Pattern matching in Scala

Question

scala> (1,5) == BigInt(12) /% 7
res3: Boolean = true

scala> BigInt(12) /% 7 match {
 | case (1,5) => true
 | }

<console>:9: error: type mismatch;
found   : Int(1)
required: scala.math.BigInt
          case (1,5) => true
                ^

Can someone perhaps explain me how to pattern match here?

score 9 · Accepted Answer · answered Nov 11 '12 at 18:55

match is more specific than equality; you can't just be equal, you must also have the same type.

In this case, BigInt is not a case class and does not have an unapply method in its companion object, so you can't match on it directly. The best you could do is

  BigInt(12) /% 7 match {
    case (a: BigInt,b: BigInt) if (a==1 && b==5) => true
    case _ => false
  }

or some variant thereof (e.g. case ab if (ab == (1,5)) =>).

Alternatively, you could create an object with an unapply method of appropriate type:

object IntBig { def unapply(b: BigInt) = Option(b.toInt) }

scala> BigInt(12) /% 7 match { case (IntBig(1), IntBig(5)) => true; case _ => false }
res1: Boolean = true

Binding to variables and a guard is what I did. I just thaught it could perhaps be done a little better. Thank you. — , Nov 11 '12 at 19:11

Brian · Answer 2 · 2012-11-11T19:01:44.410

1

1 and 5 are Int types. The pattern match is expecting a scala.math.BigInt. So, give it that by declaring a couple of vals for one and five.

scala> val OneBig = BigInt(1)
oneBig: scala.math.BigInt = 1

scala> val FiveBig = BigInt(5)
fiveBig: scala.math.BigInt = 5

scala> BigInt(12) /% 7 match {
     | case (OneBig, FiveBig) => true
     | }
res0: Boolean = true

edited Nov 11 '12 at 19:01

answered Nov 11 '12 at 18:56

Brian

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This doesn't work! `oneBig` and `fiveBig` in the pattern are new variables unrelated to the ones defined before. This can be changed by naming them with a capital letter (`OneBig` and `FiveBig`) or by surrounding the names in backticks. – Alexey Romanov Nov 11 '12 at 18:59
Thanks. Forgot about that. For those interested see http://stackoverflow.com/questions/4479474/scala-pattern-matching-with-lowercase-variable-name or page 317 of the stairway book. – Brian Nov 11 '12 at 19:02

score 0 · Answer 3 · answered Nov 11 '12 at 18:55

The problem is that 1 and 5 returned by /% are BigInts and so don't match literal Ints even though equals method (called by ==) returns true. This works but is a bit inelegant:

scala> BigInt(12) /% 7 match { case (x, y) if x == 1 && y == 5 => true }
res3: Boolean = true

Pattern matching in Scala

3 Answers3