I am trying to initlaise a list as part of a constructor argument. But I do not understand what it wants from me
distortion = List(
size = 10,
init = 0
)
The list is of type room entity
var distortion: List<DistortionCacheEntity>? = null
The init part is underlined in red. I dont know what to put in there or how to write it. Could someone help.
The function you are trying to invoke looks like this:
public inline fun <T> List(size: Int, init: (index: Int) -> T): List<T>
So init here is not an Integer but rather a function. Invoke like so:
distortion = List(size = 10) { index : Int ->
//Create object that the list needs to hold
}
var distortion: List<DistortionCacheEntity>? = null
distortion = List(10) { index -> DistortionCacheEntity()}
The simplest way to initialize a list is with kotlin library function listOf(...)
For example,
listOf(1, 2, 3) // [1, 2, 3]
Related
I want to have a class that stores three sets of 30 Foos each. I can declare them as arrays of Foo but I don't know how I can initialize them (give them default values) with 30 elements.
data class Container (
val first: Array<Foo>,
val second: Array<Foo>,
val third: Array<Foo>,
)
data class Foo (val a: Int, val b: Int)
There aren't too many ways to create an array in Kotlin and they are pretty straightforward. Depending on your needs, you can either use arrayOf() function, Array() constructor or create List first and then convert it into array.
Example with arrayOf():
val first = arrayOf(
Foo(0, 0),
Foo(1, 1),
...
)
Array():
val first = Array(30) { Foo(it, it) }
List:
val firstList = mutableListOf<Foo>()
firstList += Foo(0, 0)
firstList += Foo(1, 1)
...
first = firstList.toTypedArray()
If the code for generating these arrays is complicated, you can write it inside init {} block or create static function(s) that provides these arrays.
The Code A is from the offical sample project.
I don't understand what val tasks = remember { mutableStateListOf(*allTasks) } mean, could you tell me ?
BTW, Android Studio give me some information, you can see Image A
Code A
#Composable
fun Home() {
// String resources.
val allTasks = stringArrayResource(R.array.tasks)
val allTopics = stringArrayResource(R.array.topics).toList()
// The currently selected tab.
var tabPage by remember { mutableStateOf(TabPage.Home) }
// True if the whether data is currently loading.
var weatherLoading by remember { mutableStateOf(false) }
// Holds all the tasks currently shown on the task list.
val tasks = remember { mutableStateListOf(*allTasks) }
...
}
Image A
From the documentation of varargs:
When you call a vararg -function, you can pass arguments individually, for example asList(1, 2, 3). If you already have an array and want to pass its contents to the function, use the spread operator (prefix the array with *):
val a = arrayOf(1, 2, 3)
val list = asList(-1, 0, *a, 4)
As you see, it expands an array to multiple values for use in a vararg. If you havd an array containing the elements 1, 2, 3, you can pass *yourArray to a method that is equivalent to yourMethod(1,2,3).
In Kotlin * is the Spread Operator.
From docs :
When you call a vararg -function, you can pass arguments individually, for example asList(1, 2, 3). If you already have an array and want to pass its contents to the function, use the spread operator (prefix the array with *):
val a = arrayOf(1, 2, 3)
val list = asList(-1, 0, *a, 4)
In this case tasks will contain the list of strings from R.array.tasks
I'm trying to initialize an IntArray in Kotlin like so:
intArrayOf(1..9)
But I get a TypeError that Int is required, but I'm providing an IntRange. Is there a way to initialize the array with a range, or do I have to explicitly write out each value?
Using built in functions, this is how you could get to an IntArray from an IntRange:
val array: IntArray = (1..9).toList().toIntArray()
This is a bit wasteful, because it first constructs a list where it puts all the elements, and then it constructs an array as well. To do this directly, you could use your own extension, something like...
fun IntRange.toIntArray(): IntArray {
if (last < first)
return IntArray(0)
val result = IntArray(last - first + 1)
var index = 0
for (element in this)
result[index++] = element
return result
}
Which would give you this syntax:
val array: IntArray = (1..9).toIntArray()
Let's say I have following class:
class Person() {
var age: Pair<String, Int> = Pair("person_age", 23)
// override getValue and setValue here
}
Now I want to capsulate the actual Pair and only want the user to read/write the second value of the pair. Is it possible to override the getValue and setValue methods so I can do something like this:
val p = Person()
p.age = 25
if(p.age <= 30)
Of course I can write own getter and setter methods for each property but one nice thing about Kotlin is that you have to write such less boilerplate code which will get lost then.
