1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
|
import scala.reflect.runtime.universe._
import scala.tools.reflect.Eval
object Test extends App {
reify {
object Persons {
/** A list of persons. To create a list, we use Predef.List
* which takes a variable number of arguments and constructs
* a list out of them.
*/
val persons = List(
new Person("Bob", 17),
new Person("John", 40),
new Person("Richard", 68)
)
/** A Person class. 'val' constructor parameters become
* public members of the class.
*/
class Person(val name: String, val age: Int)
/** Return an iterator over persons that are older than 20.
*/
def olderThan20(xs: Seq[Person]): Iterator[String] =
olderThan20(xs.iterator)
/** Return an iterator over persons older than 20, given
* an iterator over persons.
*/
def olderThan20(xs: Iterator[Person]): Iterator[String] = {
// The first expression is called a 'generator' and makes
// 'p' take values from 'xs'. The second expression is
// called a 'filter' and it is a boolean expression which
// selects only persons older than 20. There can be more than
// one generator and filter. The 'yield' expression is evaluated
// for each 'p' which satisfies the filters and used to assemble
// the resulting iterator
for (p <- xs if p.age > 20) yield p.name
}
}
/** Some functions over lists of numbers which demonstrate
* the use of for comprehensions.
*/
object Numeric {
/** Return the divisors of n. */
def divisors(n: Int): List[Int] =
for (i <- List.range(1, n+1) if n % i == 0) yield i
/** Is 'n' a prime number? */
def isPrime(n: Int) = divisors(n).length == 2
/** Return pairs of numbers whose sum is prime. */
def findNums(n: Int): Iterable[(Int, Int)] = {
// a for comprehension using two generators
for (i <- 1 until n;
j <- 1 until (i-1);
if isPrime(i + j)) yield (i, j)
}
/** Return the sum of the elements of 'xs'. */
def sum(xs: List[Double]): Double =
xs.foldLeft(0.0) { (x, y) => x + y }
/** Return the sum of pairwise product of the two lists. */
def scalProd(xs: List[Double], ys: List[Double]) =
sum(for((x, y) <- xs zip ys) yield x * y);
/** Remove duplicate elements in 'xs'. */
def removeDuplicates[A](xs: List[A]): List[A] =
if (xs.isEmpty)
xs
else
xs.head :: removeDuplicates(for (x <- xs.tail if x != xs.head) yield x)
}
// import all members of object 'persons' in the current scope
import Persons._
print("Persons over 20:")
olderThan20(persons) foreach { x => print(" " + x) }
println
import Numeric._
println("divisors(34) = " + divisors(34))
print("findNums(15) =")
findNums(15) foreach { x => print(" " + x) }
println
val xs = List(3.5, 5.0, 4.5)
println("average(" + xs + ") = " + sum(xs) / xs.length)
val ys = List(2.0, 1.0, 3.0)
println("scalProd(" + xs + ", " + ys +") = " + scalProd(xs, ys))
}.eval
}
|
