Iterating over list of maps and transforming - scala

Question

I am pretty much new to Scala and trying to learn some stuff and would like to understand the best possible approach on below:
I have a list of maps as below.
List<Map<String,String>> listOfMaps = new ArrayList<>();
Map<String,String> map1 = new HashMap<>();
map1.put("a","A1");
map1.put("b","B1");
.
.
.
map10.put("a","A10");
map10.put("b","B10");
listOfMaps.add(map1);
listOfMaps.add(map2);
...
listOfMaps.add(map10);
I would like to iterate this and transform in to another list with just a certain key, value pair from the map and add a new value
List<Map<String,String>> newlistOfMaps = new ArrayList<>();
for(Map map: listOfMaps) {
Map<String,String> newMap = new HashMap<>();
newMap.put("a_b", map.get("a")+"_"+map.get("b"));
newListOfMaps.add(newMap);
}
System.out.println(newListOfMaps);
What is the best-practice to follow while doing this in Scala? Any help or suggestion would be great!

Answer 1

Does this look like what you're seeking?
Pay close attention to the syntax - you will notice it's far easier to construct a List and Map in Scala than in Java.
scala> val listOfMaps: List[Map[String, String]] = List(
Map("a" -> "A1", "b" -> "B1"),
Map("a" -> "A2", "b" -> "B2"),
Map("a" -> "A3", "b" -> "B3")
)
listOfMaps: List[Map[String,String]] = List(Map(a -> A1, b -> B1), Map(a -> A2, b -> B2), Map(a -> A3, b -> B3))
scala> val mapsCombined: List[Map[String, String]] = listOfMaps.map{ m =>
Map("a_b" -> s"${m.apply("a")}_${m.apply("b")}") // string interpolation to avoid String concatenation
}
mapsCombined: List[Map[String,String]] = List(Map(a_b -> A1_B1), Map(a_b -> A2_B2), Map(a_b -> A3_B3))
For mapsCombined we can do
listOfMaps.map{ m =>
Map("a_b" -> s"${m("a")}_${m("b")}")
}
// no .apply as calling with the paren directly is it's equivalent in Scala, also the same as .get on Java.

Answer 2

Use Scala's map function to transform the list elements (i.e. Maps) into their corresponding concatenated keys-values:
scala> val mapList: List[Map[String, String]] = List(
Map("a" -> "A1", "b" -> "B1"), Map("a" -> "A2", "b" -> "B2"), Map("a" -> "A3", "b" -> "B3")
)
mapList: List[Map[String,String]] = List(Map(a -> A1, b -> B1), Map(a -> A2, b -> B2), Map(a -> A3, b -> B3))
scala> val newMapList: List[Map[String, String]] = mapList.map( m =>
Map(m.keys.mkString("_") -> m.values.mkString("_"))
)
newMapList: List[Map[String,String]] = List(Map(a_b -> A1_B1), Map(a_b -> A2_B2), Map(a_b -> A3_B3))
Note that the same function will work if you decide to expand the original Maps to, say:
Map("a" -> "A1", "b" -> "B1", c -> "C1", ...),
Map("a" -> "A2", "b" -> "B2", c -> "C2", ...),
...