How to extract sequence elements wrapped in Option in Scala?

Question

I am learning Scala and struggling with Option[Seq[String]] object I need to process. There is a small array of strings Seq("hello", "Scala", "!") which I need to filter against charAt(0).isUpper condition.

Doing it on plain val arr = Seq("hello", "Scala", "!") is as easy as arr.filter(_.charAt(0).isUpper). However, doing the same on Option(Seq("hello", "Scala", "!")) won't work since you need to call .getOrElse on it first. But even then how can you apply the condition?

arr.filter(_.getOrElse(false).charAt(0).isUpper is wrong. I have tried a lot of variants and searching stackoverflow didn't help either and I am wondering if this is at all possible. Is there an idiomatic way to handle Option wrapped cases in Scala?

`Some(Seq("hello", "Scala", "!")).map(seq => seq.filter(_.charAt(0).isUpper))` which can even be reduced to: `.map(seq => seq.filter(_(0).isUpper))` — Xavier Guihot, Jul 05 '18 at 16:12

score 4 · Answer 1 · answered Jul 05 '18 at 16:22

If you want to apply f: X => Y to a value x of type X, you write f(x).

If you want to apply f: X => Y to a value ox of type Option[X], you write ox.map(f).

You seem to already know what you want to do with the sequence, so just put the appropriate f into map.

Example:

val ox = Option(Seq("hello", "Scala", "!"))

ox.map(_.filter(_(0).isUpper)) // Some(Seq("Scala"))

Sebastiaan van den Broek · Accepted Answer · 2018-07-05T16:21:01.030

3

You can just call foreach or map on the option, i.e. arr.map(seq => seq.filter(_.charAt(0).isUpper))

edited Jul 05 '18 at 16:21

answered Jul 05 '18 at 16:13

Sebastiaan van den Broek

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hmm, but is it good to use `foreach`? I assumed that it should be used in side effect methods only. – minerals Jul 05 '18 at 16:14
1

Right, well then just use map, that works on options too. – Sebastiaan van den Broek Jul 05 '18 at 16:15
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What? What is `.foreach.`? – Andrey Tyukin Jul 05 '18 at 16:16
I don't have a scala interpreter handy and it's been a while, so this is pseudocode :) – Sebastiaan van den Broek Jul 05 '18 at 16:17
1

That's not pseudocode. That's wrong usage of `foreach`. The method `foreach` expects one function-valued argument. `.foreach.` is an attempt to invoke `foreach` with zero arguments. One is more than zero, I can tell you that much without an interpreter. https://scastie.scala-lang.org/ – Andrey Tyukin Jul 05 '18 at 16:17
@AndreyTyukin thanks, updated. And cut me some slack here, it's been 3 years since I used Scala in my daily tasks :P – Sebastiaan van den Broek Jul 05 '18 at 16:21

Ramesh Maharjan · Answer 3 · 2018-07-05T16:36:50.597

3

You can use a pattern matching for that case as

Option(Seq("hello", "Scala", "!")) match {
      case Some(x) => x.filter(_.charAt(0).isUpper)
      case None => Seq()
    }

edited Jul 05 '18 at 16:36

answered Jul 05 '18 at 16:26

Ramesh Maharjan

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@AndreyTyukin thats something to return when Option returns None :) – Ramesh Maharjan Jul 05 '18 at 16:30
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I think that debugging code that returns `Seq("None")` might be just as fun as debugging large administrative legacy web-apps that fail to process people with [surname "Null"](http://www.bbc.com/future/story/20160325-the-names-that-break-computer-systems) properly... – Andrey Tyukin Jul 05 '18 at 16:32
No, just change it to more natural choice `Seq()`, then it's fine imho ;) – Andrey Tyukin Jul 05 '18 at 16:35

How to extract sequence elements wrapped in Option in Scala?

3 Answers3