Set functions in kotlin

This article will introduce some Kotlin Set function , These functions can greatly improve our development efficiency , It can be read as a cheat sheet at any time ~

Delete array Duplicate string in
There are many ways to remove duplicate strings from an array ：

//  Keep the original order of elements 
val devs = arrayOf("Amit", "Ali", "Amit", "Sumit", "Sumit", "Himanshu")
print(devs.distinct()) // [Amit, Ali, Sumit, Himanshu]
//  Keep the original order of elements 
val devs = arrayOf("Amit", "Ali", "Amit", "Sumit", "Sumit", "Himanshu")
print(devs.toSet()) // [Amit, Ali, Sumit, Himanshu]
//  Keep the original order of elements 
val devs = arrayOf("Amit", "Ali", "Amit", "Sumit", "Sumit", "Himanshu")
print(devs.toMutableSet()) // [Amit, Ali, Sumit, Himanshu]
//  The original order of elements is not preserved 
val devs = arrayOf("Amit", "Ali", "Amit", "Sumit", "Sumit", "Himanshu")
print(devs.toHashSet()) // [Amit, Ali, Sumit, Himanshu]

** take array To list perhaps string **
You can use joinToString Function will list To string . for example , There is a list of cities (Delhi, Mumbai, Bangalore),, Then you can turn the list into such a string ： “India is one the best country for tourism. You can visit Delhi, Mumbai, Bangalore, etc, and enjoy your holidays”.

val someKotlinCollectionFunctions = listOf(
    "distinct", "map",
    "isEmpty", "contains",
    "filter", "first",
    "last", "reduce",
    "single", "joinToString"
)
val message = someKotlinCollectionFunctions.joinToString(
    separator = ", ",
    prefix = "Kotlin has many collection functions like: ",
    postfix = "and they are awesome.",
    limit = 3,
    truncated = "etc "
)
print(message)

Kotlin There are also many great functions , such as distinct, map, isEmpty, wait

Turn a set into a single result
If you want to convert a given set into a single result , have access to rudece function . For example, you can sum all the elements in the list ：

val numList = listOf(1, 2, 3, 4, 5)
val result = numList.reduce {
     result, item ->
    result + item
}
print(result) // 15
//  Be careful : If the list is empty , Will throw out RuntimeException 

Determine whether all elements meet specific conditions
If you have an array or list , You want to know whether all the elements meet a certain condition , have access to all function ：

data class User(val id: Int, val name: String, val isCricketLover: Boolean, val isFootballLover: Boolean)
val user1 = User(id = 1, name = "Amit", isCricketLover = true, isFootballLover = true)
val user2 = User(id = 2, name = "Ali", isCricketLover = true, isFootballLover = true)
val user3 = User(id = 3, name = "Sumit", isCricketLover = true, isFootballLover = false)
val user4 = User(id = 4, name = "Himanshu", isCricketLover = true, isFootballLover = false)
val users = arrayOf(user1, user2, user3, user4)
val allLoveCricket = users.all {
     it.isCricketLover }
print(allLoveCricket) // true
val allLoveFootball = users.all {
     it.isFootballLover }
print(allLoveFootball) // false

Find an element that meets the given conditions
You can use find Function finds an element in the list that meets the given condition . such as , Find in a student list id yes 5 Of the students .find The function returns the first element that matches the given condition , If not found , Returns the null.
single The function returns the only element that matches the given condition , If there are multiple elements that meet the conditions or there are no elements that meet the conditions , It throws an exception ：

data class User(val id: Int, val name: String)
val users = arrayOf(
    User(1, "Amit"),
    User(2, "Ali"),
    User(3, "Sumit"),
    User(4, "Himanshu")
)
val userWithId3 = users.single {
     it.id == 3 }
print(userWithId3) // User(id=3, name=Sumit)
val userWithId1 = users.find {
     it.id == 1 }
print(userWithId1) // User(id=1, name=Amit)

Split the list into smaller lists
When you have a larger list , Want to split it into multiple sub lists , Then do some operations on these sub lists . At this time , have access to chunked function ：

val numList = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
val chunkedLists = numList.chunked(3)
print(chunkedLists) // [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10]]

Copy the array
You can use these functions to copy existing arrays ：

• copyInto: This function will copy the elements of the specified range of its own array to the specified position of another array , If the target array subscript is out of bounds , An array subscript out of bounds exception will be thrown ：

val arrayOne = arrayOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
val arrayTwo = arrayOf(11, 12, 13, 14, 15, 16, 17, 18, 19, 20)
arrayOne.copyInto(destination = arrayTwo, destinationOffset = 2, startIndex = 0, endIndex = 4)
arrayTwo.forEach {
    print("$it ")} // 11 12 1 2 3 4 17 18 19 20

Similarly , There are other functions that can copy array elements , for example ：

• copyOfRange(fromIndex, toIndex): Returns a new array that copies the elements of the specified range of the original array
• copyOf() or copyOf(newSize): Returns a copy of the original array with a length of newSize New array , If not specified newSize, Copy the entire array by default .

