Kotlin Tutorials

Kotlin Set indexOfFirst()
Syntax & Examples

Set.indexOfFirst() extension function

The indexOfFirst() extension function in Kotlin returns the index of the first element matching the given predicate, or -1 if the set does not contain such an element.

Syntax of Set.indexOfFirst()

The syntax of Set.indexOfFirst() extension function is:

fun <T> Set<T>.indexOfFirst(predicate: (T) -> Boolean): Int

This indexOfFirst() extension function of Set returns index of the first element matching the given predicate, or -1 if the collection does not contain such element.

Parameters

ParameterOptional/RequiredDescription
predicaterequiredA function that takes an element and returns true if the element matches the condition.

Return Type

Set.indexOfFirst() returns value of type Int.


✐ Examples

1 Finding the index of the first even number

Using indexOfFirst() to find the index of the first even number in a set.

For example,

  1. Create a set of integers.
  2. Use indexOfFirst() with a predicate function that checks for even numbers.
  3. Print the resulting index.

Kotlin Program

fun main() {
    val numbers = setOf(1, 3, 5, 6, 7, 8)
    val index = numbers.indexOfFirst { it % 2 == 0 }
    println(index)
}

Output

3

2 Finding the index of the first string with length greater than 2

Using indexOfFirst() to find the index of the first string with a length greater than 2 in a set.

For example,

  1. Create a set of strings.
  2. Use indexOfFirst() with a predicate function that checks for strings with a length greater than 2.
  3. Print the resulting index.

Kotlin Program

fun main() {
    val strings = setOf("a", "ab", "abc", "abcd")
    val index = strings.indexOfFirst { it.length > 2 }
    println(index)
}

Output

2

3 Finding the index of the first non-null element

Using indexOfFirst() to find the index of the first non-null element in a set.

For example,

  1. Create a set containing integers and null values.
  2. Use indexOfFirst() with a predicate function that checks for non-null values.
  3. Print the resulting index.

Kotlin Program

fun main() {
    val mixedSet: Set<Int?> = setOf(null, null, 3, 4, 5)
    val index = mixedSet.indexOfFirst { it != null }
    println(index)
}

Output

2

Summary

In this Kotlin tutorial, we learned about indexOfFirst() extension function of Set: the syntax and few working examples with output and detailed explanation for each example.