find method in scala's collection

Question

Per the documentation, find function in scala's collection wraps the resulting elements in Option object. In the below, the size operation ends in error; where as the endsWith produces correct result.

The second element in the list being null, both should error out / produce correct result.

val a1:String = "ABCDEF"
val a2:String = null
val res12 = List(ABCDEF, null)

res12 filter { _.size > 10 }              //errors out
java.lang.NullPointerException
  at scala.collection.immutable.StringOps$.length$extension(StringOps.scala:51)
  at scala.collection.immutable.StringOps.length(StringOps.scala:51)
  at scala.collection.SeqLike.size(SeqLike.scala:108)
  at scala.collection.SeqLike.size$(SeqLike.scala:108)
  at scala.collection.immutable.StringOps.size(StringOps.scala:33)
  at .$anonfun$res19$1(<console>:13)
  at .$anonfun$res19$1$adapted(<console>:13)
  at scala.collection.TraversableLike.$anonfun$filterImpl$1(TraversableLike.scala:251)
  at scala.collection.immutable.List.foreach(List.scala:392)
  at scala.collection.TraversableLike.filterImpl(TraversableLike.scala:250)
  at scala.collection.TraversableLike.filterImpl$(TraversableLike.scala:248)
  at scala.collection.AbstractTraversable.filterImpl(Traversable.scala:108)
  at scala.collection.TraversableLike.filter(TraversableLike.scala:262)
  at scala.collection.TraversableLike.filter$(TraversableLike.scala:262)
  at scala.collection.AbstractTraversable.filter(Traversable.scala:108)
  ... 28 elided

res12 find { _.endsWith("EF") }       //produces correct result
res20: Option[String] = Some(ABCDEF)

score 3 · Accepted Answer · answered May 06 '20 at 04:38

3

both should error out / produce correct result

No they shouldn't.

filter() has to touch every element to see if it's in or out. If there's a bomb in the collection then it will be detonated.

find() is lazy. Find the 1st element to pass the predicate test and we're outa here. One and done.

answered May 06 '20 at 04:38

jwvh

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Thanks jwvh, I just swapped the element order in the list and I am getting the exception now. On a different note, I am going through Option usage in Scala collection. When we have a collection made of only Option objects (Some and None) and if we apply any methods on the collection (e.g: map , filter, fold), the methods will not process None. Am I correct ? – user3103957 May 06 '20 at 05:35
2

@user3103957, no. Which elements the method will process (in this case apply the supplied function) depends on the method. As @jwvh pointed out `find()` is lazy and stops iterating after the first match. Methods like `filter`, `map` and `fold` apply the function to all elements in the collection – CervEd May 06 '20 at 06:07
2

@user3103957; `None` elements get processed just like any other: `listOfOptions.count(_.isEmpty)` – jwvh May 06 '20 at 06:58
Thanks @CervEd and jwvh., I got another example that flatten will not process None. val x = List(Some(1), Some(2), None, Some(3), None); x.flatten gives List(1, 2, 3). So None is safely skipped by flatten. Hence it is very obvious that processing of None, depends on the called collection's method. – user3103957 May 07 '20 at 02:57
2

@user3103957; When you `flatten` a collection of options, the `None` elements are not skipped, they are removed, and for that to happen they have to be interrogated (processed) to determine if they are worthy of removal. It's the same with `List(List(2,3),List(),List(9,1)).flatten`. The empty `List` is not "skipped," it is processed like the others. – jwvh May 07 '20 at 03:30
Thank you, I am understanding now. I come across another scenario. In this piece of code: val x= Option("ABC"); x map { e => Option("XYZ") } --> returns Some(Some(XYZ)) which is surprising. If it was other collections (like: List, Set), it should return Some("XYZ"). So I understand in Option monoid, the map method is written differently. – user3103957 May 07 '20 at 05:21
2

`List("ABC").map(e => List("XYZ"))` returns `List(List(XYZ))` Doesn't look very different to me. – jwvh May 07 '20 at 06:04
Thanks jwvh. That was a brain fade moment when I write my above comment.I realized it now. Thanks again – user3103957 May 09 '20 at 06:58

score 1 · Answer 2 · answered May 06 '20 at 04:28

This produces an error because you are calling size on a null String which is in fact a NullPointerException

scala> val s: String = null
val s: String = null

scala> s.size
java.lang.NullPointerException
  at scala.collection.StringOps$.size$extension(StringOps.scala:165)
  ... 32 elided

find method in scala's collection

2 Answers2