How to replace nth character of a string with another

Solutions that use NSString methods will fail for any strings with multi-byte Unicode characters. Here are two Swift-native ways to approach the problem:

You can use the fact that a String is a sequence of Character to convert the string to an array, modify it, and convert the array back:

func replace(myString: String, _ index: Int, _ newChar: Character) -> String {
    var chars = Array(myString)     // gets an array of characters
    chars[index] = newChar
    let modifiedString = String(chars)
    return modifiedString
}

replace("House", 2, "r")
// Horse

Alternately, you can step through the string yourself:

func replace(myString: String, _ index: Int, _ newChar: Character) -> String {
    var modifiedString = String()
    for (i, char) in myString.characters.enumerate() {
        modifiedString += String((i == index) ? newChar : char)
    }
    return modifiedString
}

Since these stay entirely within Swift, they’re both Unicode-safe:

replace("🏠🏡🏠🏡🏠", 2, "🐴")
// 🏠🏡🐴🏡🏠