The following should probably already suffice:
class Person() {
var age : Int = 23 // public by default
private /* or internal */ fun toAgePair() = "person_age" to age // narrow visibility
}
So all your code accesses the age as you have shown:
val p = Person()
p.age = 25
if (p.age <= 30) ...
But if you require your Pair you just do the following instead:
p.toAgePair() // or skip that method and use: '"person_age" to p.age' instead
Alternatives to access the Pair content are: Pair.first, Pair.second or destructured, e.g.:
val myPair = Pair("person_age", 23)
// myPair.second = 25 // setting will not work however
myPair.let { (name, age) -> /* do something with it */ }
Or alternatively:
val p = Person()
val (name, age) = p.toAgePair()
// age = 25 // setting will not work however (and it wouldn't set the actual value inside the Pair if it would contain vars)
if (age < 30) // accessing is OK
However then you get access to both values which you probably didn't want in the first place, if I understood you correctly.
You could overcome the setting part using your own data class with a var but then again, you do not really gain something from it.
I wouldn't recommend you to use Pair at all. Maybe you could modify it (inherit from it, use extension functions) to suit your needs, but why try to change something as simple as Pair?. It is much easier and in this case also cleaner to just create your own class which suits your needs:
data class MyPair<out A, B>(
val first: A,
var second: B
)
val pair = MyPair("age", 1)
pair.second = 2
pair.first = 1 // error
This class has all important features which Pair has: generic types for first and second, and you can use destructuring declarations.
Now I want to capselate the actual Pair and only want the user to read/write the second value of the pair.
Assuming this means you want the first value to be final, but not the second one, there are some options.
If you only want one of the values to be writeable and readable, don't use a pair. It's not designed to be used like that. All the items of a Pair are vals.
If you want a Pair either way, can do this:
class Person(var age: Int = 23){
val pair: Pair<String, Int>
get() = Pair("person_age", age)
//Alternatively, if you don't want to use a property:
//fun getPair() = "person_age" to age
}
What this does is creating a final pair where the first value can't be modified, but the second can.
So now:
fun example(){
val person = Person()
person.age = 25;//Fine: Age is an int, and a var
//person.pair = Pair("something", 45)//fails: "Val cannot be reassigned
val pair = person.pair // Allowed. Accessing the pair still works
assert(pair.second == person.age) // This is true
}
However, if you're fine with a non-Pair solution, this works too:
data class Person (var age: Int, val string: String = "person_age")
fun example(){
val person = Person(23)
val (name, string) = person// Allowed! Just like with Pairs
person.age = 25; // Also allowed
//person.string = "something"//Not allowed
}
The n-touple unpacking is supported for data classes. If you don't have a data class, you need to declare an operator fun for each component you want to unpack. Example:
class Person (val string: String = "person_age", var age: Int){
operator fun component1() = string
operator fun component2() = age
}
But tbh, it sounds like the data class solution is the one you're looking for. It would lock the String to what it's initialized with, and because of the default value and its position, you can initialize it with a single positioned argument*
You could also use generics if you want to use the same class for multiple types.
* Assumes the code is in Kotlin. Positioned and default arguments don't work from Java code.
Here's how to overwrite a getter method in Kotlin
class Person {
var age: Int = 0
get() = if (field < 0) 0 else field
}
The attribute is accessed directly
fun main(args: Array<String>) {
val p = Person()
p.age = -28
println(p.age) //0
}
How can I use Stack (from java) in Kotlin?
Or theres any other alternative?
