writing a custom get method for a scala map - scala

Question

I have a map which is something like
val m = Map("foo" -> "bar", "faz" -> "baz")
I need to write a custom get method, so that the key can be the key in the map with a number in the end.
So for example:
m.get("foo1") should return "bar"
I am looking for a good scala pattern to solve this problem.
Also I am generating the above map from a for loop using yield, so I can't do something like this
val m = CustomMap("foo" -> "bar")
Any solutions will be appreciated.
Thanks

Answer 1

First of all, you can generate a map from a for comprehension, and then convert it to CustomMap. You just need to define a
def apply(map: Map[String, String]) = CustomMap(map.toSeq :_*) in CustomMap - then you can do val m = CustomMap( for { ... } yield ... )
Secondly, if it doesn't have to be named get (it probably shouldn't be anyway), you can do this sort of thing with an implicit:
object PimpMyMap {
val pref = ".*?(\\d+)".r
implicit class Pimped[V](val map: Map[String,V]) extends AnyVal {
def getPrefix(key: String): Option[V] = map.get(key).orElse { key match {
case pref(k) => map.get(k)
case _ => None
}
}
Now you can write things like:
import PimpMyMap._
val map = Map("foo" -> 1)
val one = map.getPrefix("foo123") // Some(1)
val anotherOne = map.getPrefix("foo") // also Some(1);

Answer 2

You can do this with an implicit class and implicit conversion:
import scala.language.implicitConversions
object MapHelpers {
implicit def optionStringToString(maybeS: Option[String]): String = maybeS.getOrElse("")
implicit class MapWithIntKey(val m: Map[String, String]) extends Map[String, String] {
override def get(key: String): Option[String] = {
val intRegex = """(\d+)""".r
val keyWithoutInt = intRegex
.findFirstMatchIn(key)
.map(int => {
val idx = key.indexOf(int.toString)
key.slice(0, idx)
})
.getOrElse(key)
m.get(keyWithoutInt)
}
def +[V1 >: String](
kv: (String, V1)): scala.collection.immutable.Map[String, V1] = m + kv
def -(key: String): scala.collection.immutable.Map[String, String] = m - key
def iterator: Iterator[(String, String)] = m.iterator
}
}
object App {
import MapHelpers._
def testMapImplicit(): Unit = {
val myMap: MapWithIntKey = Map("foo" -> "bar", "faz" -> "baz")
val result: String = myMap.get("foo1")
println("result", result) // bar
}
}
Working Scastie

Answer 3

If you have a sure way to get the real key from the fake key, you can do this with Map.withDefault:
class CustomMap[K, +V] private (underlying: Map[K, Option[V]]) {
def get(k: K): Option[V] = underlying(k)
}
object CustomMap {
def apply[K, V](original: Map[K, V], keyReducer: K => K) = new CustomMap(originalMap.
mapValues(Some(_)).
withDefault(k => originalMap.get(keyReducer(k))
)
}
In your case, you can use this with
val stringKeyReducer: String => String = k.reverse.dropWhile(_.isDigit).reverse
to drop the digits at the end of your strings, so
CustomMap(Map("foo" -> "bar"), stringKeyReducer).get("foo1") = Some("bar")

Answer 4

Here is solution which combines both the answers.
import scala.language.implicitConversions
object MapHelpers {
implicit def optionStringToString(maybeS: Option[String]): String = maybeS.getOrElse("")
implicit class MapWithIntKey(val m: Map[String, String]) extends Map[String, String] {
override def get(key: String): Option[String] = {
val prefix = "(.*?)\\d+".r
m.get(key).orElse{
key match {
case prefix(p) => m.get(p)
case _ => None
}
}
}
def +[V1 >: String](kv: (String, V1)): scala.collection.immutable.Map[String, V1] = m + kv
def -(key: String): scala.collection.immutable.Map[String, String] = m - key
def iterator: Iterator[(String, String)] = m.iterator
}
}
object App {
import MapHelpers._
def testMapImplicit(): Unit = {
val myMap: MapWithIntKey = Map("foo" -> "bar", "faz" -> "baz")
println("result - number match ", myMap.get("foo1"))
println("result - exact match ", myMap.get("foo"))
}
}
App.testMapImplicit()
Working Scastie