I'm trying to wrap my head around map and reduce operations in Kotlin. At least, I guess it's reduce what I'm trying to do.
Let's say that I have a class called Car that takes any number (varargs constructor) of CarPart. Then, I have a list of CarPart which I'll do a map operation and from the result of the operation I need to build one Car using each subelement, something along these lines:
class CarPart(val description: String)
class Car(vararg val carPart: CarPart)
val carParts = listOf(CarPart("Engine"), CarPart("Steering Wheel")))
carParts.map { it.description.toUpperCase() }
.map { CarPart(it) }
.reduce { acc, carPart -> Car(carPart) } <--- I'm struggling here, is reduce what I should be doing
to construct one car from all the subelement?
PS.1: I know that the class design could be better and not take a varargs, this is just an example of a legacy application I'm refactoring and originally that's a Java class taking varargs which I can't change now.
PS.2: The example of mapping to a String and then creating an object out of that String is just for the sake of the example. The actual code grabs an object within the list.
You can simply use a the spread operator (*) over an array:
val mappedCarParts = carParts
.map { it.description.toUpperCase() }
.map { CarPart(it) }
.toTypedArray()
val car = Car(*mappedCarParts)
// Or even:
val car = carParts
.map { it.description.toUpperCase() }
.map { CarPart(it) }
.toTypedArray()
.let{ Car(*it) }
You could just extract the constructor of the Car outside of the creation of the list. I don't see any reason as to why you'd want it inside.
val car = Car(
*carParts
.map { CarPart(it.description.uppercase(Locale.getDefault())) } //keep the .toUpperCase() if you are using an old version of Kotlin
.toTypedArray()
)
We need the spread operator there in order for the vararg to know that we are passing it the elements of the list and not the list itself.
Related
I have a list for example of type People. My list can contain only elements of type Student or only elements of type Worker:
interface People {
val name: String
val age: Int
}
data class Student(
override val name: String,
override val age: Int,
val course: Int
) : People
data class Worker(
override val name: String,
override val age: Int,
val position: String
) : People
At some point I need to know the exact type of the list (student or worker).
Can I safely find out the exact type? So far I've written this code, but it doesn't look very good:
fun someLogic(items: List<People>): List<People> {
return (items as? List<Student>) ?: (items as? List<Worker>)
?.filter {}
....
}
Also, I get a warning:
Unchecked cast
Can you please tell me how to perform such transformations correctly?
At runtime, the type parameter you used to create the list is not available. e.g. it is impossible to distinguish between the following two situations:
val students: List<People> = listOf<Student>(student1, student2)
val people: List<People> = listOf<People>(student1, student2)
This is because of type erasure.
The only information you have at runtime that can help determine a list's element type is the type of its elements.
So if a list has no elements, there is no way of knowing what type of list it is. Though in most situations, you don't need to anyway.
So assuming the list can only be a list of all students, or a list of all workers, but not a list containing a mixture of students and workers, you can determine the type of the list by checking the first element.
when (items.firstOrNull()) {
null -> { /* cannot determine the type */ }
is Student -> { /* is a list of students */ }
is Worker -> { /* is a list of worker */ }
// you can remove this branch by making the interface sealed
else -> { /* someone made another class implementing People! */ }
}
If you want to get a List<Student> or List<Worker> out of this on the other hand, you can just use filterIsInstance:
val students = items.filterIsInstance<Student>()
val worker = items.filterIsInstance<Worker>()
whichever list is not empty, then the type of items is the type of that list.
If you want to check that List<People> is List<Student> you can use this extension function:
fun List<People>.isStudentList(): Boolean {
// returns true if no element is not Student, so all elements are Student
return all { it is Student }
}
And if you want to cast List<People> to List<Student>, you can use map, and this cast is safe so let's say that there is some People that the are not Student so the cast is going to return null instead of Student because of as? and the mapNotNull is going to exclude null elements so in worst cases where you pass a list that doesn't contain any Student this function is going to return an empty list:
fun List<People>.toStudentList(): List<Student> {
// This is going to loop through the list and cast each People to Student
return mapNotNull { it as? Student }
}
Or you can just use filterIsInstance<Student> this will work the same as toStudentList above:
list.filterIsInstance<Student>()
And the same approach can be used for Worker
I would solve the problem with more specific classes.
You can define:
interface PeopleList<P : People> : List<P>
class StudentList : PeopleList<Student> {
// add implementation
}
class WorkerList : PeopleList<Worker> {
// add implementation
}
You can then easily check the types of these lists. Each of those classes can then provide guarantees that you are not mixing Student and Worker objects in the same List, something you can't do with plain List<People> objects.
