Kotlin - Lists

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Kotlin list is an ordered collection of items. A Kotlin list can be either mutable (mutableListOf) or read-only (listOf). The elements of list can be accessed using indices. Kotlin mutable or immutable lists can have duplicate elements.

Creating Kotlin Lists

For list creation, use the standard library functions listOf() for read-only lists and mutableListOf() for mutable lists.

To prevent unwanted modifications, obtain read-only views of mutable lists by casting them to List.

Example

fun main() {
    val theList = listOf("one", "two", "three", "four")
    println(theList)
    
    val theMutableList = mutableListOf("one", "two", "three", "four")
    println(theMutableList)
}

When you run the above Kotlin program, it will generate the following output:

[one, two, three, four]
[one, two, three, four]

Loop through Kotlin Lists

There are various ways to loop through a Kotlin list. Lets study them one by one:

Using toString() function

fun main() {
    val theList = listOf("one", "two", "three", "four")
    println(theList.toString())
}

When you run the above Kotlin program, it will generate the following output:

[one, two, three, four]

Using Iterator

fun main() {
    val theList = listOf("one", "two", "three", "four")
    
    val itr = theList.listIterator() 
    while (itr.hasNext()) {
        println(itr.next())
    }
}

When you run the above Kotlin program, it will generate the following output:

one
two
three
four

Using for loop

fun main() {
   val theList = listOf("one", "two", "three", "four")
    
   for (i in theList.indices) {
      println(theList[i])
   }
}

When you run the above Kotlin program, it will generate the following output:

one
two
three
four

Using forEach

fun main() {
   val theList = listOf("one", "two", "three", "four")
    
   theList.forEach { println(it) }
}

When you run the above Kotlin program, it will generate the following output:

one
two
three
four

Note - here it works like this operator in Java.

Size of Kotlin List

We can use size property to get the total number of elements in a list:

fun main() {
    val theList = listOf("one", "two", null, "four", "five")
    
    println("Size of the list " + theList.size)
}

When you run the above Kotlin program, it will generate the following output:

Size of the list 5

The "in" Operator

The in operator can be used to check the existence of an element in a list.

Example

fun main() {
   val theList = listOf("one", "two", "three", "four")
    
   if("two" in theList){
      println(true)
   }else{
      println(false)
   }
}

When you run the above Kotlin program, it will generate the following output:

true

The contain() Method

The contain() method can also be used to check the existence of an element in a list.

Example

fun main() {
   val theList = listOf("one", "two", "three", "four")

   if(theList.contains("two")){
      println(true)
   }else{
      println(false)
   }
    
}

When you run the above Kotlin program, it will generate the following output:

true

The isEmpty() Method

The isEmpty() method returns true if the collection is empty (contains no elements), false otherwise.

Example

fun main() {
   val theList = listOf("one", "two", "three", "four")
    
   if(theList.isEmpty()){
      println(true)
   }else{
      println(false)
   }
}

When you run the above Kotlin program, it will generate the following output:

false

The indexOf() Method

The indexOf() method returns the index of the first occurrence of the specified element in the list, or -1 if the specified element is not contained in the list.

Example

fun main() {
   val theList = listOf("one", "two", "three", "four")
    
   println("Index of 'two' :  " + theList.indexOf("two"))
}

When you run the above Kotlin program, it will generate the following output:

Index of 'two' :  1

The get() Method

The get() method can be used to get the element at the specified index in the list. First element index will be zero.

Example

fun main() {
   val theList = listOf("one", "two", "three", "four")

   println("Element at 3rd position " + theList.get(2))
}

When you run the above Kotlin program, it will generate the following output:

Element at 3rd position three

List Addition

We can use + operator to add two or more lists into a single list. This will add second list into first list, even duplicate elements will also be added.

Example

fun main() {
    val firstList = listOf("one", "two", "three")
    val secondList = listOf("four", "five", "six")
    val resultList = firstList + secondList
    
    println(resultList)
}

When you run the above Kotlin program, it will generate the following output:

[one, two, three, four, five, six]

List Subtraction

We can use - operator to subtract a list from another list. This operation will remove the common elements from the first list and will return the result.

