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Finding elements in a scala list and also know which predicate has been satisfied

Asked 2010-02-01T10:09:34.080
10

I have the following problem in scala. I have to find the first element in al list which satisfies a predicate function with two conditions in OR. The problem is that I would like to get the element but also know which of the two conditions has been satisfied. Here is a simple example:

val l1 = List("A", "B", "AA", "BB")
val l2 = List("AA", "BB", "A", "B")

def c1(s: String) = s.startsWith("B")
def c2(s: String) = s.length == 2

println(l1.find(s => c1(s) || c2(s)))
println(l2.find(s => c1(s) || c2(s)))

result is:

Some(B)
Some(AA)

For the l1 case I would like to have some return value (a String for example) indicating that c1 was satisfied (c2 for the l2 case). A possible solution could be to define a var before the test and set it within the c1 and c2 functions, but I would like to find a more "functional style" solution, maybe something that return a Tuple like: (element found, condition satisfied).

Thanks in advance for the help

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1 Answer

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Here's a variant on Daniel's (and Retronym's) answer(s).

If you just want the predicate (out of a list) that succeeded, then you can use

def findP[T](list: Iterable[T], preds: Iterable[T=>Boolean]) = {
  list.view.map( x => (x , preds.find( _(x) )) ).find( _._2.isDefined )
}

Alternatively, you could use a list of named predicates:

def findP[T](list: Iterable[T],preds: Iterable[(T=>Boolean,String)]) = {
  list.view.map(x => (x , preds.find( _._1(x) ))).find( _._2.isDefined )
}

scala> findP(
     |   List(1,2,3,4,5,6),
     |   List( ((i:Int)=>i>4,"Fred") , ((i:Int)=>(i%6)==0,"Barney"))
     | )
res2: Option[(Int, Option[((Int) => Boolean, String)])] =
  Some((5,Some((<function1>,Fred))))

The result a little cluttered, but can be unwrapped easily enough to give exactly what you asked for:

def findP[T](list: Iterable[T],preds: Iterable[(T=>Boolean,String)]) = {
  list.view.map(x => (x , preds.find( _._1(x) ))).find( _._2.isDefined ) match {
    case Some((i,Some((_,s)))) => Some((i,s))
    case _ => None
  }
}

(This is code for 2.8; switch "view" to "projection" for 2.7.)

answered 2010-02-01T16:38:20.693

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