How to Find Index of Character in String Kotlin

In Kotlin, use the indexOf() function on a string to find the index of a character. It returns the first position of the character or -1 if not found.

Syntax

The indexOf() function is called on a string and takes a character as an argument. It returns an integer representing the first index where the character appears, or -1 if the character is not found.

• string.indexOf(char): Finds the first occurrence of char.
• Returns Int: index position starting at 0, or -1 if missing.

kotlin

val text = "hello"
val index = text.indexOf('e')

Example

This example shows how to find the index of a character in a string and handle the case when the character is not found.

kotlin

fun main() {
    val word = "kotlin"
    val charToFind = 't'
    val index = word.indexOf(charToFind)

    if (index != -1) {
        println("Character '$charToFind' found at index: $index")
    } else {
        println("Character '$charToFind' not found in the string.")
    }
}

Output

Character 't' found at index: 2

Common Pitfalls

One common mistake is expecting indexOf() to return null if the character is not found, but it actually returns -1. Always check for -1 before using the index.

Another pitfall is confusing character 'a' with string "a". indexOf() for characters requires a Char argument, not a String.

kotlin

fun main() {
    val text = "apple"
    // Wrong: passing String instead of Char
    // val wrongIndex = text.indexOf("a") // This causes a compile error

    // Correct:
    val correctIndex = text.indexOf('a')
    println(correctIndex)  // Output: 0

    // Checking for character not found
    val missingIndex = text.indexOf('z')
    if (missingIndex == -1) {
        println("Character not found")
    }
}

Output

0
Character not found

Quick Reference

Function	Description	Returns
indexOf(char)	Finds first index of character	Int index or -1 if not found
indexOf(char, startIndex)	Finds first index of character starting from startIndex	Int index or -1
lastIndexOf(char)	Finds last index of character	Int index or -1

Key Takeaways

Use indexOf() to find the first position of a character in a Kotlin string.

The function returns -1 if the character is not found, not null.

Pass a Char (single quotes) to indexOf(), not a String.

Check the returned index before using it to avoid errors.

You can also use lastIndexOf() to find the last occurrence of a character.