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Flashcard 149630773

Tags

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

Question

How can you make the function literal more concise?
someNumbers.filter(x => x > 0)

Answer

someNumbers.filter(_ > 0)

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Flashcard 149630784

Tags

#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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Annotation 149630804

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

Multiple underscores mean multiple parameters, not reuse of a single parameter repeatedly. The first underscore represents the first parameter, the second underscore the second parameter, the third underscore the third parameter, and so on.

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Annotation 149630820

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

you can also replace an entire parameter list with an underscore

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Flashcard 149630836

Tags

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

Question

Although you can’t assign a method or nested function to a variable, or pass it as an argument to another function, you can do these things if you wrap the method or nested function in a function value by [...].

Answer

placing an underscore after its name

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Flashcard 149630852

Tags

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

Question

Now, although sum _ is indeed a partially applied function, it may not be obvious to you why it is called this. It has this name because you are not applying that function to all of its arguments. In the case of sum _, you are applying it to [...].

Answer

none of its arguments

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Flashcard 149630859

Tags

#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

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Article 149630870

#odersky-programming-in-scala-2ed #p194 #scala

If you are writing a partially applied function expression in which you leave off all parameters, such as println _ or sum _, you can express it more concisely by leaving off the underscore if a function is required at that point in the code, e.g. instead of someNumbers.foreach(println _) you could just write someNumbers.foreach(println)

Flashcard 149630879

Tags

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

Question

If you are writing a partially applied function expression in which you leave off all parameters, such as println _ or sum _, you can express it more concisely by [...] if a function is required at that point in the code, e.g. instead of
someNumbers.foreach(println _)
you could just write
[...]

Answer

leaving off the underscore

you could just write
someNumbers.foreach(println)

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If you are writing a partially applied function expression in which you leave off all parameters, such as println _ or sum _, you can express it more concisely by leaving off the underscore if a function is required at that point in the code, e.g. instead of someNumbers.foreach(println _) you could just write someNumbers.foreach(println) <

Flashcard 149630890

Tags

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

Question

If you are writing a partially applied function expression in which you leave off all parameters, such as println _ or sum _, you can express it more concisely by leaving off the underscore if a [...], e.g. instead of
someNumbers.foreach(println _)
you could just write
someNumbers.foreach(println)

Answer

function is required at that point in the code (remember that when anything, not specifically a function is required, like in assignment val b = sum _, leaving off _ won't work).

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Annotation 149630910

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

the type of args inside the function, which is declared as type “String*” is actually Array[String]

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Flashcard 149630917

Tags

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

Question

How to call this function?

def echo(args: String*) =
  for (arg <- args) println(arg)

Answer

with any number of comma separated strings, e.g. echo("hello", "world!")

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Flashcard 149630928

Tags

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

Question

How to apply array of strings to echo?

val arr = Array("What's", "up", "doc?")

def echo(args: String*) =
  for (arg <- args) println(arg)

Answer

add colon and _* symbol to the argument:

echo(arr: _*)

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Flashcard 149630943

Tags

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

Question

How to call this function with named parameters instead of speed(100, 10)?

def speed(distance: Float, time: Float): Float =
  distance / time

Answer

Use equal sign: speed(distance = 100, time = 10)

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Annotation 149630963

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

It is also to mix positional and named arguments. In that case, the positional arguments come first.

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Flashcard 149630970

Tags

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

Question

How to add Console.out as a default value for out parameter?

def printTime(out: java.io.PrintStream) =
  out.println("time = "+ System.currentTimeMillis())

Answer

def printTime(out: java.io.PrintStream = Console.out) =
  out.println("time = "+ System.currentTimeMillis())

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Annotation 149630990

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

tail-call optimization is limited to situations in which a method or nested function calls itself directly as its last operation, without going through a function value or some other intermediary.

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Flashcard 149630997

Tags

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

Question

What is function value?

Answer

A result of partial application of a function without supplying any arguments, e.g.
val funValue = nestedFun _

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Flashcard 149631008

Tags

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

Question

Explain the second argument of the filesMatching function

def filesMatching(query: String,
    matcher: (String, String) => Boolean) = {
  for (file <- filesHere; if matcher(file.getName, query))
    yield file
}

Answer

It is a function (specifically function value) that takes 2 Strings as arguments and return Boolean

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Flashcard 149631019

Tags

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

Question

How can I simplify the syntax of a call to filesMatching inside filesEnding?


def filesEnding(query: String) =
  filesMatching(query, (fileName: String, query: String) => fileName.endsWith(query))

def filesMatching(query: String,
    matcher: (String, String) => Boolean) = {
  for (file <- filesHere; if matcher(file.getName, query))
    yield file
}

Answer

def filesEnding(query: String) =
  filesMatching(query, _.endsWith(_))

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Flashcard 149631030

Tags

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

Question

How can you simplify this code using closure to avoid passing query as an argument to filesMatching, where filesEnding is the public API client function?


def filesEnding(query: String) =
  filesMatching(query, _.endsWith(_))

def filesMatching(query: String,
    matcher: (String, String) => Boolean) = {
  for (file <- filesHere; if matcher(file.getName, query))
    yield file
}

Answer


def filesEnding(query: String) =
  filesMatching(_.endsWith(query))

def filesMatching(matcher: String => Boolean) =
  for (file <- filesHere; if matcher(file.getName))
    yield file

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Flashcard 149631041

Tags

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

Question

How to rewrite this using curried function (using compact notation)?

scala> def plainOldSum(x: Int, y: Int) = x + y
plainOldSum: (x: Int,y: Int)Int
scala> plainOldSum(1, 2)
res4: Int = 3

Answer

scala> def curriedSum(x: Int)(y: Int) = x + y
curriedSum: (x: Int)(y: Int)Int
scala> curriedSum(1)(2)
res5: Int = 3

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Flashcard 149631069

Tags

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

Question

In any method invocation in Scala in which [...], you can opt to use curly braces to surround the argument instead of parentheses.