Change the type of set
You can change the type of set according to the situation . You can create a new set of other types by referencing the old set :
toIntArray, toBooleanArray, toLongArray, toShortArray, toByteArray, toDoubleArray, toList, toMap, toSet, toPair, And other functions can change the type of set etc can be used to change the type of one collection to another type.

var uIntArray = UIntArray(5) {
     1U }
var intArray = uIntArray.toIntArray()
intArray[0] = 0
print(uIntArray.toList()) // [1, 1, 1, 1, 1]
print(intArray.toList()) // [0, 1, 1, 1, 1]

Here we create a new set and merge and change the elements in the new set , This will not affect the collection . meanwhile , We can also hold references to old sets to change the set type , Changing one set will affect the other set . Like this with as Function of prefix ：asIntArray, asLongArray, asShortArray, asByteArray, asList wait ：

var uIntArray = UIntArray(5) {
     1U }
var intArray = uIntArray.asIntArray()
intArray[0] = 0
print(uIntArray.toList()) // [0, 1, 1, 1, 1]
print(intArray.toList()) // [0, 1, 1, 1, 1]

take key Associated with data
If you have a list of data , I want to compare the data with key Connect , have access to associateBy function ：

data class Contact(val name: String, val phoneNumber: String)
val contactList = listOf(
    Contact("Amit", "+9199XXXX1111"),
    Contact("Ali", "+9199XXXX2222"),
    Contact("Himanshu", "+9199XXXX3333"),
    Contact("Sumit", "+9199XXXX4444")
)
val phoneNumberToContactMap = contactList.associateBy {
     it.phoneNumber }
print(phoneNumberToContactMap)
// Map with key: phoneNumber and value: Contact
// {
    
// +9199XXXX1111=Contact(name=Amit, phoneNumber=+9199XXXX1111),
// +9199XXXX2222=Contact(name=Ali, phoneNumber=+9199XXXX2222),
// +9199XXXX3333=Contact(name=Himanshu, phoneNumber=+9199XXXX3333),
// +9199XXXX4444=Contact(name=Sumit, phoneNumber=+9199XXXX4444)
// }

In the example above ,key yes phoneNumber ,value yes Contact. If you don't want to put the whole Contact As value, Then you can pass on your expectations like this ：

val phoneNumberToContactMap = contactList.associateBy({
    it.phoneNumber}, {
    it.name})
print(phoneNumberToContactMap)
// Map with key: phoneNumber and value: name
// {
    
// +9199XXXX1111=Amit, 
// +9199XXXX2222=Ali, 
// +9199XXXX3333=Himanshu, 
// +9199XXXX4444=Sumit}
// }

Find the only element in the collection
We can call distinct Function to get the unique set of elements in the list

val list = listOf(1, 2, 2, 3, 3, 3, 4, 4, 4, 4)
println(list.distinct()) // [1, 2, 3, 4]

Joint set
call union Function can get the only element in two sets , The order of elements in the original set will also be preserved , The elements of the second set will be added to the elements of the first set

val listOne = listOf(1, 2, 3, 3, 4, 5, 6)
val listTwo = listOf(2, 2, 4, 5, 6, 7, 8)
println(listOne.union(listTwo)) // [1, 2, 3, 4, 5, 6, 7, 8]

Take the intersection of sets
In order to get the elements common to two sets , have access to intersect function , It returns the set of common elements that appear in both sets

val listOne = listOf(1, 2, 3, 3, 4, 5, 6)
val listTwo = listOf(2, 2, 4, 5, 6, 7, 8)
println(listOne.intersect(listTwo)) // [2, 4, 5, 6]

Keep only specific elements
Use retainAll Functions retain specific elements , This function will modify the original list , So make sure that the list or array can be modified
If an element is deleted from the list ,retainAll Returns the true, Otherwise it will return to false

val listOne = mutableListOf(1, 2, 3, 3, 4, 5, 6)
val listTwo = listOf(1, 2, 3, 3, 4, 5, 6)
val listThree = listOf(1, 2, 3, 3, 4, 5, 7)
println(listOne.retainAll(listTwo)) // false
println(listOne.retainAll(listThree)) // true
println(listOne) // [1, 2, 3, 3, 4, 5]

Similarly , Use removeAll Function can delete elements in a set that appear in another set .

