Kotlin How to Convert String to Lowercase Example

In Kotlin, convert a string to lowercase using the lowercase() function like this: val lower = yourString.lowercase().

Examples

InputHELLO

Outputhello

InputKotlin Is Fun

Outputkotlin is fun

Input123 ABC xyz!

Output123 abc xyz!

How to Think About It

To convert a string to lowercase in Kotlin, think of changing every uppercase letter to its lowercase version while leaving other characters unchanged. The lowercase() function does this easily by scanning the string and converting letters as needed.

Algorithm

1

Get the input string.

2

Use the lowercase() function to convert all uppercase letters to lowercase.

3

Return or print the converted lowercase string.

Code

kotlin

fun main() {
    val original = "Hello Kotlin!"
    val lower = original.lowercase()
    println(lower)
}

Output

hello kotlin!

Dry Run

Let's trace the string "Hello Kotlin!" through the code

1

Original string

original = "Hello Kotlin!"

2

Convert to lowercase

lower = original.lowercase() -> "hello kotlin!"

3

Print result

Output: "hello kotlin!"

Step	Value
original	Hello Kotlin!
lower	hello kotlin!

Why This Works

Step 1: Use of lowercase()

The lowercase() function scans the string and converts each uppercase letter to its lowercase equivalent.

Step 2: Non-letter characters unchanged

Characters like spaces, punctuation, and numbers remain the same because they have no lowercase form.

Step 3: Returns new string

lowercase() returns a new string and does not change the original string.

Alternative Approaches

lowercase(Locale)

kotlin

import java.util.Locale

fun main() {
    val original = "Hello Kotlin!"
    val lower = original.lowercase(Locale.getDefault())
    println(lower)
}

This method allows specifying locale for language-specific lowercase rules.

map with lowercaseChar() (legacy)

kotlin

fun main() {
    val original = "Hello Kotlin!"
    val lower = original.map { it.lowercaseChar() }.joinToString("")
    println(lower)
}

This manually converts each character; less efficient and more complex than lowercase().

Complexity: O(n) time, O(n) space

Time Complexity

The function processes each character once, so time grows linearly with string length.

Space Complexity

A new string is created for the result, so space also grows linearly with input size.

Which Approach is Fastest?

lowercase() is optimized and simpler than manual character mapping, making it the best choice.

Approach	Time	Space	Best For
lowercase()	O(n)	O(n)	Simple, efficient lowercase conversion
lowercase(Locale)	O(n)	O(n)	Locale-aware lowercase conversion
map with lowercaseChar()	O(n)	O(n)	Manual control but less efficient