Note also you are better off writing your code avoiding checking types if at all possible. Much better to add methods to the PeopleList interface and force the subclasses to implement them, for example:
interface PeopleList<P : People> : List<P> {
fun doSomethingGood()
}
Then you can call these methods at the appropriate time, instead of checking the type. This approach keeps the functionality associated with the subtypes alongside those subtypes and not scattered through the code at the various points where you have to check the type of PeopleList.
I've been using kotlin arrow quite a bit recently, and I've ran into a specific use case that has me stuck.
Let's say I have a collection of some object that I want to convert to another datatype using a convert function. Let's also say that this convert function has an ability to fail-- but instead of throwing an exception, it will just return an Either, where Either.Left() is a failure and Either.Right() is the mapped object. What is the best way to handle this use case? Some sample code below:
val list: Collection<Object> // some collection
val eithers: List<Either<ConvertError, NewObject>> = list.map { convert(it) } // through some logic, convert each object in the collection
val desired: Either<ConvertError, Collection<NewObject>> = eithers.map { ??? }
fun convert(o: Object) : Either<ConvertError, NewObject> { ... }
Essentially, I'd like to call a mapping function on a collection of data, and if any of the mappings respond with a failure, I'd like to have an Either.Left() containing the error. And then otherwise, I'd like the Either.Right() to contain all of the mapped objects.
Any ideas for a clean way to do this? Ideally, I'd like to make a chain of function calls, but have the ability to percolate an error up through the function calls.
You can use Arrow's computation blocks to unwrap Either inside map like so:
import arrow.core.Either
import arrow.core.computations.either
val list: ListObject> // some collection
val eithers: List<Either<ConvertError, NewObject>> = list.map { convert(it) } // through some logic, convert each object in the collection
val desired: Either<ConvertError, Collection<NewObject>> = either.eager {
eithers.map { convert(it).bind() }
}
fun convert(o: Object) : Either<ConvertError, NewObject> { ... }
Here bind() will either unwrap Either into NewObject in the case Either is Right, or it will exit the either.eager block in case it finds Left with ConvertError. Here we're using the eager { } variant since we're assigning it to a val immediately. The main suspend fun either { } block supports suspend functions inside but is itself also a suspend function.
This is an alternative to the traverse operator.
The traverse operation will be simplified in Arrow 0.12.0 to the following:
import arrow.core.traverseEither
eithers.traverseEither(::convert)
The traverse operator is also available in Arrow Fx Coroutines with support for traversing in parallel, and some powerful derivatives of this operation.
import arrow.fx.coroutines.parTraverseEither
eithers.parTraverseEither(Dispatcheres.IO, ::convert)
This is a frequent one, what you're looking for is called traverse. It's like map, except it collects the results following the aggregation rules of the content.
So, list.k().traverse(Either.applicative()) { convert(it) } will return Either.Left is any of the operations return Left, and Right<List< otherwise.
How about arrow.core.IterableKt#sequenceEither?
val desired: Either<ConvertError, Collection<NewObject>> = eithers.sequenceEither()
I'm new to Kotlin and lambdas and I'm trying to understand it. I'm trying to generate a list of 100 random numbers.
This works:
private val maxRandomValues = (1..100).toList()
But I want to do something like that:
private val maxRandomValues = (1..100).forEach { RandomGenerator().nextDouble() }.toList()
But this is not working. I'm trying to figure out how to use the values generated into forEach are used in the toList()
It's way better to use kotlin.collections function to do this:
List(100) {
Random.nextInt()
}
According to Collections.kt
inline fun <T> List(size: Int, init: (index: Int) -> T): List<T> = MutableList(size, init)
It's also possible to generate using range like in your case:
(1..100).map { Random.nextInt() }
The reason you can't use forEach is that it return Unit (which is sort of like void in Java, C#, etc.). map operates Iterable (in this case the range of numbers from 1 to 100) to map them to a different value. It then returns a list with all those values. In this case, it makes more sense to use the List constructor above because you're not really "mapping" the values, but creating a list
Beside using constructor: List(10, { Random.nextInt() }),
Kotlin also has a built-in function for this purpose: buildList { repeat(10) { add(Random.nextInt()) } }
See: https://kotlinlang.org/docs/constructing-collections.html
I am using Kotlin for a project, I write this code can complete the requirement:
val rewards = ArrayList<Map<String, Int>>()
rangeExams
.forEach { examAnswer ->
var reward = hashMapOf("Score" to examAnswer.answerScore)
var questionIds = examAnswer
.answers
.map { it.id }
reward.put("NewQuestion", questionIds.size)
rewards.add(reward)
}
"rangeExams" is a list of collection.