Filter sets based on conditions
filter Function can filter out elements in the set that do not meet the specified conditions , It will return a list of all elements that meet this condition ：

val list = listOf(1, 2, 3, 4, 5, 6, 7, 8)
val filteredList = list.filter {
     it % 2 == 0 }
print(filteredList) // [2, 4, 6, 8]

filterIndexed You can filter elements based on subscripts . If you want to save the filtered elements in a collection , that , use filterIndexedTo Function ：

val list = listOf(1, 2, 3, 4, 5, 6, 7, 8)
val filteredList = mutableListOf<Int>()
list.filterIndexedTo(filteredList) {
     index, i -> list[index] % 2 == 0 }
print(filteredList) // [2, 4, 6, 8]

You can use filterIsInstance Function to filter out elements that meet specific types of instances ：

val mixedList = listOf(1, 2, 3, "one", "two", 4, "three", "four", 5, 6, "five", 7)
val strList = mixedList.filterIsInstance<String>()
print(strList) // [one, two, three, four, five]

Compress the set
zip Function returns a list of two tuples , The first element of a two tuple is taken from the first set , The second element song, the second set . The length of the returned list is equal to the length of the shorter of the two sets ：

val listOne = listOf(1, 2, 3, 4, 5)
val listTwo = listOf("a", "b", "c", "d", "e", "f")
print(listOne zip listTwo) // [(1, a), (2, b), (3, c), (4, d), (5, e)]

zipWithNext Function returns a list of two tuples , The elements of two tuples are composed of adjacent elements in the set ：

val list = listOf(1, 2, 3, 4, 5)
print(list.zipWithNext()) // [(1, 2), (2, 3), (3, 4), (4, 5)]

Decompress the collection
unzup Function returns a list Binary , The first list in a tuple consists of the first element of a two tuple in the original set , The second element consists of the second tuple in the original set ：

val list = listOf("Amit" to 8, "Ali" to 10, "Sumit" to 4, "Himanshu" to 2)
val (players, footballSkills) = list.unzip()
print(players) // [Amit, Ali, Sumit, Himanshu]
print(footballSkills) // [8, 10, 4, 2]

Divide the array into two parts
If you want to divide an array into two parts by some conditions ,partition Function can meet the requirements ：

data class User(val id: Int, val name: String, val isFootballLover: Boolean)
val users = listOf(
    User(1, "Amit", true),
    User(2, "Ali", true),
    User(3, "Sumit", false),
    User(4, "Himanshu", false)
)

val (footballLovers, nonFootballLovers) = users.partition {
     it.isFootballLover }

print(footballLovers) // [User(id=1, name=Amit, isFootballLover=true), User(id=2, name=Ali, isFootballLover=true)]

print(nonFootballLovers) // [User(id=3, name=Sumit, isFootballLover=false), User(id=4, name=Himanshu, isFootballLover=false)]

Reverse list
reversed Functions and asReversed Function can reverse the list . The difference is ：reversed It can be applied to Array, List, and MutableList On , Function returns a new list after inversion ; asReversed It can be used for List, and MutableList On , The list returned by the function still refers to the old list , Change one of them , Will affect another list ：

val list = listOf(1, 2, 3, 4, 5)
print(list.reversed()) // [5, 4, 3, 2, 1]
print(list.asReversed()) // [5, 4, 3, 2, 1]

Similarly , also reversedArray,reverse Two functions .

Group collection elements
groupBy Function can group set elements based on a certain condition ：

val list = listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
print(list.groupBy {
     it % 4 })
// {
    
// 1=[1, 5, 9], 
// 2=[2, 6, 10], 
// 3=[3, 7], 
// 0=[4, 8]
// }

Sort collection elements
sorted Function can sort collection elements , And return a sorted list ：

val list = listOf(10, 4, 1, 3, 7, 2, 6)
print(list.sorted()) // [1, 2, 3, 4, 6, 7, 10]

besides , There are also many sorting functions , Such as ：sortedArray, sortedArrayWith, sortedBy, sortedByDescending, sortedArraydescending, sortedWith wait .