Matching in for-comprehensions

At a glance a for-comprehension appears to be equivalent to a Java for-loop, but it is much much more than that. As shown in post: for-comprehensions, for-comprehensions can have guards which filter out which elements are processed:

1. scala> for ( x <- 1 to 10; if (x >4) ) println(x)
2. 5
3. 6
4. 7
5. 8
6. 9
7. 10

They can be used to construct new collections:

1. scala>?for(?i?<-?List(?"a",?"b",?"c")?)?yield?"Word:?"+i
2. res1:?List[java.lang.String]?=?List(Word:?a,?Word:?b,?Word:?c)

They can contain multiple generators:

1. scala> for {x <- 1 to 10                           
2.      |      if(x%2 == 0)
3.      |      y <- 1 to 5} yield (x,y)
4. res1: scala.collection.immutable.IndexedSeq[(Int, Int)] = IndexedSeq((2,1), (2,2), (2,3), (2,4), (2,5), (4,1), (4,2), (4,3), (4,4), (4,5), (6,1), (6,2), (6,3), (6,4), (6,5), (8,1), (8,2), (8,3), (8,4), (8,5), (10,1), (10,2), (10,3), (10,4), (10,5))

What has not been covered is that the assignments also does pattern matching:

1. scala> for ( (x,y) <- (6 to 1 by -2).zipWithIndex) println (x,y) 
2. (6,0)
3. (4,1)
4. (2,2)

This is not surprising as this also occurs during normal assignment. But what is interesting is that the pattern matching can act as a guard as well. See Extractor examples and Assignment and Parameter Objects for more information of pattern matching and extractors.

1. scala> val args = Array( "h=2", "b=3")
2. args: Array[java.lang.String] = Array(h=2, b=3)
4. scala> val Property = """(.+)=(.+)""".r 
5. Property: scala.util.matching.Regex = (.+)=(.+)
7. scala> for {Property(key,value) <- args } yield (key,value)
8. res0: Array[(String, String)] = Array((h,2), (b,3))
10. scala> Map(res0:_*)
11. res1: scala.collection.immutable.Map[String,String] = Map(h -> 2, b -> 3)
13. scala> res1("h")
14. res3: String = 2

Now just for fun here is a similar example but using symbols instead of strings for the key values:

1. scala> val args = Array( "h=2", "b=3")
2. args: Array[java.lang.String] = Array(h=2, b=3)
4. scala> val Property = """(.+)=(.+)""".r 
5. Property: scala.util.matching.Regex = (.+)=(.+)
7. scala> for {Property(key,value) <- args } yield (Symbol(key),value)
8. res0: Array[(Symbol, String)] = Array(('h,2), ('b,3))
10. scala> Map(res0:_*)
11. res1: scala.collection.immutable.Map[Symbol,String] = Map('h -> 2, 'b -> 3)
13. scala> res1('h)
14. res2: String = 2