How to find Index of Substring in String in Kotlin?

In this tutorial, you shall learn how to get the index of a specific search string in a given string in Kotlin, using String.indexOf() function, with examples.

Kotlin – Index of Substring in String

To find index of substring in this string in Kotlin, call indexOf() method on this string, and pass the substring as argument.

String.indexOf() returns an integer representing the index of first occurrence of the match for the given substring. If there is no match found for the given substring, then indexOf() returns -1.

Syntax

The syntax of indexOf() method is

fun CharSequence.indexOf(
    string: String,
    startIndex: Int = 0,
    ignoreCase: Boolean = false
): Int

Examples

1. Find index of substring “apple”

In the following example, we take a string in str and find the index of the substring "apple" in str string.

Main.kt

fun main(args: Array<String>) {
    var str = "an apple a day. Some apples is red."
    val index = str.indexOf("apple")
    println("Index : $index")
}

Output

Index : 3

2. Find index of substring with search from a specific start

We can also provide an optional start index for second argument. The search for the substring in this string happens from this start index.

In the following example, we find the index of the substring "apple" in the string str from the start index 8.

Main.kt

fun main(args: Array<String>) {
    var str = "an apple a day. Some apples is red."
    val index = str.indexOf("apple", startIndex = 8)
    println("Index : $index")
}

Output

Index : 21

Since we are searching from start index 8, the first occurrence of the given substring is ignored.

3. Find index of substring with search ignoring case

We can also try to find the match by ignoring the case.

In the following example, we find the index of the substring "APPLE" in the string str.

Main.kt

fun main(args: Array<String>) {
    var str = "an apple a day. Some apples is red."
    val index = str.indexOf("APPLE", ignoreCase = true)
    println("Index : $index")
}

Output

Index : 3

4. Find index of substring – Negative Scenario

Now, let us take a substring which is not present in the string, and find the index returned by indexOf() method.

Main.kt

fun main(args: Array<String>) {
    var str = "an apple a day. Some apples is red."
    val index = str.indexOf("mango")
    println("Index : $index")
}

Output

Index : -1

Conclusion

In this Kotlin Tutorial, we learned how to find the index of a substring in the given string using String.indexOf(), with examples.