I would like to combinate Kotlin Functions of Collection,
to put elements of rangeExams into a map
and put this map to a new list,
how can I simplify this code by Kotlin ?
ExamAnswer is a pojo:
class ExamAnswer (val id: String, val answerScore: Int, val answers:List<Answer>)
Thank you for your reply
Since you add an item to the rewards for each element of rangeExams, the .forEach { ... } call can be transformed to .map { ... }.
Also, you only use the result of examAnswer.answers.map { it.id } to get its size, so you can remove .map { it.id } and use the size of the original collection.
If you don't need to mutate the maps afterwards, you can replace hashMapOf(...) with mapOf(...).
val rewards = rangeExams.map {
mapOf(
"Score" to it.answerScore,
"NewQuestion" to it.answers.size)
}
If you need to mutate the rewards list after it's created, add .toMutableList() in the end.
There is a little potential to simplify this.
Firstly, I would suggest a more functional approach, which could turn the mutable list rewards into an immutable one.
Secondly, infer the creation of the hash-map reward with the put into one line. You than can use the also the immutable version of the map, instead of the mutable one created by hashMapOf (if you need mutability, than you can just keep hashMapOf).
thirdly, you just use the questionIds to the the size. For that, you don't have to map anything, just call examAnswer.ansers.size. This short call can be inferred as well
fourthly, you can use it instead of explicitly name the param examAnswer because this block is now quite short anyway
This would lead to this code:
val rewards = rangeExams.map {
mapOf("Score" to it.answerScore,
"NewQuestion" to it.answers.size)
}
I wish to have a good example for each function run, let, apply, also, with
I have read this article but still lack of an example
All these functions are used for switching the scope of the current function / the variable. They are used to keep things that belong together in one place (mostly initializations).
Here are some examples:
run - returns anything you want and re-scopes the variable it's used on to this
val password: Password = PasswordGenerator().run {
seed = "someString"
hash = {s -> someHash(s)}
hashRepetitions = 1000
generate()
}
The password generator is now rescoped as this and we can therefore set seed, hash and hashRepetitions without using a variable.
generate() will return an instance of Password.
apply is similar, but it will return this:
val generator = PasswordGenerator().apply {
seed = "someString"
hash = {s -> someHash(s)}
hashRepetitions = 1000
}
val pasword = generator.generate()
That's particularly useful as a replacement for the Builder pattern, and if you want to re-use certain configurations.
let - mostly used to avoid null checks, but can also be used as a replacement for run. The difference is, that this will still be the same as before and you access the re-scoped variable using it:
val fruitBasket = ...
apple?.let {
println("adding a ${it.color} apple!")
fruitBasket.add(it)
}
The code above will add the apple to the basket only if it's not null. Also notice that it is now not optional anymore so you won't run into a NullPointerException here (aka. you don't need to use ?. to access its attributes)
also - use it when you want to use apply, but don't want to shadow this
class FruitBasket {
private var weight = 0
fun addFrom(appleTree: AppleTree) {
val apple = appleTree.pick().also { apple ->
this.weight += apple.weight
add(apple)
}
...
}
...
fun add(fruit: Fruit) = ...
}
Using apply here would shadow this, so that this.weight would refer to the apple, and not to the fruit basket.
Note: I shamelessly took the examples from my blog
There are a few more articles like here, and here that are worth to take a look.
I think it is down to when you need a shorter, more concise within a few lines, and to avoid branching or conditional statement checking (such as if not null, then do this).
I love this simple chart, so I linked it here. You can see it from this as written by Sebastiano Gottardo.
Please also look at the chart accompanying my explanation below.
Concept
I think it as a role playing way inside your code block when you call those functions + whether you want yourself back (to chain call functions, or set to result variable, etc).
Above is what I think.
Concept Example
Let's see examples for all of them here
1.) myComputer.apply { } means you want to act as a main actor (you want to think that you're computer), and you want yourself back (computer) so you can do
var crashedComputer = myComputer.apply {
// you're the computer, you yourself install the apps
// note: installFancyApps is one of methods of computer
installFancyApps()
}.crash()
Yup, you yourself just install the apps, crash yourself, and saved yourself as reference to allow others to see and do something with it.