I'm trying to convert list to Stack
Kotlin 1.3.70 introduced the kotlin.collections.ArrayDeque class, which functions as both a queue and a stack, like Java's java.util.Deque (Deque meaning "double-ended queue"). It was created out of a necessity for a multiplatform ArrayDeque implementation.
val stack = ArrayDeque(listOf(1, 2, 3)) // stack: [1, 2, 3]
stack.addLast(0) // stack: [1, 2, 3, 0] (push)
val value = stack.removeLast() // value: 0, stack: [1, 2, 3] (pop)
Note that if an ArrayDeque is empty when you call removeFirst or removeLast, it will throw a kotlin.NoSuchElementException. If you don't want to check the size of your deque every time you need to access it, then you should use the removeFirstOrNull and removeLastOrNull functions.
Optional Snippets
ArrayDeque constructor function:
inline fun <T> arrayDequeOf(vararg elements: T) = ArrayDeque(elements.toList())
// ...
val stack = arrayDequeOf(1, 2, 3)
Stack-like ArrayDeque calls:
inline fun <T> ArrayDeque<T>.push(element: T) = addLast(element) // returns Unit
inline fun <T> ArrayDeque<T>.pop() = removeLastOrNull() // returns T?
import java.util.ArrayDeque
var stack = ArrayDeque<Int>()
stack.push(1)
stack.push(2)
stack.push(3)
stack.push(4)
println(stack) // --> [4, 3, 2, 1]
println(stack.isEmpty()) // --> false
println(stack.peek()) // --> 4
println(stack) // --> [4, 3, 2, 1]
println(stack.pop()) // --> 4
println(stack) // --> [3, 2, 1]
stack.push(9)
println(stack) // --> [9, 3, 2, 1]
You can use following:
/**
* Stack as type alias of Mutable List
*/
typealias Stack<T> = MutableList<T>
/**
* Pushes item to [Stack]
* #param item Item to be pushed
*/
inline fun <T> Stack<T>.push(item: T) = add(item)
/**
* Pops (removes and return) last item from [Stack]
* #return item Last item if [Stack] is not empty, null otherwise
*/
fun <T> Stack<T>.pop(): T? = if (isNotEmpty()) removeAt(lastIndex) else null
/**
* Peeks (return) last item from [Stack]
* #return item Last item if [Stack] is not empty, null otherwise
*/
fun <T> Stack<T>.peek(): T? = if (isNotEmpty()) this[lastIndex] else null
This is done in the same way as you would in Java, but with Kotlin syntax - notably different are the val keyword and lack of new keyword. For example:
import java.util.Stack
...
val someList = ArrayList()
...
val stack = Stack()
stack.addAll(someList)
This is a few years old but I suspect there's room for a different approach. If you want to use a stack structure in Kotlin you certainly don't need to resort to Java. You could easily just create a new class with an internal Kotlin list and stack-like public functions, or use Kotlin's extension methods to give an existing Kotlin collection "stack-like" functionality, for example:
fun <T> MutableList<T>.push(item: T) = this.add(this.count(), item)
fun <T> MutableList<T>.pop(): T? = if(this.count() > 0) this.removeAt(this.count() - 1) else null
fun <T> MutableList<T>.peek(): T? = if(this.count() > 0) this[this.count() - 1] else null
fun <T> MutableList<T>.hasMore() = this.count() > 0
Then, optionally, you could use a typealias to make it more obvious what you're trying to do when using those functions:
typealias Stack = MutableList<MyClass>
Then create one and use it:
val myStack: Stack = mutableListOf()
myStack.push(MyClass())
myStack.pop()
etc
you can define Stack like it.
val stack = Stack<YourStackType>()
notice that set the data type of your stack, for example stack of Int is like this:
val stack = Stack<Int>()
after that you can use push , pop , or other stack operations
example for Int stack :
a:Int = 10
stack.push(a)
a = stack.pop()
I don't believe there's a specific separate implementation of Stack in Kotlin. You could definitely use the answer by Ed.
Alternatively, you could use a mutableListOf<DataType> construct, and then have custom methods that are on top of this.
It would be something like this :
var stackDemo = mutableListOf<String>()
To push an element
var count = stackDemo.count()
stackDemo.add(count,"One")
To pop an element
var count = stackDemo.count()
stackDemo.removeAt(count)
You can refer to this Github link for a model implementation