Daily scala

A quick tip for generating some random testdata.

Note: This is a poor man's solution to using ScalaCheck. If you can handle the dependency I would really recommend using that library.

1. scala> object Options extends Enumeration {                       
2.      | val ONE, TWO, THREE, FOUR = Value                          
3.      | }
4. defined module Options
5. /*
6. Randomly select zero or more elements from the options enumeration
7. */
8. scala> Options.values.filter {_ => util.Random.nextBoolean} mkString ", "
9. res2: String = TWO, FOUR
10. /*
11. Select a random string.  
13. Warning:  there is no restriction on the characters so control characters are likely
14. */
15. scala> util.Random.nextString(10)
16. res5: String = ??????????
17. /*
18. ASCII string is oftern more useful for test data.  This selects a random string up to 13 characters long
19. */
20. scala> util.Random.nextASCIIString(13)
21. res6: java.lang.String = RVPD\#_HqJ8:o
22. /*
23. This creates a sequence of 10 random strings
24. */
25. scala> 1 to 10 map {_ => util.Random.nextASCIIString(13)}
26. res7: scala.collection.immutable.IndexedSeq[java.lang.String] = IndexedSeq(;E8|Q8H8RI;Q=, vM-X;"ksBr\:c, SKyz{uXNQ5E]X, =Jd8_ll08)s%e, gRCs)6wj%C-YF, `x;2Zru?l*c%@, XE*/Rx9:qPfpm, s|u,e.un+-Xm(, M,TpX9Dq-6$+^, w;exER&#0|}Ya)

The for-comprehension construct is a very powerful way of iterating over collections. In its most basic form it is the java for( var: collection){} loop. As with all flow constructs in Scala, the scala for loop (or more correctly for-comprehension) can return a value. In the case of the for-comprehension it returns Unit (similar to void in Java terms) or a Sequence if yield is used.

1. scala> val range = 1 to 5
2. range: Range.Inclusive = Range(1, 2, 3, 4, 5)
3. // no return value if there is no 'yield' keyword
4. scala> for( i <- 1 to 10 ) { i + 1 }
5. // if there is a yield a collection is returned
6. // the type of collection depends on the input
7. // here a Range is returned
8. scala> for( i <- range ) yield i+1
9. res1: RandomAccessSeq.Projection[Int] = RangeM(2, 3, 4, 5, 6)
10. // here a list is returned
11. scala> for( i <- list( "a", "b", "c") ) yield "Word: "+i
12. res1: List[java.lang.String] = List(Word: a, Word: b, Word: c)
13. // you can filter the elements that visited in the loop
14. scala> for( i <- range; if( i % 2 == 0) ) yield i
15. res2: Seq.Projection[Int] = RangeFM(2, 4)
16. // this    is more    about creating ranges than loops
17. scala> for ( i <- 20 until (10,-2) ) yield i
18. res3: RandomAccessSeq.Projection[Int] = RangeM(20, 18, 16, 14, 12)
19. // you can string together multiple "generators"
20. scala> for( i <- range; j <- range) yield (i,j)
21. res4: Seq.Projection[(Int, Int)] = RangeG((1,1), (1,2), (1,3), (1,4), (1,5), (2,1), (2,2), (2,3), (2,4), (2,5), (3,1), (3,2), (3,3), (3,4), (3,5), (4,1), (4,2), (4,3), (4,4), (4,5), (5,1), (5,2), (5,3), (5\
22. ,4), (5,5))
23. // you can also    declar variables as part of the    loop declaration
24. scala> for( i <- range; j <- 1 to i; k = i-j) yield k
25. res5: Seq.Projection[Int] = RangeG(0, 1, 0, 2, 1, 0, 3, 2, 1, 0, 4, 3, 2, 1, 0)
26. // with round brackets '(' and ')' multiple lines will require semi-colons
27. scala> for (
28.      | i <- range;
29.      | j <- 1 to i;
30.      | k = i-j) yield k
31. res6: Seq.Projection[Int] = RangeG(0, 1, 0, 2, 1, 0, 3, 2, 1, 0, 4, 3, 2, 1, 0)
32. // with curly brackets '{' and '}' multiple lines you do not require semi-colons
33. scala> for {
34.      | i <- range
35.      | j <- 1 to i
36.      | k = i-j}
37.      | yield{
38.      | k
39.      | }
40. res7: Seq.Projection[Int] = RangeG(0, 1, 0, 2, 1, 0, 3, 2, 1, 0, 4, 3, 2, 1, 0)
41. scala> for( i <- "enjoy" ) print(i)
42. enjoy