Using collections in functional style
Let us now review some of the most common “utility methods” in the collection classes.
You can also revisit Chapter 6.2 of the CS-A1110 Programming 1 course.
For the programming assignments, the following methods may also prove useful:
splitAtkeysfiltertoSettoMaptoVectorindexWheretakefilterKeysminandmax
To study these and the collection classes further, please see
Let us proceed to review some of the utility methods with examples.
forall
Checks if all the elements in a collection satisfy a predicate (recall that a predicate is a function whose range is Boolean).
For instance, we can test whether none of the elements in a set is divisible by 4:
scala> val s = Set(2,5,7,11,12)
val s: Set[Int] = HashSet(5, 2, 12, 7, 11)
scala> s.forall(_ % 4 != 0)
res: Boolean = false
Note that the order on which the predicate is applied to the elements is not necessarily “the intuitive one” and that the predicate is not necessarily applied to all the elements:
scala> val s = Set(12,2,5,7,11)
val s: Set[Int] = HashSet(5, 2, 12, 7, 11)
scala> s.forall(e => {println(e); e % 4 != 0})
5
2
12
res: Boolean = false
This matters if the predicate has side effects: the result can be very difficult to understand in case something unexpected happens!
foreach
This method applies the argument function to all the elements in a collection. The return values of the function evaluations are ignored. As an example
scala> val points = Map("40355T"->30, "346823"->70, "826822"->65)
points: Map[String,Int] = Map(40355T -> 30, 346823 -> 70, 826822 -> 65)
scala> points.foreach((id,score) => println(s"Student ${id} " + (if score > 50 then "passed" else "did not pass")))
Student '40355T' did not pass
Student '346823' passed
Student '826822' passed
Observe that the parameter x of the anonymous function is of type
Tuple2[String,Int],
that is, a (key,value) -pair.
If you want to consider only keys or values of a Map,
take a look at the keys and values methods.
For example:
scala> val avgPoints = points.values.sum / points.size
avgPoints: Int = 55
map
This is one of the most commonly used utility methods that transforms a collection into another by applying the function given as parameter to each element in the collection. There is no requirement for the domain and range types of the function to be the same; for example,
scala> val names = Seq("Turing", "Godel", "Shannon", "Shamir")
names: Seq[String] = List(Turing, Godel, Shannon, Shamir)
scala> val lengths = names.map(name => name.length)
lengths: Seq[Int] = List(6, 5, 7, 6)
flatMap
Like map but the applied function must return a traversable collection,
such as List or Vector, and
the resulting collection will then contain the concatenation
of the function applications.
For example:
scala> val l = List(1,2,3)
l: List[Int] = List(1, 2, 3)
scala> l.map(v => List(v*2, v*2 + 1))
res: List[List[Int]] = List(List(2, 3), List(4, 5), List(6, 7))
scala> l.flatMap(v => List(v*2, v*2 + 1))
res: List[Int] = List(2, 3, 4, 5, 6, 7)
groupBy
The groupBy(f) method applies the function f to each element
in the collection and returns an association array that maps each
value of the function encountered to the list of elements that evaluated to that value.
This is a very convenient method when one wants to classify elements in a collection according to some criteria.
For instance,
we can make an association array that groups all the names in a list that start with the same letter together as follows:
scala> val names = List("abba", "Turing", "Alabama", "Celsius")
names: List[String] = List(abba, Turing, Alabama, Celsius)
scala> names.groupBy(name => name(0).toLower)
res0: scala.collection.immutable.Map[Char,List[String]] = Map(t -> List(Turing), a -> List(abba, Alabama), c -> List(Celsius))
As a second example,
we can use groupBy to compute an association map that tells how many names of certain length occurs in a list:
scala> val names = Seq("Turing", "Godel", "Shannon", "Shamir")
names: Seq[String] = List(Turing, Godel, Shannon, Shamir)
scala> val namesOfLength = names.groupBy(name => name.length)
namesOfLength: scala.collection.immutable.Map[Int,Seq[String]] = Map(5 -> List(Godel), 7 -> List(Shannon), 6 -> List(Turing, Shamir))
scala> val distributionMap = namesOfLength.map((k,v) => (k,v.length))
distributionMap: scala.collection.immutable.Map[Int,Int] = Map(5 -> 1, 7 -> 1, 6 -> 2)
Also observe the last statement: the Map trait includes the mapValues
method that works like map but is only applied to
the values of the association array.
foldLeft
An invocation of foldLeft(start)(f),
where f is a function with two arguments,
computes the value f(...f(f(start, v1), v2)...,vn)
over the elements v1,v2,...,vn in the collection.
For instance, we could write the mathematical vector length method
with foldLeft:
def len(vec: Vector[Double]) = math.sqrt(vec.foldLeft(0.0)((s, v) => s+v*v))
This computes the same result as
def len(vec: Vector[Double]) = math.sqrt(vec.map(v => v*v).sum)
zip
Combines, or “zips”, two collections into a collection of pairs in an element-by-element manner. For example,
scala> val l = List("a", "b", "c", "d")
l: List[String] = List(a, b, c, d)
scala> val n = Vector(1, 2, 3, 4, 5)
n: scala.collection.immutable.Vector[Int] = Vector(1, 2, 3, 4, 5)
scala> l zip n
res: List[(String, Int)] = List((a,1), (b,2), (c,3), (d,4))
As we can see, if either collection has more elements than the other,
then its “extra” elements are ignored.
And l zip n is just another way of writing l.zip(n).
By the way, a highly convenient method for building maps
from other collections of pairs is to use the toMap method:
scala> (l zip n).toMap
res: scala.collection.immutable.Map[String,Int] = Map(a -> 1, b -> 2, c -> 3, d -> 4)
indices and zipWithIndex
The method indices returns the indices of a sequence while
zipWithIndex returns a new sequence of pairs of form (e, i),
where e is an element in the original sequence and i is its
index there:
scala> val s = "Turing"
s: String = Turing
scala> s.indices
res: scala.collection.immutable.Range = Range(0, 1, 2, 3, 4, 5)
scala> s.zipWithIndex
res: scala.collection.immutable.IndexedSeq[(Char, Int)] = Vector((T,0), (u,1), (r,2), (i,3), (n,4), (g,5))
For example, there are now many alternatives to print out the characters in a string one-by-one:
val s = "Turing"
// Imperative style with vars and while loops
var i = 0
while i < s.length do
println("s("+i+") = "+s(i))
i += 1
end while
// With for-loops: this actually is internally implemented roughly as the next one
for i <- s.indices do println("s("+i+") = "+s(i))
// Functional style alternatives, pick your favorite
s.indices.foreach(i => println("s("+i+") = "+s(i)))
s.zipWithIndex.foreach(p => println("s("+p._2+") = "+p._1))
s.zipWithIndex.foreach((char, index) => println("s("+index+") = "+char))