In this article , lets discuss about Kotlin Collections operation - Filter functions

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Filtering a collection is an everyday task in dev life. In Kotlin , Filtering conditions are defined by predicates

predicate

• takes a collection value and returns a boolean value

  if it returns true -> given element matches predicate

  if it returns false -> given element not matches predicate

Filter by predicate

filter()

• when called with predicate , filter() returns the list of elements that matches the condition

• For both List and Set , the result is List

• For Map , the result is Map

   val numbers = listOf(62, 51, 67, 32, 45, 12)

    //filter for list and set
    val result = numbers.filter { it > 50 }
    println(result)
    //output: [62, 51, 67]

   val studentInfo = mapOf(
        "Student 1" to 18,
        "Student 99" to 19,
        "Student 101" to 21,
        "Student 199" to 23
    )
    //filter for map
    val seniorStudent = studentInfo.filter { (student, age) ->
        student.endsWith("9") && age > 20
    }
    println(seniorStudent)
    //output: {Student 199=23}

filterIndexed()

• If we need the element position in filter we can use filterIndexed()

• It takes the predicate with two arguments : index and value

   val numbers = listOf(62, 51, 67, 32, 45, 12)

    // filer indexed
    val filterIndexedResult = numbers.filterIndexed { index, i ->
        index != 0 && i > 60
    }
    println(filterIndexedResult)
    //output : [67]

filterNot()

• filter collections by negative conditions

• returns the list of all elements that predicate returned false

   val numbers = listOf(62, 51, 67, 32, 45, 12)

   //filterNot
    val filterNotResult = numbers.filterNot {
        it > 50
    }
    println(filterNotResult)
    //output : [32, 45, 12]

filterIsInstance()

• returns the collection of given type

• if we call on List<Any> with filterIsInstance<String>() , the result is list of strings i.e List<String>

    val typesList = listOf(1, null, "one", 'f', 3, null, 4.56, 78.9087, "twenty")

    //filterIsInstance
    val filterInstanceResult = typesList.filterIsInstance<Double>()
    println(filterInstanceResult)
    //output : [4.56, 78.9087]

filterNotNull()

• returns all non-nullable elements in the collection

• if we call on List<T> , it returns List<T : Any>

    val typesList = listOf(1, null, "one", 'f', 3, null, 4.56, 78.9087, "twenty")

    // filterNotNull
    val filterNotNullResult = typesList.filterNotNull()
    println(filterNotNullResult)
    //output: [1, one, f, 3, 4.56, 78.9087, twenty]

Partition

partition()

• filters the given collection by predicate and keeps the elements that does not match in a separate list

• returns pair of list , first one containing the elements matching the predicate and the second one not matching the predicate

    val numbers = listOf(23, 56, 34, 67, 12, 73, 8, 82, 45, 90)

    val partition = numbers.partition {
        it > 50
    }

    println(partition.first)
    //output : [56, 67, 73, 82, 90]
    println(partition.second)
    //output : [23, 34, 12, 8, 45]

Please leave your comments to improve.

Happy and Enjoy coding