For-comprehensions

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)
5. // no return value if there is no 'yield' keyword
6. scala> for( i <- 1 to 10 ) { i + 1 }
8. // if there is a yield a collection is returned
9. // the type of collection depends on the input
10. // here a Range is returned
11. scala> for( i <- range ) yield i+1
12. res1: RandomAccessSeq.Projection[Int] = RangeM(2, 3, 4, 5, 6)
14. // here a list is returned
15. scala> for( i <- List( "a", "b", "c") ) yield "Word: "+i
16. res1: List[java.lang.String] = List(Word: a, Word: b, Word: c)
18. // you can filter the elements that visited in the loop
19. scala> for( i <- range; if( i % 2 == 0) ) yield i
20. res2: Seq.Projection[Int] = RangeFM(2, 4)
22. // this    is more    about creating ranges than loops
23. scala> for ( i <- 20 until (10,-2) ) yield i
24. res3: RandomAccessSeq.Projection[Int] = RangeM(20, 18, 16, 14, 12)
26. // you can string together multiple "generators"
27. scala> for( i <- range; j <- range) yield (i,j)
28. 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\
29. ,4), (5,5))
31. // you can also    declar variables as part of the    loop declaration
32. scala> for( i <- range; j <- 1 to i; k = i-j) yield k
33. res5: Seq.Projection[Int] = RangeG(0, 1, 0, 2, 1, 0, 3, 2, 1, 0, 4, 3, 2, 1, 0)
35. // with round brackets '(' and ')' multiple lines will require semi-colons
36. scala> for (
37.      | i <- range;
38.      | j <- 1 to i;
39.      | k = i-j) yield k
40. res6: Seq.Projection[Int] = RangeG(0, 1, 0, 2, 1, 0, 3, 2, 1, 0, 4, 3, 2, 1, 0)
42. // with curly brackets '{' and '}' multiple lines you do not require semi-colons
43. scala> for {
44.      | i <- range
45.      | j <- 1 to i
46.      | k = i-j}
47.      | yield{
48.      | k
49.      | }
50. res7: Seq.Projection[Int] = RangeG(0, 1, 0, 2, 1, 0, 3, 2, 1, 0, 4, 3, 2, 1, 0)
52. scala> for( i <- "enjoy" ) print(i)
53. enjoy