标签：over contains arch 4.0 es6 cep ica default exp
6.1 Other Collections
Operations on Vectors:
Vectors are created analogously to lists:
val nums = Vector(1, 2, 3, -88)
val peoplr = Vector("Bob", "James", "Peter")
They support the same operations as lists, with the exception of ::
Instrad of x :: xs, there is
x +: xs Create a new vector with leading element x, followed by all elements of xs
xs :+ x Create a new vector with trailing element x, preceded by all elements of xs
object test {
val xs = Array(1, 2, 3, 44) //> xs : Array[Int] = Array(1, 2, 3, 44)
xs map (x => x*2) //> res0: Array[Int] = Array(2, 4, 6, 88)
val s = "Hello World" //> s : String = Hello World
s filter (c => c.isUpper) //> res1: String = HW
s exists (c => c.isUpper) //> res2: Boolean = true
s forall (c => c.isUpper) //> res3: Boolean = false
val pairs = List(1, 2, 3) zip s //> pairs : List[(Int, Char)] = List((1,H), (2,e), (3,l))
pairs.unzip //> res4: (List[Int], List[Char]) = (List(1, 2, 3),List(H, e, l))
s flatMap (c => List(‘.‘, c)) //> res5: String = .H.e.l.l.o. .W.o.r.l.d
xs.sum //> res6: Int = 50
xs.max //> res7: Int = 44
xs.min //> res8: Int = 1
}
6.2 Combinatorial Search and For-Expressions
For-Expression Example:
Let persons be a list of elements of class Person, with fields name and age.
case class Person(name: String, age: Int)
To obtain the names of persons over 20 years old, you can write:
for (p <- persons if p.age > 20) yield p.name
which is equivalent to:
persons filter (p => p.age > 20) map (p => p.name)
Syntax of For
A for-expression is of the form
for ( s ) yield e
where s is a sequence of generators and filters, and e is an expression whose value is returned by an iteration.
Instead of ( s ), braces { s } can also be used, and then the sequence of generators and filters can be written on multiple lines without require semicolons.
6.3 Combinatorial Search Example
Sets vs Sequences
The principal differences between sets and sequences are:
6.4 Maps
++:
object polynominals {
class Poly(terms0: Map[Int, Double]) {
def this(bindings: (Int, Double)*) = this(bindings.toMap)
val terms = terms0 withDefaultValue 0.0
def + (other: Poly) = new Poly(terms ++ (other.terms map adjust))
def adjust(term: (Int,Double)) : (Int, Double) = {
val (exp, coeff) = term
exp -> (coeff + terms(exp))
}
override def toString =
(for ((exp , coeff) <- terms.toList.sorted.reverse) yield coeff + "x^" + exp) mkString " + "
}
val p1 = new Poly(1 -> 2.0, 3 -> 4.0, 5 -> 6.2)
//> p1 : week6.polynominals.Poly = 6.2x^5 + 4.0x^3 + 2.0x^1
val p2 = new Poly(0 -> 3.0, 3 -> 7.0) //> p2 : week6.polynominals.Poly = 7.0x^3 + 3.0x^0
p1 + p2 //> res0: week6.polynominals.Poly = 6.2x^5 + 11.0x^3 + 2.0x^1 + 3.0x^0
p1.terms(7) //> res1: Double = 0.0
}
foldLeft(more efficiency):
object polynominals {
class Poly(terms0: Map[Int, Double]) {
def this(bindings: (Int, Double)*) = this(bindings.toMap)
val terms = terms0 withDefaultValue 0.0
def + (other: Poly) = new Poly((other.terms foldLeft terms)(addTerm))
def addTerm(terms: Map[Int, Double], term: (Int, Double)): Map[Int, Double] = {
val (exp, coeff) = term
terms + (exp -> (coeff+terms(exp)))
}
override def toString =
(for ((exp , coeff) <- terms.toList.sorted.reverse) yield coeff + "x^" + exp) mkString " + "
}
val p1 = new Poly(1 -> 2.0, 3 -> 4.0, 5 -> 6.2)
//> p1 : week6.polynominals.Poly = 6.2x^5 + 4.0x^3 + 2.0x^1
val p2 = new Poly(0 -> 3.0, 3 -> 7.0) //> p2 : week6.polynominals.Poly = 7.0x^3 + 3.0x^0
p1 + p2 //> res0: week6.polynominals.Poly = 6.2x^5 + 11.0x^3 + 2.0x^1 + 3.0x^0
p1.terms(7) //> res1: Double = 0.0
}
Scala 函数式程序设计原理(6)--Collections
标签：over contains arch 4.0 es6 cep ica default exp
原文地址：http://www.cnblogs.com/PaulingZhou/p/6905355.html
