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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. println(furniture) println(result) Output 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) 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) Output 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") println(codes) // Convert all strings to uppercase. // ... Then get distinct strings. val result = println(result) Output 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.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.
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