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Cheat Sheet

This cheat sheet has been adapted from the awesome cheatsheet from the Coursera Scala Course. Thank you to @mbren for the suggestion.

The scope is intentionally small, but if there are other core library methods we are missing examples / explanations of, please contribute!

TODO:

  • add function syntax guide
  • type parameters
  • pattern matching
  • more examples!
  • audit list examples
  • what else?

Evaluation Rules

  • Call by value: evaluates the function arguments before calling the function
  • Call by name: evaluates the function first, and then evaluates the arguments if need be
    def example = 2      // evaluated when called
    val example = 2      // evaluated immediately
    lazy val example = 2 // evaluated once when needed
    
    def square(x: Double)    // call by value
    def square(x: => Double) // call by name

Operators

myObject myMethod 1 is the same as calling myObject.myMethod(1)

Operator (i.e. function) names can be alphanumeric, symbolic (e.g. x1, *, +?%&, vector_++, counter_=)

Hello World

To create a runnable application in Scala:

    object Hello {  
      def main(args: Array[String]) = println("Hello world")  
    }

or

    object Hello extends App {
      println("Hello World")
    }

Code Organization

  • Code is organized in packages (package myPackage).

  • They can be referenced through import statements (import myPackage.MyClass, import myPackage._, import myPackage.{MyClass1, MyClass2}, import myPackage.{MyClass1 => A})

  • All members of packages scala and java.lang as well as all members of the object scala.Predef are automatically imported.

Type Parameters

TODO

Pattern Matching

TODO

Options

Some functions (like Map.get) return a value of type Option[T] which is either a value of type Some[T] or the value None where T can be any type:

    val myMap = Map("a" -> 42, "b" -> 43)
    def getMapValue(s: String): String = {
      myMap get s match {
        case Some(nb) => "Value found: " + nb
        case None => "No value found"
      }
    }
    getMapValue("a")  // "Value found: 42"
    getMapValue("c")  // "No value found"

Collections

Scala defines several collection classes:

Base Classes

  • Iterable (collections you can iterate on)
  • Seq (ordered sequences)
  • List (linked list, provides fast sequential access)
  • Vector (array-like type, implemented as tree of blocks, provides fast random access)
  • Range (ordered sequence of integers with equal spacing)
  • String (Java type, implicitly converted to a character sequence, so you can treat every string like a Seq[Char])
  • Map (collection that maps keys to values)
  • Set (collection without duplicate elements)

Examples

TODO evaluate these examples for clarity

    val fruitList = List("apples", "oranges", "pears")
    // Alternative syntax for lists
    val fruit = "apples" :: ("oranges" :: ("pears" :: Nil)) // parens optional, :: is right-associative
    fruit.head   // "apples"
    fruit.tail   // List("oranges", "pears")
    val empty = List()
    val empty = Nil

    val nums = Vector("louis", "frank", "hiromi")
    nums(1)                     // element at index 1, returns "frank", complexity O(log(n))
    nums.updated(2, "helena")   // new vector with a different string at index 2, complexity O(log(n))
    
    val fruitSet = Set("apple", "banana", "pear", "banana")
    fruitSet.size    // returns 3: there are no duplicates, only one banana

    val r: Range = 1 until 5 // 1, 2, 3, 4
    val s: Range = 1 to 5    // 1, 2, 3, 4, 5
    1 to 10 by 3  // 1, 4, 7, 10
    6 to 1 by -2  // 6, 4, 2

    val s = (1 to 6).toSet
    s map (_ + 2) // adds 2 to each element of the set

    val s = "Hello World"
    s filter (c => c.isUpper) // returns "HW"; strings can be treated as Seq[Char]

    // Operations on sequences
    val xs = List(...)
    xs.length   // number of elements, complexity O(n)
    xs.last     // last element (exception if xs is empty), complexity O(n)
    xs.init     // all elements of xs but the last (exception if xs is empty), complexity O(n)
    xs take n   // first n elements of xs
    xs drop n   // the rest of the collection after taking n elements
    xs(n)       // the nth element of xs, complexity O(n)
    xs ++ ys    // concatenation, complexity O(n)
    xs.reverse  // reverse the order, complexity O(n)
    xs updated(n, x)  // same list than xs, except at index n where it contains x, complexity O(n)
    xs indexOf x      // the index of the first element equal to x (-1 otherwise)
    xs contains x     // same as xs indexOf x >= 0
    xs filter p       // returns a list of the elements that satisfy the predicate p
    xs filterNot p    // filter with negated p 
    xs partition p    // same as (xs filter p, xs filterNot p)
    xs takeWhile p    // the longest prefix consisting of elements that satisfy p
    xs dropWhile p    // the remainder of the list after any leading element satisfying p have been removed
    xs span p         // same as (xs takeWhile p, xs dropWhile p)
    
    List(x1, ..., xn) reduceLeft op    // (...(x1 op x2) op x3) op ...) op xn
    List(x1, ..., xn).foldLeft(z)(op)  // (...( z op x1) op x2) op ...) op xn
    List(x1, ..., xn) reduceRight op   // x1 op (... (x{n-1} op xn) ...)
    List(x1, ..., xn).foldRight(z)(op) // x1 op (... (    xn op  z) ...)
    
    xs exists p    // true if there is at least one element for which predicate p is true
    xs forall p    // true if p(x) is true for all elements
    xs zip ys      // returns a list of pairs which groups elements with same index together
    xs unzip       // opposite of zip: returns a pair of two lists
    xs.flatMap f   // applies the function to all elements and concatenates the result
    xs.sum         // sum of elements of the numeric collection
    xs.product     // product of elements of the numeric collection
    xs.max         // maximum of collection
    xs.min         // minimum of collection
    xs.flatten     // flattens a collection of collection into a single-level collection
    xs groupBy f   // returns a map which points to a list of elements
    xs distinct    // sequence of distinct entries (removes duplicates)

    x +: xs  // creates a new collection with leading element x
    xs :+ x  // creates a new collection with trailing element x

    // Operations on maps
    val myMap = Map("I" -> 1, "V" -> 5, "X" -> 10)  // create a map
    myMap("I")      // => 1  
    myMap("A")      // => java.util.NoSuchElementException  
    myMap get "A"   // => None 
    myMap get "I"   // => Some(1)
    myMap.updated("V", 15)  // returns a new map where "V" maps to 15 (entry is updated)
                            // if the key ("V" here) does not exist, a new entry is added

    // Operations on Streams
    val xs = Stream(1, 2, 3)
    val xs = Stream.cons(1, Stream.cons(2, Stream.cons(3, Stream.empty))) // same as above
    (1 to 1000).toStream // => Stream(1, ?)
    x #:: xs // Same as Stream.cons(x, xs)
             // In the Stream's cons operator, the second parameter (the tail)
             // is defined as a "call by name" parameter.
             // Note that x::xs always produces a List

Pairs (similar for larger Tuples)

    val pair = ("answer", 42)   // type: (String, Int)
    val (label, value) = pair   // label = "answer", value = 42  
    pair._1 // "answer"  
    pair._2 // 42