foreach loop in scala

Question

In scala foreach loop if I have list

val a = List("a","b","c","d")

I can print them without a pattern matching like this

a.foreach(c => println(c))

But, if I have a tuple like this

val v = Vector((1,9), (2,8), (3,7), (4,6), (5,5))

why should I have to use

v.foreach{ case(i,j) => println(i, j) }

a pattern matching case
{ brackets

Please explain what happens when the two foreach loops are executed.

It is design flaw in scala. There is probably explanation for this, but it will be long and mainly useless. — talex, Jul 18 '17 at 12:50
According to the Dotty team in Scala 3 this will be valid: `v.foreach((i,j) => println(i, j))` — kanbagoly, Jul 18 '17 at 22:27

Yuval Itzchakov · Accepted Answer · 2017-07-18T11:39:41.250

You don't have to, you choose to. The problem is that the current Scala compiler doesn't deconstruct tuples, you can do:

v.foreach(tup => println(tup._1, tup._2))

But, if you want to be able to refer to each element on it's own with a fresh variable name, you have to resort to a partial function with pattern matching which can deconstruct the tuple.

This is what the compiler does when you use case like that:

def main(args: Array[String]): Unit = {
  val v: List[(Int, Int)] = scala.collection.immutable.List.apply[(Int, Int)](scala.Tuple2.apply[Int, Int](1, 2), scala.Tuple2.apply[Int, Int](2, 3));
  v.foreach[Unit](((x0$1: (Int, Int)) => x0$1 match {
    case (_1: Int, _2: Int)(Int, Int)((i @ _), (j @ _)) => scala.Predef.println(scala.Tuple2.apply[Int, Int](i, j))
  }))
}

You see that it pattern matches on unnamed x0$1 and puts _1 and _2 inside i and j, respectively.

score 6 · Answer 2 · edited Jul 18 '17 at 12:30

6

According to http://alvinalexander.com/scala/iterating-scala-lists-foreach-for-comprehension:

val names = Vector("Bob", "Fred", "Joe", "Julia", "Kim")

for (name <- names)
    println(name)

edited Jul 18 '17 at 12:30

Melebius

6,183
4
39
52

answered Jul 18 '17 at 11:26

grnc

477
6
13

score 2 · Answer 3 · answered Jul 18 '17 at 11:32

2

To answer #2: You can only use case in braces. A more complete answer about braces is located here.

answered Jul 18 '17 at 11:32

pJunger

41
6

score 1 · Answer 4 · answered Dec 25 '18 at 20:15

Vector is working a bit differently, you using function literals using case...

In Scala, we using brackets{} which accept case...

{
  case pattern1 => "xxx"
  case pattern2 => "yyy"
}

So, in this case, we using it with foreach loop...

Print all values using the below pattern then:

val nums = Vector((1,9), (2,8), (3,7), (4,6), (5,5))

nums.foreach {
    case(key, value) => println(s"key: $key, value: $value")
}

Also you can check other loops like for loop as well if you think this is not something which you are comfortable with...

foreach loop in scala

4 Answers4