How to initialize an array in Kotlin with values?

问题:

In Java an array can be initialized such as:

int numbers[] = new int[] {10, 20, 30, 40, 50}

How does Kotlin's array initialization look like?

回答1:

val numbers: IntArray = intArrayOf(10, 20, 30, 40, 50)

See Kotlin - Basic Types for details.

回答2:

Worth mentioning that when using kotlin builtines (e.g. intArrayOf(), longArrayOf(), arrayOf(), etc) you are not able to initialize the array with default values (or all values to desired value) for a given size, instead you need to do initialize via calling according to class constructor.

// Array of integers of a size of N
val arr = IntArray(N)

// Array of integers of a size of N initialized with a default value of 2
val arr = IntArray(N) { i -> 2 }

回答3:

Here's an example:

fun main(args: Array<String>) {
    val arr = arrayOf(1, 2, 3);
    for (item in arr) {
        println(item);
    }
}

You can also use a playground to test language features.

回答4:

In Kotlin There are Several Ways.

var arr = IntArray(size) // construct with only size

Then simply initial value from users or from another collection or wherever you want.

var arr = IntArray(size, { 0 } ) // construct with size and fill array with 0
var arr = IntArray(size, { it * 1 } ) // construct with size and fill with its index

We also can create array with built in function like-

var arr = intArrayOf(1, 2, 3, 4, 5) // create an array with 5 values

Another way

var arr = Array(size, { 0 } ) // it will create an integer array
var arr = Array<String>(size, { "$it" } ) // this will create array with "0", "1", "2" and so on.

You also can use doubleArrayOf() or DoubleArray() or any primitive type instead of Int.

回答5:

In Kotlin we can create array using arrayOf(), intArrayOf(), charArrayOf(), booleanArrayOf(), longArrayOf() functions.

For example:

var Arr1 = arrayOf(1,10,4,6,15)  
var Arr2 = arrayOf<Int>(1,10,4,6,15)  
var Arr3 = arrayOf<String>("Surat","Mumbai","Rajkot")  
var Arr4 = arrayOf(1,10,4, "Ajay","Prakesh")  
var Arr5: IntArray = intArrayOf(5,10,15,20)  

回答6:

Old question, but if you'd like to use a range:

var numbers: IntArray = IntRange(10, 50).step(10).toList().toIntArray()

Yields nearly the same result as:

var numbers = Array(5, { i -> i*10 + 10 })

result: 10, 20, 30, 40, 50

I think the first option is a little more readable. Both work.

回答7:

you can use this methods

var numbers=Array<Int>(size,init)
var numbers=IntArray(size,init)
var numbers= intArrayOf(1,2,3)

example

var numbers = Array<Int>(5, { i -> 0 })

init represents the default value ( initialize )

回答8:

I think one thing that is worth mentioning and isn't intuitive enough from the documentation is that, when you use a factory function to create an array and you specify it's size, the array is initialized with values that are equal to their index values. For example, in an array such as this: val array = Array(5, { i -> i }), the initial values assigned are [0,1,2,3,4] and not say, [0,0,0,0,0]. That is why from the documentation, val asc = Array(5, { i -> (i * i).toString() }) produces an answer of ["0", "1", "4", "9", "16"]

回答9:

You can create an Int Array like this:

val numbers = IntArray(5, { 10 * (it + 1) })

5 is the Int Array size. the lambda function is the element init function. 'it' range in [0,4], plus 1 make range in [1,5]

origin function is:

 /**
 * An array of ints. When targeting the JVM, instances of this class are 
 * represented as `int[]`.
 * @constructor Creates a new array of the specified [size], with all elements 
 *  initialized to zero.
 */
 public class IntArray(size: Int) {
       /**
        * Creates a new array of the specified [size], where each element is 
        * calculated by calling the specified
        * [init] function. The [init] function returns an array element given 
        * its index.
        */
      public inline constructor(size: Int, init: (Int) -> Int)
  ...
 }

IntArray class defined in the Arrays.kt

回答10:

Kotlin language has specialised classes for representing arrays of primitive types without boxing overhead: for instance – IntArray, ShortArray, ByteArray, etc. I need to say that these classes have no inheritance relation to the parent Array class, but they have the same set of methods and properties. Each of them also has a corresponding factory function. So, to initialise an array with values in Kotlin you just need to type this:

val myArr: IntArray = intArrayOf(10, 20, 30, 40, 50)

...or this way:

val myArr = Array<Int>(5, { i -> ((i+1) * 10) })

myArr.forEach { println(it) }                                // 10, 20, 30, 40, 50

Now you can use it:

myArr[0] = (myArr[1] + myArr[2]) - myArr[3]

Hope this helps.

回答11:

You can try this:

var a = Array<Int>(5){0}

回答12:

You can simply use the existing standard library methods as shown here:

val numbers = intArrayOf(10, 20, 30, 40, 50)

It might make sense to use a special constructor though:

val numbers2 = IntArray(5) { (it + 1) * 10 }

You pass a size and a lambda that describes how to init the values. Here's the documentation:

/**
 * Creates a new array of the specified [size], where each element is calculated by calling the specified
 * [init] function. The [init] function returns an array element given its index.
 */
public inline constructor(size: Int, init: (Int) -> Int)

回答13:

I'm wondering why nobody just gave the most simple of answers:

val array: Array<Int> = [1, 2, 3]

As per one of the comments to my original answer, I realized this only works when used in annotations arguments (which was really unexpected for me).

Looks like Kotlin doesn't allow to create array literals outside annotations.

For instance, look at this code using @Option from args4j library:

    @Option(
        name = "-h",
        aliases = ["--help", "-?"],
        usage = "Show this help"
    )
    var help: Boolean = false

The option argument "aliases" is of type Array<String>

回答14:

In my case I need to initialise my drawer items. I fill data by below code.

    val iconsArr : IntArray = resources.getIntArray(R.array.navigation_drawer_items_icon)
    val names : Array<String> = resources.getStringArray(R.array.navigation_drawer_items_name)


    // Use lambda function to add data in my custom model class i.e. DrawerItem
    val drawerItems = Array<DrawerItem>(iconsArr.size, init = 
                         { index -> DrawerItem(iconsArr[index], names[index])})
    Log.d(LOGGER_TAG, "Number of items in drawer is: "+ drawerItems.size)

Custom Model class-

class DrawerItem(var icon: Int, var name: String) {

}

回答15:

Declare int array at global

var numbers= intArrayOf()

next onCreate method initialize your array with value

override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)
    //create your int array here
    numbers= intArrayOf(10,20,30,40,50)
}

回答16:

In Java an array can be initialized such as:

int numbers[] = new int[] {10, 20, 30, 40, 50}

But In Kotlin an array initialized many way such as:

Any generic type of array you can use arrayOf() function :

val arr = arrayOf(10, 20, 30, 40, 50)

val genericArray = arrayOf(10, "Stack", 30.00, 40, "Fifty")

Using utility functions of Kotlin an array can be initialized

val intArray = intArrayOf(10, 20, 30, 40, 50)

回答17:

intialize array in this way : val paramValueList : Array<String?> = arrayOfNulls<String>(5)