Example

fun main() {
    val firstList = listOf("one", "two", "three")
    val secondList = listOf("one", "five", "six")
    val resultList = firstList - secondList
    
    println(resultList)
}

When you run the above Kotlin program, it will generate the following output:

[two, three]

Slicing a List

We can obtain a sublist from a given list using slice() method which makes use of range of the elements indices.

Example

fun main() {
    val theList = listOf("one", "two", "three", "four", "five")
    val resultList = theList.slice( 2..4)
    
    println(resultList)
}

When you run the above Kotlin program, it will generate the following output:

[three, four, five]

Removing null a List

We can use filterNotNull() method to remove null elements from a Kotlin list.

fun main() {
    val theList = listOf("one", "two", null, "four", "five")
    val resultList = theList.filterNotNull()
    
    println(resultList)
}

When you run the above Kotlin program, it will generate the following output:

[one, two, four, five]

Filtering Elements

We can use filter() method to filter out the elements matching with the given predicate.

fun main() {
    val theList = listOf(10, 20, 30, 31, 40, 50, -1, 0)
    val resultList = theList.filter{ it > 30}
    
    println(resultList)
}

When you run the above Kotlin program, it will generate the following output:

[31, 40, 50]

Dropping First N Elements

We can use drop() method to drop first N elements from the list.

fun main() {
    val theList = listOf(10, 20, 30, 31, 40, 50, -1, 0)
    val resultList = theList.drop(3)
    
    println(resultList)
}

When you run the above Kotlin program, it will generate the following output:

[31, 40, 50, -1, 0]

Grouping List Elements

We can use groupBy() method to group the elements matching with the given predicate.

fun main() {
    val theList = listOf(10, 12, 30, 31, 40, 9, -3, 0)
    val resultList = theList.groupBy{ it % 3}
    
    println(resultList)
}

When you run the above Kotlin program, it will generate the following output:

{1=[10, 31, 40], 0=[12, 30, 9, -3, 0]}

Mapping List Elements

We can use map() method to map all elements using the provided function:.

fun main() {
    val theList = listOf(10, 12, 30, 31, 40, 9, -3, 0)
    val resultList = theList.map{ it / 3 }
    
    println(resultList)
}

When you run the above Kotlin program, it will generate the following output:

[3, 4, 10, 10, 13, 3, -1, 0]

Chunking List Elements

We can use chunked() method to create chunks of the given size from a list. Last chunk may not have the elements equal to the number of chunk size based on the total number of elements in the list.

fun main() {
    val theList = listOf(10, 12, 30, 31, 40, 9, -3, 0)
    val resultList = theList.chunked(3)
    
    println(resultList)
}

When you run the above Kotlin program, it will generate the following output:

[[10, 12, 30], [31, 40, 9], [-3, 0]]

Windowing List Elements

We can use windowed() method to a list of element ranges by moving a sliding window of a given size over a collection of elements.

fun main() {
    val theList = listOf(10, 12, 30, 31, 40, 9, -3, 0)
    val resultList = theList.windowed(3)
    
    println(resultList)
}

When you run the above Kotlin program, it will generate the following output:

[[10, 12, 30], [12, 30, 31], [30, 31, 40], [31, 40, 9], [40, 9, -3], [9, -3, 0]]

By default, the sliding window moves one step further each time but we can change that by passing a custom step value:

fun main() {
    val theList = listOf(10, 12, 30, 31, 40, 9, -3, 0)
    val resultList = theList.windowed(3, 3)
    
    println(resultList)
}

When you run the above Kotlin program, it will generate the following output:

[[10, 12, 30], [31, 40, 9]]

Kotlin mutable List

We can create mutable list using mutableListOf(), later we can use add() to add more elements in the same list, and we can use remove() method to remove the elements from the list.

fun main() {
    val theList = mutableSetOf(10, 20, 30)

    theList.add(40)
    theList.add(50)
    println(theList)

    theList.remove(10)
    theList.remove(30)
    println(theList)
}

When you run the above Kotlin program, it will generate the following output:

[10, 20, 30, 40, 50]
[20, 40, 50]