Power with control: Scala control structures and abstractions

Note: this coverage doesn’t claim, or even attempt, to be authoritative, but tries to cover some of the iteration styles and constructs I have found most useful.

So without further ado, let the click-clacking of chains purr along as we start our ascent..

if()

e.g. if() as a ternary expression

val result = if (1 > 2) true else false

e.g. if as a ternary with just one return value declared

var result2 = if(1>2) 1 // assigns result2 = AnyVal

while and do.. while

e.g. while() and do.. while()

var i = 0
while(i < 3) println("Hello " + {i = i + 1; i} )

or

do { println("Hello " + { i = i + 1; i } ) } while(i < 3)

for(i to/until)

e.g. Generators and filters in a for() comprehension

for(i <- 1 to 4; j <- 1 until 4 if (i * j % 2 == 0)) { println(i * j) }

• clauses contained in for expressions are actually closures and, as such, they fail to ‘break’ in the expected imperative sense.
• eager evaluation is the default implementation of most for() expressions.
• ‘real-time’ (i.e. lazy) evaluation of break clauses is possible using the .view of the collection. (Note: view replaces the [now] deprecated .projection method)
• however… to actually exit the closure early, the break condition should be evaluated against the projected surrogate object.

e.g. An example for() comprehension using the functional form:

var multiplesOfTwo = for(i <- 1 to 10 if i % 2 == 0) yield i

e.g. A lazy evaluated for() comprehension with a break gate

var gate = true // note this has to be a 'var' type, as we hope to reassign the variable during iteration
for(i <- testList.view.takeWhile(i => gate)) if(i < 5) println(i) else gate = false

e.g. using a while() to escape non-Collection iteration

var index = 1
var escapeGate = false
while(index < 10000000 && !escapeGate) {
    if(index > 5) escapeGate = true
    println(index)
    i += 1
}

Useful operations on Lists

e.g. Creating a List

val testList = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

or

val testList = 1 :: 2 :: 3 :: 4 :: 5 :: 6 :: 7 :: 8 :: 9 :: 10 :: Nil

e.g. Inheritence amongst List elements

class Car; class Vovlo extends Car; class VW extends Car;
var lcdList = List(new Volvo()) ::: List(new VW()) // returns a List of Type 'Car'

foreach()

Use: Applies a function to every element of a list. This looping construct is used for the side effects that are created
Example:

testList foreach(x => println(x*2))

e.g. foreach with inferred parameters

testList foreach(println)

and

testList foreach(println _)

filter()  

Use: Selects all elements from a List that satisfy a given predicate.
Example:

val evensList = testList filter (_ % 2 == 0)

Note: Order of elements in the returned List is retained from the Source List.

partition()

Use: Splits a list into 2 lists based upon a predicate submitted, and returns a Tuple2 containing the returned Lists.
Example:

val tupleWrappedOddAndEvenLists = testList partition (_ % 2 == 0)
// the individual elements of the Tuple can be accessed in the usual way, e.g.:
println(tupleWrappedOddAndEvenLists._1)

Note: Tuple support and usage in Scala extends beyond acting as a simple wrapper to input/return multiple variables, as they can also be used in extractors, and hence reused in pattern matches. Both topics are beyond the scope of this article.

forall()

Use: Returns a boolean indicating whether all elements of a List pass a given predicate
Example:

testList forall(a => a.isInstanceOf[Int])

exists()

Use: Tests whether any element of a given List is true according to a given predicate
Example:

testList exists(a => a == 4)

map()  

Use: Applies a function to all elements of a List and returns a new List containing the results.
Example:

val doubledList = testList map (_ * 2)

flatMap()

Use: Similar to the map operation, but will concatenate the results into a single List. For example, given a List of Lists and a function as input parameters, a single (transformed) List is returned.
Example:

val palindromeList = List(testList, testList reverse) flatMap (_.toList)

fold[Left/Right]()

Example:

(1 foldLeft testList) (_-_) // This takes the seed value for the consolidated return as 1, before applying the muliplication operation on all elements of the palindromeList (effectively adding the results into a running total)

// to see the difference in foldLeft and foldRight for noncommutative operations, let's look at the minus operations applied with a foldLeft and a foldRight (using the shorthand syntax), and observe how the results differ:

(1 /: testList) (_-_) // left fold on the minus operation returns -54
(testList :\ 1) (_-_) // right fold on the minus operation returns -4

Related: Catamorphism

reduce[Left/Right]()

Use: Recursively applies a given function to elements from of a List. The resulting value from the first iteration is used as the primary input for the subsequent iteration, and this is pattern of substitution is then applied recursively.
Example:

testList reduceLeft ((a, b) => if (a > b) a else b)

Reference: Power with control… control structures and abstractions in Scala from our JCG partner Kingsley Davies at the Scalabound blog.