Answer

call to superclass constructor passing this class constructor argument s