Scala pattern matching - match multiple successful cases

Question

I am fairly new to Scala, and would like to know if it is possible for a match to execute multiple matching cases at once. Without getting into too much detail, I am basically working on a function that "scores" a certain piece of text according to various traits; these traits can overlap, and multiple traits can be true for one given string.

To illustrate what I want in code, it would look something like this:

Say we have a String, str, with a value of "Hello World". I would like something along the lines of the following:

str match {
    case i if !i.isEmpty => 2
    case i if i.startsWith("world") => 5
    case i if i.contains("world") => 3
    case _ => 0
}

I would like the above code to trigger both the first and third conditions, effectively returning both 2 and 3 (as a tuple or in any other way).

Is this possible?

Edit: I know this can be done with a chain of if's, which is the approach I took. I'm just curious if something like the above implementation is possible.

As far as i'm concerned pattern matching only aplies to 1st correct case. — Haito
– Haito, Commented Sep 14, 2015 at 8:37
@Haito Yup... That's why I was wondering if there is any way that I'm not familiar with to make something like this work. — Ruslan
– Ruslan, Commented Sep 14, 2015 at 8:38
It is not possible. What would the return type be? You could use something like a List[(String => Boolean, Int)] and then use predicateList.collect {case (p, i) if p(myWordToTest) => i} — Marth
– Marth, Commented Sep 14, 2015 at 8:42
I ended up doing ((if (!i.isEmpty) 2) :: (if (i.startsWith("world")) 5) :: (if (i.contains("world")) 3) :: Nil) map {case i: Int => i case _ => 0} sum but wanted to see if there was a more elegant way. @Marth I'd have expected the return types to possibly be a Tuple, if something like that was possible. — Ruslan
– Ruslan, Commented Sep 14, 2015 at 8:51

ccheneson · Accepted Answer · 2015-09-14 08:57:34Z

2

You can turn your case statements to functions

val isEmpty = (str: String) => if ( !str.isEmpty) 2 else 0
val startsWith = (str: String) => if ( str.startsWith("world"))  5  else 0
val isContains = (str: String) => if (str.toLowerCase.contains("world")) 3  else 0

val str = "Hello World"

val ret = List(isEmpty, startsWith, isContains).foldLeft(List.empty[Int])( ( a, b ) =>  a :+ b(str)   )

ret.foreach(println)
//2
//0
//3

You can filter the 0 values with filter

 val ret0 = ret.filter( _ > 0)
 ret0.foreach(println)

answered Sep 14, 2015 at 8:57

ccheneson

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Nyavro · Accepted Answer · 2015-09-14 08:43:26Z

1

Please, consider this solution:

val matches = Map[Int, String => Boolean](2 -> {_.isEmpty}, 3 -> {_.contains("world")}, 5 -> {_.startsWith("world")})
val scores = matches.filter {case (k, v) => v(str)}.keys

answered Sep 14, 2015 at 8:43

Nyavro

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