2.) myComputer.also {} means you're completely sure you aren't computer, you're outsider that wants to do something with it, and also wants it computer as a returned result.
var crashedComputer = myComputer.also {
// now your grandpa does something with it
myGrandpa.installVirusOn(it)
}.crash()
3.) with(myComputer) { } means you're main actor (computer), and you don't want yourself as a result back.
with(myComputer) {
// you're the computer, you yourself install the apps
installFancyApps()
}
4.) myComputer.run { } means you're main actor (computer), and you don't want yourself as a result back.
myComputer.run {
// you're the computer, you yourself install the apps
installFancyApps()
}
but it's different from with { } in a very subtle sense that you can chain call run { } like the following
myComputer.run {
installFancyApps()
}.run {
// computer object isn't passed through here. So you cannot call installFancyApps() here again.
println("woop!")
}
This is due to run {} is extension function, but with { } is not. So you call run { } and this inside the code block will be reflected to the caller type of object. You can see this for an excellent explanation for the difference between run {} and with {}.
5.) myComputer.let { } means you're outsider that looks at the computer, and want to do something about it without any care for computer instance to be returned back to you again.
myComputer.let {
myGrandpa.installVirusOn(it)
}
The Way to Look At It
I tend to look at also and let as something which is external, outside. Whenever you say these two words, it's like you try to act up on something. let install virus on this computer, and also crash it. So this nails down the part of whether you're an actor or not.
For the result part, it's clearly there. also expresses that it's also another thing, so you still retain the availability of object itself. Thus it returns it as a result.
Everything else associates with this. Additionally run/with clearly doesn't interest in return object-self back. Now you can differentiate all of them.
I think sometimes when we step away from 100% programming/logic-based of examples, then we are in better position to conceptualize things. But that depends right :)
There are 6 different scoping functions:
T.run
T.let
T.apply
T.also
with
run
I prepared a visual note as the below to show the differences :
data class Citizen(var name: String, var age: Int, var residence: String)
Decision depends on your needs. The use cases of different functions overlap, so that you can choose the functions based on the specific conventions used in your project or team.
Although the scope functions are a way of making the code more concise, avoid overusing them: it can decrease your code readability and lead to errors. Avoid nesting scope functions and be careful when chaining them: it's easy to get confused about the current context object and the value of this or it.
Here is another diagram for deciding which one to use from https://medium.com/#elye.project/mastering-kotlin-standard-functions-run-with-let-also-and-apply-9cd334b0ef84
Some conventions are as the following :
Use also for additional actions that don't alter the object, such as logging or printing debug information.
val numbers = mutableListOf("one", "two", "three")
numbers
.also { println("The list elements before adding new one: $it") }
.add("four")
The common case for apply is the object configuration.
val adam = Person("Adam").apply {
age = 32
city = "London"
}
println(adam)
If you need shadowing, use run
fun test() {
var mood = "I am sad"
run {
val mood = "I am happy"
println(mood) // I am happy
}
println(mood) // I am sad
}
If you need to return receiver object itself, use apply or also
let, also, apply, takeIf, takeUnless are extension functions in Kotlin.
To understand these function you have to understand Extension functions and Lambda functions in Kotlin.
Extension Function:
By the use of extension function, we can create a function for a class without inheriting a class.
Kotlin, similar to C# and Gosu, provides the ability to extend a class
with new functionality without having to inherit from the class or use
any type of design pattern such as Decorator. This is done via special
declarations called extensions. Kotlin supports extension functions
and extension properties.
So, to find if only numbers in the String, you can create a method like below without inheriting String class.
fun String.isNumber(): Boolean = this.matches("[0-9]+".toRegex())
you can use the above extension function like this,
val phoneNumber = "8899665544"
println(phoneNumber.isNumber)
which is prints true.
Lambda Functions:
Lambda functions are just like Interface in Java. But in Kotlin, lambda functions can be passed as a parameter in functions.
Example:
fun String.isNumber(block: () -> Unit): Boolean {
return if (this.matches("[0-9]+".toRegex())) {
block()
true
} else false
}
You can see, the block is a lambda function and it is passed as a parameter. You can use the above function like this,
val phoneNumber = "8899665544"
println(phoneNumber.isNumber {
println("Block executed")
})
The above function will print like this,
Block executed
true
I hope, now you got an idea about Extension functions and Lambda functions. Now we can go to Extension functions one by one.
let
public inline fun <T, R> T.let(block: (T) -> R): R = block(this)
Two Types T and R used in the above function.
