Finding an item that matches predicate in Scala

Question

I'm trying to search a scala collection for an item in a list that matches some predicate. I don't necessarily need the return value, just testing if the list contains it.

In Java, I might do something like:

for ( Object item : collection ) {     if ( condition1(item) && condition2(item) ) {        return true;     } } return false; 

In Groovy, I can do something like:

return collection.find { condition1(it) && condition2(it) } != null 

What's the idiomatic way to do this in Scala? I could of course convert the Java loop style to Scala, but I feel like there's a more functional way to do this.

Answer 1

Use filter:

scala> val collection = List(1,2,3,4,5) collection: List[Int] = List(1, 2, 3, 4, 5)  // take only that values that both are even and greater than 3  scala> collection.filter(x => (x % 2 == 0) && (x > 3)) res1: List[Int] = List(4)  // you can return this in order to check that there such values scala> res1.isEmpty res2: Boolean = false  // now query for elements that definitely not in collection scala> collection.filter(x => (x % 2 == 0) && (x > 5)) res3: List[Int] = List()  scala> res3.isEmpty res4: Boolean = true 

But if all you need is to check use exists:

scala> collection.exists( x => x % 2 == 0 ) res6: Boolean = true 

Answer 2

Testing if value matching predicate exists

If you're just interested in testing if a value exists, you can do it with.... exists

scala> val l=(1 to 4) toList l: List[Int] = List(1, 2, 3, 4)  scala> l exists (_>5) res1: Boolean = false  scala> l exists (_<2) res2: Boolean = true  scala> l exists (a => a<2 || a>5) res3: Boolean = true 

Other methods (some based on comments):

Counting matching elements

Count elements that satisfy predicate (and check if count > 0)

scala> (l count (_ < 3)) > 0 res4: Boolean = true 

Returning first matching element

Find the first element that satisfies predicate (as suggested by Tomer Gabel and Luigi Plinge this should be more efficient because it returns as soon as it finds one element that satisfies the predicate, rather than traversing the whole List anyway)

scala> l find (_ < 3) res5: Option[Int] = Some(1)   // also see if we found some element by // checking if the returned Option has a value in it scala> l.find(_ < 3) isDefined res6: Boolean = true 

Testing if exact value exists

For the simple case where we're actually only checking if one specific element is in the list

scala> l contains 2 res7: Boolean = true