Iterating Over Strings P2

Beyond basic iteration, strings offer several useful methods for building new strings from existing ones. A common task is transforming characters while iterating and collecting the results.

You can build a new string by appending characters to a StringBuilder or simply concatenating to a string variable:

val original = "hello"
var result = ""
for (char in original) {
    result += char.uppercaseChar()
}
println(result)  // HELLO

The uppercaseChar() method converts a single character to uppercase, while lowercaseChar() does the opposite. These work on individual Char values, unlike uppercase() which works on entire strings.

Another practical technique is filtering characters while iterating. For example, extracting only digits from a mixed string:

val mixed = "a1b2c3"
var digits = ""
for (char in mixed) {
    if (char.isDigit()) {
        digits += char
    }
}
println(digits)  // 123

Kotlin provides helpful character-checking methods like isDigit(), isLetter(), and isWhitespace() that return true or false based on the character type. These make it easy to filter or categorize characters during iteration.