How to extract the elements of an Array[String] in Scala?

Question

Suppose we have an Array:

val arr = Array("Id:1; Apple; Red; 2; Out",
                "Id:2; Banana; Yellow; 5",
                "Id:3; Peach; Red; 3",
                "Id:4; Grape; Green; 5; Out")

I want to apply a function on each element of the array, which extracts the fruit type and the number and returns a Map. The output in this case would be:

(Apple, 2)
(Banana, 5)
(Peach, 3) 
(Grape, 5)

I tried:

val pairMap = arr.foreach(r => r.split(";")(1) zip r.split(";")(3))

but I always obtain Unit

That's because foreach returns Unit. Use map instead. You might also want to parse the numbers into integers (right now you will get strings). — Jesper
– Jesper, Commented Jul 7, 2017 at 13:06

thwiegan · Accepted Answer · 2017-07-07 20:22:54Z

6

You actually have two errors in your code.

First as pointed out already you are using foreach (which returns Unit) instead of map (which returns an Object).

Second, you are using zip in your foreach function, which combines two collections pair wise, like this:

val arr1 = Seq("Apple", "Peach", "Banana")
val arr2 = Seq("Red", "Red", "Yellow")

val arr3 = arr1 zip arr2 // = Seq(("Apple", "Red"), ("Peach", "Red"), ("Banana", "Yellow"))

Your code should look like this:

val arr = Array("Id:1; Apple; Red; 2; Out",
  "Id:2; Banana; Yellow; 5",
  "Id:3; Peach; Red; 3",
  "Id:4; Grape; Green; 5; Out")

arr.map(r => (r.split(";")(1), r.split(";")(3)))

Or to be a tiny bit more efficient, by splitting only once:

arr.map { r =>
  val t = r.split(";")
  (t(1), t(2))
}

And you probably don't want the white spaces, so:

arr.map { r =>
  val t = r.split(";")
  (t(1).trim(), t(2).trim())
}

Just adding this because I think regex is also a good option for string processing and a neat feature together with scala's pattern match:

val regEx = "[^;]+; ([^;]+); [^;]+; ([^;]+).*".r

arr collect {
  case regEx(fruit, number) => (fruit, number)
}

Also leads to the desired output, but might be a bit overkill for this simple use case.

edited Jul 7, 2017 at 20:22

answered Jul 7, 2017 at 13:10

thwiegan

2,17312 silver badges19 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

sanyi14ka Over a year ago

Yes, I was looking for this. Thank you!

stefanobaghino Over a year ago

This is a correct solution, in the sake of efficiency one further thing would be to not traverse the collection twice by using map two times.

sanyi14ka Over a year ago

For the regex solution, do you just have to add parentheses around the element which you want to extract?

thwiegan Over a year ago

Yes, exactly. It's called a capture group ( a general regex thing ). But you need to be careful in case you have an element that does not match, you might want to add a default case in the pattern matching. Otherwise you will get a scala.MatchError exception. I changed the code to use collect, which will just go over non matching elements and ignore them

stefanobaghino · Accepted Answer · 2017-07-07 13:16:37Z

1

Close, but you should have used map instead of foreach.

Both these higher-order functions take a function and apply that to each element of the collection.

The difference between the two can be seen in the (simplified, not actual) signature:

trait Collection[A] {

  def map[B](f: A => B): Collection[B]

  def foreach(f: A => Unit): Unit

}

The difference is that foreach is used to make some sort of action or side effect happen (e.g. printing), while mapping yields a collection whose elements are those of the starting collection with the function applied:

List(1, 2, 3, 4).map(_ * 2).forech(print) // prints 2468

Furthermore, the function has to be edited a little bit in order to work as intended:

arr.map {
  row =>
    val split = row.split(";") // split only once
    (split(1).trim, split(3).trim) // no zipping necessary
}

zip is not necessary because you are already processing each item, one at a time, while zip allows you to zip together two collection.

Seq(1, 2, 3) zip Seq('a', 'b', 'c') == Seq((1, 'a'), (2, 'b'), (3, 'c')) // true

edited Jul 7, 2017 at 13:16

answered Jul 7, 2017 at 13:06

stefanobaghino

13.4k4 gold badges44 silver badges69 bronze badges

1 Comment

sanyi14ka Over a year ago

If I use map, I receive:

pairMap: Array[scala.collection.immutable.IndexedSeq[(Char, Char)]] = Array(Vector(( , ), (A,2)), Vector(( , ), (B,5)), Vector(( , ), (P,3)), Vector(( , ), (G,5)))

Collectives™ on Stack Overflow

How to extract the elements of an Array[String] in Scala?

2 Answers 2

4 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related