Kotlin List.indexOfFirst() Examples

Kotlin List indexOfFirst()

The indexOfFirst() function in Kotlin is used to find the index of the first element in a list that matches a given predicate.

Syntax

The indexOfFirst() function takes a predicate as input and returns the index of the first element in the list that satisfies the predicate. If no element matches the predicate, it returns -1.

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

Parameters

Parameter	Description
predicate	A function that takes an element of type T and returns true if the element matches the criteria, false otherwise.

Return Value

The indexOfFirst() function returns the index of the first element that matches the predicate. If no element matches, it returns -1.

Example 1: Finding the Index of First Even Number

This example demonstrates how to use indexOfFirst() to find the index of the first even number in a list.

In this example,

1. Create a list of numbers in the variable numbers.
2. Define a predicate function that checks if a number is even.
3. Call indexOfFirst() on the list numbers with the predicate function.
4. The indexOfFirst() function returns the index of the first even number in the list.

Kotlin Program

fun main() {
    val numbers = listOf(1, 3, 5, 2, 4, 6)
    val index = numbers.indexOfFirst { it % 2 == 0 }
    println("Index of first even number: $index")
}

Output

Example 2: Finding Index of First String Starting with “A”

In this example, we find the index of the first string in a list that starts with the letter “A”.

In this example,

1. Create a list of strings in the variable names.
2. Define a predicate function that checks if a string starts with the letter “A”.
3. Call indexOfFirst() on the list names with the predicate function.
4. The indexOfFirst() function returns the index of the first string starting with “A” in the list.

Kotlin Program

fun main() {
    val names = listOf("Alice", "Bob", "Anna", "John")
    val index = names.indexOfFirst { it.startsWith("A") }
    println("Index of first name starting with 'A': $index") 
}

Output

Summary

The indexOfFirst() function in Kotlin is useful for finding the index of the first element in a list that matches a given predicate. We have seen examples of using it with different criteria.