In this article , lets discuss about Kotlin Collections operation - Plus-Minus and grouping functions

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Plus and Minus

• In Kotlin , + and - operators are used in collection

• The first operand is a collection where as second operand can either be an element or another collection

• returns a read only collection

Plus ( + )

• return a collection containing elements from both first and second operand

Minus ( - )

• returns a collection containing elements from original collection except the elements of second operand

• If the second operand is

  an element , minus remove its first occurrence

  a collection , all occurrence of its elements are removed

    val numberList = listOf(1, 3, 5, 7, 9)
    val numberOperand = listOf(3, 5)

    val plusResultList = numberList.plus(numberOperand)
    println(plusResultList)
    // output : [1, 3, 5, 7, 9, 3, 5]

    val minusResultList = numberList.minus(numberOperand)
    println(minusResultList)
    //output : [1, 7, 9]

    val number = 7
    val resultList = numberList.minus(number)
    println(resultList)
    //output : [1, 3, 5, 9]

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Grouping

groupBy()

• takes a lambda function and returns a map

• In the Result

  key - lambda result

  value- list of elements on which the lambda result is obtained

       val numbers = listOf(22, 56, 44, 67, 88, 45, 77)
  
        // groupBy
        val groupByResult = numbers.groupBy {
            it % 11 == 0
        }
        println(groupByResult)
        //output : {true=[22, 44, 88, 77], false=[56, 67, 45]}
  

groupBy() - 2 Lambda argument

• Second lambda argument is a value transformation function

• In Result

  key - produced by keySelector function

  value - result of valueTransform function

    val numbers = listOf(22, 56, 44, 67, 88, 45, 77)

    val result = numbers.groupBy(
        keySelector = { it % 2 == 0 },
        valueTransform = {
            if (it % 2 == 0) {
                "$it is an Even number"
            } else {
                "$it is not an Even number"
            }
        })
    println(result)
    //output : {
//           true=[22 is an Even number,
//              56 is an Even number,
//              44 is an Even number,
//              88 is an Even number],
//           false=[67 is not an Even number,
//              45 is not an Even number,
//              77 is not an Even number]}

groupingBy()

• helps to group elements and apply an operation to all groups at one time

• returns an instance of Grouping type

• applies the operation in lazy manner : the groups are actually built right before the operation execution

• supports the following execution

  1.eachCount() - counts the element in each group

  2.fold() & reduce() - perform fold and reduce operations on each group as separate collection and return results

  3.aggregate() - applies a given operation subsequently to all elements in each group and return results

    val oddNumbers = listOf(1, 7, 5, 7, 9)
    val groupingByResult = oddNumbers.groupingBy {
        it * 2
    }.eachCount()

    println(groupingByResult)
    // output : {2=1, 14=2, 10=1, 18=1}

Please leave your comments to improve.

Happy and Enjoy coding