create a map from list in Scala

Question

I need to create a HashMap of directory-to-file in scala while I list all files in the directory. How can I achieve this in scala?

val directoryToFile = awsClient.listFiles(uploadPath).collect {
  case path if !path.endsWith("/") => {
    path match {
      // do some regex matching to get directory & file names
      case regex(dir, date) => {
          // NEED TO CREATE A HASH MAP OF dir -> date. How???
      }
      case _ => None
    }
  }
}

The method listFiles(path: String) returns a Seq[String] of absolute path of all files in the path passed as argument to the function

@YuvalItzchakov Seq[String] which is a list of absolute paths for all files in the directory — Darth.Vader, Nov 06 '16 at 15:26

Yuval Itzchakov · Answer 1 · 2016-11-06T17:32:00.107

0

You can filter and then foldLeft:

val l = List("""/opt/file1.txt""", """/opt/file2.txt""")
val finalMap = l
                .filter(!_.endsWith("/"))
                .foldLeft(Map.empty[String, LocalDateTime])((map, s) =>
  s match {
    case regex(dir, date) => map + (dir -> date)
    case _ => map
  }
)

edited Nov 06 '16 at 17:32

answered Nov 06 '16 at 15:33

Yuval Itzchakov

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thanks! What about the "collect" method where I was filtering out some elements based on a certain criteria in the original code? – Darth.Vader Nov 06 '16 at 15:42
You can apply a `filter` before using `foldLeft`. Edited the answer. – Yuval Itzchakov Nov 06 '16 at 15:44
thank you! Now, I minor update: there could be a situation that the same directory may have multiple files denoted by date: "dir1/2016-01-01.txt" and "dir1/2013-01-01/txt". In that case, I want the hashmap to only keep the value with the latest file date.. how can I use "max" function to do that? – Darth.Vader Nov 06 '16 at 15:56
1

`+` on `immutable.Map` with the same key will override the previous entry. – Yuval Itzchakov Nov 06 '16 at 15:59
1

See my solutions. I have added your "max" function. – Rok Kralj Nov 06 '16 at 16:00
`// Stuff` means `map`? – Victor Moroz Nov 06 '16 at 17:10

Rok Kralj · Answer 2 · 2016-11-06T16:00:22.807

0

Try to write more idiomatic Scala. Something like this:

val directoryToFile = (for {
    path <- awsClient.listFiles(uploadPath)
    if !path.endsWith("/")
    regex(dir, date) <- regex.findFirstIn(path)
} yield dir -> date).sortBy(_._2).toMap

edited Nov 06 '16 at 16:00

answered Nov 06 '16 at 15:49

Rok Kralj

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What `sortBy` is for? `Map` in Scala is unordered. – Victor Moroz Nov 06 '16 at 16:45
You said you wanted only the last (most recent) date associated with each dir. This is the purpose of the sortBy statement. You can also remove that, if you do not need it. – Rok Kralj Nov 06 '16 at 16:56

Victor Moroz · Answer 3 · 2016-11-06T17:37:14.910

You can try something like this:

val regex =  """(\d)-(\d)""".r
val paths = List("1-2", "3-4", "555")

for {

  // Hint to Scala to produce specific type
  _ <- Map("" -> "")

  // Not sure why your !path.endsWith("/") is not part of regex
  path@regex(a, b) <- paths
  if path.startsWith("1")

} yield (a, b)

//> scala.collection.immutable.Map[String,String] = Map(1 -> 2)

Slightly more complicated if you need max:

val regex =  """(\d)-(\d)""".r
val paths = List("1-2", "3-4", "555", "1-3")

for {
  (_, ps) <-
    ( for {
        path@regex(a, b) <- paths
        if path.startsWith("1")
      } yield (a, b)
    ).groupBy(_._1)
} yield ps.maxBy(_._2)

//> scala.collection.immutable.Map[String,String] = Map(1 -> 3)

create a map from list in Scala

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