Exercise: Adding element at position

We can add an element at a specific position to a mutable list using the add method. For instance:

fun main() {
    val list = mutableListOf(1, 2, 3)
    list.add(1, 4)
    println(list) // [1, 4, 2, 3]
}

Unfortunately, there is no similar function that would allow us to add an element at a specific position to an immutable list. Your task is to define it, and name it plusAt.

fun <T> List<T>.plusAt(index: Int, element: T): List<T> {
    TODO()
}

It should be used in the following way:

val list = listOf(1, 2, 3)
println(list.plusAt(1, 4)) // [1, 4, 2, 3]
println(list.plusAt(0, 5)) // [5, 1, 2, 3]
println(list.plusAt(3, 6)) // [1, 2, 3, 6]

val list2 = listOf("A", "B", "C")
println(list2.plusAt(1, "D")) // [A, D, B, C]

This function should check if the index is correct. If it is not, it should throw an IllegalArgumentException. Should consider 0 as a correct index, and add this element to the beginning of the list. Should also consider size as a correct index, and add this element to the end of the list.

I know at least three significantly different ways to implement this function. One uses a mutable collection, while others use collection processing functions and + operator. Try to implement it in all three ways.

This problem can either be solved in the below playground or you can clone kotlin-exercises project and solve it locally. In the project, you can find code template for this exercise in functional/collections/PlusAt.kt. You can find there starting code, example usage and unit tests.

Once you are done with the exercise, you can check your solution here.

Playground

import org.junit.Test
import kotlin.test.assertEquals

fun <T> List<T>.plusAt(index: Int, element: T): List<T> {
    TODO()
}

fun main() {
    val list = listOf(1, 2, 3)
    println(list.plusAt(1, 4)) // [1, 4, 2, 3]
    println(list.plusAt(0, 5)) // [5, 1, 2, 3]
    println(list.plusAt(3, 6)) // [1, 2, 3, 6]
    
    val list2 = listOf("A", "B", "C")
    println(list2.plusAt(1, "D")) // [A, D, B, C]
}

class PlusAtTest {

    @Test
    fun `Simple addition to the middle adds correctly at the position`() {
        assertEquals(listOf(1, 2, 7, 3), listOf(1, 2, 3).plusAt(2, 7))
        assertEquals(listOf("A", "B", "D", "C"), listOf("A", "B", "C").plusAt(2, "D"))
    }

    @Test
    fun `When we add at size position, element is added at the end`() {
        assertEquals(listOf(1, 2, 3, 7), listOf(1, 2, 3).plusAt(3, 7))
        assertEquals(listOf("A", "B", "C", "D"), listOf("A", "B", "C").plusAt(3, "D"))
    }

    @Test
    fun `When we add at 0, element is added at the beginning`() {
        assertEquals(listOf(7, 1, 2, 3), listOf(1, 2, 3).plusAt(0, 7))
        assertEquals(listOf("D", "A", "B", "C"), listOf("A", "B", "C").plusAt(0, "D"))
    }

    @Test
    fun `When we try to insert at illegal position, IllegalArgumentException error is thrown`() {
        assertThrows<IllegalArgumentException> { listOf(1, 2, 3).plusAt(-1, 7) }
        assertThrows<IllegalArgumentException> { listOf(1, 2, 3).plusAt(8, 7) }
        assertThrows<IllegalArgumentException> { listOf(1, 2, 3).plusAt(10, 7) }
        assertThrows<IllegalArgumentException> { listOf(1, 2, 3).plusAt(100, 7) }
    }
}

inline fun <reified T: Throwable> assertThrows(operation: () -> Unit) {
    val result = runCatching { operation() }
    assert(result.isFailure) { "Operation has not failed with exception" }
    val exception = result.exceptionOrNull()
    assert(exception is T) { "Incorrect exception type, it should be ${T::class}, but it is $exception" }
}

Exercise: Adding element at position

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