T.let
T could be any object like String class. so you can invoke this function with any objects.
block: (T) -> R
In parameter of let, you can see the above lambda function. Also, the invoking object is passed as a parameter of the function. So you can use the invoking class object inside the function. then it returns the R (another object).
Example:
val phoneNumber = "8899665544"
val numberAndCount: Pair<Int, Int> = phoneNumber.let { it.toInt() to it.count() }
In above example let takes String as a parameter of its lambda function and it returns Pair in return.
In the same way, other extension function works.
also
public inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this }
extension function also takes the invoking class as a lambda function parameter and returns nothing.
Example:
val phoneNumber = "8899665544"
phoneNumber.also { number ->
println(number.contains("8"))
println(number.length)
}
apply
public inline fun <T> T.apply(block: T.() -> Unit): T { block(); return this }
Same as also but the same invoking object passed as the function so you can use the functions and other properties without calling it or parameter name.
Example:
val phoneNumber = "8899665544"
phoneNumber.apply {
println(contains("8"))
println(length)
}
You can see in the above example the functions of String class directly invoked inside the lambda funtion.
takeIf
public inline fun <T> T.takeIf(predicate: (T) -> Boolean): T? = if (predicate(this)) this else null
Example:
val phoneNumber = "8899665544"
val number = phoneNumber.takeIf { it.matches("[0-9]+".toRegex()) }
In above example number will have a string of phoneNumber only it matches the regex. Otherwise, it will be null.
takeUnless
public inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? = if (!predicate(this)) this else null
It is the reverse of takeIf.
Example:
val phoneNumber = "8899665544"
val number = phoneNumber.takeUnless { it.matches("[0-9]+".toRegex()) }
number will have a string of phoneNumber only if not matches the regex. Otherwise, it will be null.
You can view similar answers which is usefull here difference between kotlin also, apply, let, use, takeIf and takeUnless in Kotlin
According to my experience, since such functions are inline syntactic sugar with no performance difference, you should always choose the one that requires writing the least amount of code in the lamda.
To do this, first determine whether you want the lambda to return its result (choose run/let) or the object itself (choose apply/also); then in most cases when the lambda is a single expression, choose the ones with the same block function type as that expression, because when it's a receiver expression, this can be omitted, when it's a parameter expression, it is shorter than this:
val a: Type = ...
fun Type.receiverFunction(...): ReturnType { ... }
a.run/*apply*/ { receiverFunction(...) } // shorter because "this" can be omitted
a.let/*also*/ { it.receiverFunction(...) } // longer
fun parameterFunction(parameter: Type, ...): ReturnType { ... }
a.run/*apply*/ { parameterFunction(this, ...) } // longer
a.let/*also*/ { parameterFunction(it, ...) } // shorter because "it" is shorter than "this"
However, when the lambda consists of a mix of them, it's up to you then to choose the one that fits better into the context or you feel more comfortable with.
Also, use the ones with parameter block function when deconstruction is needed:
val pair: Pair<TypeA, TypeB> = ...
pair.run/*apply*/ {
val (first, second) = this
...
} // longer
pair.let/*also*/ { (first, second) -> ... } // shorter
Here is a brief comparison among all these functions from JetBrains's official Kotlin course on Coursera Kotlin for Java Developers:
I must admit that the difference is not so obvious at first glance, among other things because these 5 functions are often interchangeable. Here is my understanding :
APPLY -> Initialize an object with theses properties and wait for the object
val paint = Paint().apply {
this.style = Paint.Style.FILL
this.color = Color.WHITE
}
LET -> Isolate a piece of code and wait for the result
val result = let {
val b = 3
val c = 2
b + c
}
or
val a = 1
val result = a.let {
val b = 3
val c = 2
it + b + c
}
or
val paint: Paint? = Paint()
paint?.let {
// here, paint is always NOT NULL
// paint is "Paint", not "Paint?"
}
ALSO -> Execute 2 operations at the same time and wait for the result
var a = 1
var b = 3
a = b.also { b = a }
WITH -> Do something with this variable/object and don't wait for a result (chaining NOT allowed )
with(canvas) {
this.draw(x)
this.draw(y)
}
RUN -> Do something with this variable/object and don't wait for a result (chaining allowed)
canvas.run {
this.draw(x)
this.draw(y)
}
or
canvas.run {this.draw(x)}.run {this.draw(x)}