Scala Remove Duplicates From List: Distinct Example
Use the distinct function, along with map, to remove duplicate elements from lists.
Remove duplicates. A list contains strings: chair, table, table. It has a duplicate string. With Scala we have many ways to remove duplicates from our list.
With distinct, a method on the List type, we eliminate duplicates and retain a list's order. Other approaches are possible. We can convert the list to a set: this also dedupes a list.
An example. Let us begin with this example. We create a constant, immutable List of furniture strings. The list has two instances of the string "table."
Distinct: We invoke the distinct function on the furniture list. A new list, with duplicates removed, is returned.
Scala program that removes duplicates from list// Create a list of strings.
val furniture = List("chair", "bed", "table", "table", "couch")
// Get distinct strings.
// ... This removes duplicates but retains order.
val result = furniture.distinct
// Print results.
List(chair, bed, table, table, couch)
List(chair, bed, table, couch)
ToSet, toList. Here we use toSet and toList to strip duplicate Ints. We create a list that has six Ints, and two duplicate Ints. We then remove those duplicates.
ToSet: This converts the list to a set. Duplicates are removed because a set cannot store duplicates.
ToList: We convert the set back into a list. Please note that ordering may be changed by the set.
Scala program that uses toSet, toList// Create a list of Ints.
val ids = List(10, 10, 1, 2, 3, 3)
// Convert list to set.
// ... Duplicate elements are removed at this step.
val set = ids.toSet
// Convert set to list.
val ids2 = set.toList
List(10, 10, 1, 2, 3, 3)
Set(10, 1, 2, 3)
List(10, 1, 2, 3)
Map, distinct. With the map method we can transform all elements into a standard form. This may result in duplicates. With distinct we can remove the dupes.
Here: We use a lambda expression to map all strings to uppercase forms. A more complex method could be applied.
Scala program that uses map and distinct
val codes = List("abC", "Abc", "ABC", "xyz", "XyZ")
// Convert all strings to uppercase.
// ... Then get distinct strings.
val result = codes.map(_.toUpperCase()).distinct
List(abC, Abc, ABC, xyz, XyZ)
Sets, maps. Some collections enforce unique elements. These never need to have duplicates removed—duplicates never occur. Sets and maps cannot store duplicates.SetMap
A summary. Removing duplicates from a list is an essential task in programming. It helps us understand basic manipulations of lists. And it is useful in real programs.