Filtering inside `for` with pattern matching - scala

Question

I am reading a TSV file and using using something like this:
case class Entry(entryType: Int, value: Int)
def filterEntries(): Iterator[Entry] = {
for {
line <- scala.io.Source.fromFile("filename").getLines()
} yield new Entry(line.split("\t").map(x => x.toInt))
}
Now I am both interested in filtering out entries whose entryType are set to 0 and ignoring lines with column count greater or lesser than 2 (that does not match the constructor). I was wondering if there's an idiomatic way to achieve this may be using pattern matching and unapply method in a companion object. The only thing I can think of is using .filter on the resulting iterator.
I will also accept solution not involving for loop but that returns Iterator[Entry]. They solutions must be tolerant to malformed inputs.

Answer 1

This is more state-of-arty:
package object liner {
implicit class R(val sc: StringContext) {
object r {
def unapplySeq(s: String): Option[Seq[String]] = sc.parts.mkString.r unapplySeq s
}
}
}
package liner {
case class Entry(entryType: Int, value: Int)
object I {
def unapply(s: String): Option[Int] = util.Try(s.toInt).toOption
}
object Test extends App {
def lines = List("1 2", "3", "", " 4 5 ", "junk", "0, 100000", "6 7 8")
def entries = lines flatMap {
case r"""\s*${I(i)}(\d+)\s+${I(j)}(\d+)\s*""" if i != 0 => Some(Entry(i, j))
case __________________________________________________ => None
}
Console println entries
}
}
Hopefully, the regex interpolator will make it into the standard distro soon, but this shows how easy it is to rig up. Also hopefully, a scanf-style interpolator will allow easy extraction with case f"$i%d".
I just started using the "elongated wildcard" in patterns to align the arrows.
There is a pupal or maybe larval regex macro:
https://github.com/som-snytt/regextractor

Answer 2

You can create variables in the head of the for-comprehension and then use a guard:
edit: ensure length of array
for {
line <- scala.io.Source.fromFile("filename").getLines()
arr = line.split("\t").map(x => x.toInt)
if arr.size == 2 && arr(0) != 0
} yield new Entry(arr(0), arr(1))

Answer 3

I have solved it using the following code:
import scala.util.{Try, Success}
val lines = List(
"1\t2",
"1\t",
"2",
"hello",
"1\t3"
)
case class Entry(val entryType: Int, val value: Int)
object Entry {
def unapply(line: String) = {
line.split("\t").map(x => Try(x.toInt)) match {
case Array(Success(entryType: Int), Success(value: Int)) => Some(Entry(entryType, value))
case _ =>
println("Malformed line: " + line)
None
}
}
}
for {
line <- lines
entryOption = Entry.unapply(line)
if entryOption.isDefined
} yield entryOption.get

Answer 4

The left hand side of a <- or = in a for-loop may be a fully-fledged pattern. So you may write this:
def filterEntries(): Iterator[Int] = for {
line <- scala.io.Source.fromFile("filename").getLines()
arr = line.split("\t").map(x => x.toInt)
if arr.size == 2
// now you may use pattern matching to extract the array
Array(entryType, value) = arr
if entryType == 0
} yield Entry(entryType, value)
Note that this solution will throw a NumberFormatException if a field is not convertible to an Int. If you do not want that, you'll have to encapsulate x.toInt with a Try and pattern match again.