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."
Detail We invoke the distinct function on the furniture list. A new list, with duplicates removed, is returned.
// 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.
println(furniture)
println(result)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.
Note ToSet converts the list to a set. Duplicates are removed because a set cannot store duplicates.
Note 2 We use ToList to convert the set back into a list. The ordering may be changed by the set.
// Create a list of Ints.
val ids = List(10, 10, 1, 2, 3, 3)
println(ids)
// Convert list to set.// ... Duplicate elements are removed at this step.
val set = ids.toSet
println(set)
// Convert set to list.
val ids2 = set.toList
println(ids2)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.
val codes = List("abC", "Abc", "ABC", "xyz", "XyZ")
println(codes)
// Convert all strings to uppercase.// ... Then get distinct strings.
val result = codes.map(_.toUpperCase()).distinct
println(result)List(abC, Abc, ABC, xyz, XyZ)
List(ABC, XYZ)
Sets, maps. Some collections enforce unique elements. These never need to have duplicates removed—duplicates never occur. Sets and maps cannot store duplicates.
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.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Apr 21, 2023 (edit).