Answer

you’re passing in exactly one argument

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Flashcard 149631076

Tags

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

Question

What do you think the header of withPrintWriter looks like?

val file = new File("date.txt")
withPrintWriter(file) {
  writer => writer.println(new java.util.Date)
}

Answer

def withPrintWriter(file: File)(op: PrintWriter => Unit) {
  // ...
}

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Flashcard 149631091

Tags

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

Question

In this code you pass in a zero-arg function to delay evaluation; how would you rewrite it to use by-name parameters?


def myAssert(predicate: () => Boolean) =
  if (!predicate())
    throw new AssertionError
    
myAssert(() => 5 > 3)

Answer

change type () => Boolean to => Boolean:
def myAssert(predicate: => Boolean) =
  if (!predicate())
    throw new AssertionError

myAssert(5 > 3)

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Flashcard 149631106

Tags

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

Question

Why is it an abstract method, not a variable (field) declaration?

abstract class Element {
def contents: Array[String]
}

Answer

because it is def not var or val

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Flashcard 149631130

Tags

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

Question

If you leave out an extends clause, the Scala compiler implicitly assumes your class extends from [...], which on the Java platform is the same as class java.lang.Object.

Answer

scala.AnyRef

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Annotation 149631146

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

In Scala, fields and methods belong to the same namespace. This makes it possible for a field to override a parameterless method.

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Annotation 149631162

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

In Scala it is forbidden to define a field and method with the same name in the same class, whereas it is allowed in Java.

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Article 149631169

#odersky-programming-in-scala-2ed #p230 #scala

Scala has just two namespaces for definitions in place of Java’s four. Java’s four namespaces are fields, methods, types, and packages. By contrast, Scala’s two namespaces are: values (fields, methods, packages, and singleton objects)types (class and trait names)

Flashcard 149631178

Tags

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

Question

Scala has just two namespaces for definitions in place of Java’s four. Java’s four namespaces are [...], methods, types, and packages. By contrast, Scala’s two namespaces are:

values (fields, methods, packages, and singleton objects)
types (class and trait names)

Answer

fields

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Scala has just two namespaces for definitions in place of Java’s four. Java’s four namespaces are fields, methods, types, and packages. By contrast, Scala’s two namespaces are: values (fields, methods, packages, and singleton objects)types (class and trait names) </

Flashcard 149631185

Tags

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

Question

Scala has just two namespaces for definitions in place of Java’s four. Java’s four namespaces are fields, [...], types, and packages. By contrast, Scala’s two namespaces are:

values (fields, methods, packages, and singleton objects)
types (class and trait names)

Answer

methods

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Flashcard 149631192

Tags

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

Question

Scala has just two namespaces for definitions in place of Java’s four. Java’s four namespaces are fields, methods, [...], and packages. By contrast, Scala’s two namespaces are:

values (fields, methods, packages, and singleton objects)
types (class and trait names)

Answer

types

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Flashcard 149631199

Tags

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

Question

Scala has just two namespaces for definitions in place of Java’s four. Java’s four namespaces are fields, methods, types, and [...]. By contrast, Scala’s two namespaces are:

values (fields, methods, packages, and singleton objects)
types (class and trait names)

Answer

packages

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Flashcard 149631215

Tags

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

Question

Scala has just two namespaces for definitions in place of Java’s four. Java’s four namespaces are fields, methods, types, and packages. By contrast, Scala’s two namespaces are:

[...]
types (class and trait names)

Answer

values (fields, methods, packages, and singleton objects)

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Flashcard 149631222

Tags

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

Question

Scala has just two namespaces for definitions in place of Java’s four. Java’s four namespaces are fields, methods, types, and packages. By contrast, Scala’s two namespaces are:

values (fields, methods, packages, and singleton objects)
[...]

Answer

types (class and trait names)

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pan>Scala has just two namespaces for definitions in place of Java’s four. Java’s four namespaces are fields, methods, types, and packages. By contrast, Scala’s two namespaces are: values (fields, methods, packages, and singleton objects)<span>types (class and trait names) <span><body><html>

Flashcard 149631233

Tags

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

Question

If a class has a parameter whose sole purpose is to be copied into a field, you can use a [...] instead

Answer

parametric field

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Flashcard 149631244

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

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Annotation 149631268

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

it is possible to add modifiers such as private, protected, or override to parametric field

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Flashcard 149631352

Tags

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

Question

To invoke a superclass constructor, you simply place

Answer

the argument or arguments you want to pass in parentheses following the name of the superclass.

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Flashcard 149631359

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

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Flashcard 149631372

Tags

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

Question

Scala requires override modifier for [...].

Answer

all members that override a concrete member in a parent class

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Flashcard 149631383

Tags

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

Question

The override modifier is optional if [...].

Answer

a member implements an abstract member with the same name

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Edited, memorised or added to reading queue

on 24-Aug-2014 (Sun)

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