How to reverse the case of a string in Kotlin?

Question

What's the most efficient way to reverse the case of a string?

input = "Hello World"
output = "hELLO wORLD"

Here upper case character will be converted to lower case and vice versa.

Is there any default library support to do this?

Answer 1

In Kotlin I would propose something like this:

fun String.reverseCase(): String = map { if (it.isUpperCase()) it.toLowerCase() else it.toUpperCase() }.joinToString("")

Convert every Char in the String to its opposite and then join to String with no delimiter.

Which can be used as follows:

val input = "Hello WorLD"
val expected = "hELLO wORld"
input.reverseCase() shouldBeEqualTo expected

Answer 2

String extension function to reverse the case of a string:

Way 1: Using CharArray

fun String.reverseCaseOfString(): String {
    val inputCharArr = toCharArray() // Converting the input to char array
    var output = ""
    for (i in 0 until inputCharArr.size) {
        output += if (inputCharArr[i].isUpperCase()) { // Checking the character is in uppercase or not
            inputCharArr[i].toLowerCase() // Converting the char to lower case
        } else {
            inputCharArr[i].toUpperCase() // Converting the char to upper case
        }
    }
    return output
}

Way 2: Using CharIterator

private fun String.reverseCaseOfString(): String {
    val charIterator = iterator() // CharIterator of string
    var output = ""
    for (char in charIterator) {
        output += if (char.isUpperCase()) { // Checking the character is in uppercase or not
            char.toLowerCase() // Converting the char to lower case
        } else {
            char.toUpperCase() // Converting the char to upper case
        }
    }
    return output
}

Then you can use this extension function by the below code:

fun main(args: Array<String>) {
    print("Hello World".reverseCaseOfString())
}

Output: hELLO wORLD

Default library support is not available till now to reverse the case of a string.

Answer 3

Here's my attempt at this, with the goal of keeping the code easy to read but also performant.

fun String.invertCase(): String {
    val array = this.toCharArray()
    this.forEachIndexed { index, char ->
        when {
            char.isUpperCase() -> array[index] = char.toLowerCase()
            char.isLowerCase() -> array[index] = char.toUpperCase()
        }
    }
    return String(array)
}

val input = "Hello World"
println(input.invertCase()) // hELLO wORLD

Answer 4

For me an elegant way is:

1. to create an extension function for Char:

   fun Char.reverseCase(): Char = if (isUpperCase()) toLowerCase() else toUpperCase()

2. and then use it to create the extension for String:
   

   fun String.reverseCase(): String = toCharArray().map { it.reverseCase() }.joinToString("")