CODEWITHSUNDEEP
algorithmkotlin
jturolla
jturolla

Reputation: 6746

How to remove duplicate strings from an array in Kotlin

How to remove duplicates from an Array<String?> in Kotlin?

Upvotes: 179

Views: 110550

Answers (2)

waterstopper
waterstopper

Reputation: 568

Algorithm

If you need to remove duplicates in-place use the following extension function:

fun <T : Comparable<T>> Array<T?>.distinctInPlace(): Int {
    this.sortBy { it }
    var placed = 1
    var removed = 0
    var i = 1
    while (i < size) {
        if (this[i] == this[i - 1])
            removed++
        else {
            this[placed] = this[i]
            placed++
        }
        i++
    }
    for (iter in size - removed..lastIndex)
        this[iter] = null
    return size - removed
}

This method will return the amount of unique elements in O(n log(n)) time. All of them will be sorted. Last for loop is used to set all other elements to null.

Note: if you had a null element in the array, it will be placed at the 0 index - so you can distinguish whether you had any nulls or they were added after.

Examples

fun main() {
    val arr = arrayOf("a", null, "b", null, "c", "ab", "ab")
    arr.distinctInPlace() // returns 5, arr is now [null, "a", "ab", "b", "c", null, null]

    val withoutNulls = arrayOf("a", "a", "aa", "aaa", "aa")
    withoutNulls.distinctInPlace() // returns 3, arr is now ["a", "aa", "aaa"]
}

Upvotes: 1

hotkey
hotkey

Reputation: 147901

Use the distinct extension function:

val a = arrayOf("a", "a", "b", "c", "c")
val b = a.distinct() // ["a", "b", "c"]

There's also distinctBy function that allows one to specify how to distinguish the items:

val a = listOf("a", "b", "ab", "ba", "abc")
val b = a.distinctBy { it.length } // ["a", "ab", "abc"]

As @mfulton26 suggested, you can also use toSet, toMutableSet and, if you don't need the original ordering to be preserved, toHashSet. These functions produce a Set instead of a List and should be a little bit more efficient than distinct.

You may find useful:

Upvotes: 349

Related Questions