Swift extensions: String function fixIndex

In this post I will keep track of general purpose extensions for the Swift String type.

fixIndex

extension String {

    

    /// This function fixes the associated endIndex of the given index by returning a new index with the same value and a new associated endIndex.

    

    func fixIndex(index: String.Index) -> String.Index {

        let kludge = distance(startIndex, index)

        return advance(startIndex, kludge)

    }

}

Example usage:

var string = "12"

var index = string.startIndex.successor().successor()

string += "34"

// index = index.successor().successor() ==>> gives us an "EXC_BAD_INSTRUCTION"

index = string.fixIndex(index)

index = index.successor().successor() // OK

isLowercaseCharacterAtIndex

extension String {

    

    /// Returns true if the character at the given index is a lowercase character as defined in the lowercase character set. Works for UTF16 characters only.

    func isLowercaseCharacterAtIndex(index: String.Index) -> Bool {

        return NSCharacterSet.lowercaseLetterCharacterSet().characterIsMember(String(self[index]).utf16[String.UTF16Index(0)])

    }

}

Example usage:

let str = "1 gF🌵"

for i in str.startIndex ... str.endIndex.predecessor() {

    print("\(str.isLowercaseCharacterAtIndex(i)), ")

}

Prints:

false, false, true, false, false,

isUppercaseCharacterAtIndex

extension String {

    

    /// Returns true if the character at the given index is an uppercase character as defined in the uppercase character set. Works for UTF16 characters only.

    func isUppercaseCharacterAtIndex(index: String.Index) -> Bool {

        return NSCharacterSet.uppercaseLetterCharacterSet().characterIsMember(String(self[index]).utf16[String.UTF16Index(0)])

    }

}

Example usage:

let str = "1 Fg🌵"

for i in str.startIndex ... str.endIndex.predecessor() {

    print("\(str.isUppercaseCharacterAtIndex(i)), ")

}

Prints:

false, false, true, false, false,

replaceCharacterAtIndex

extension String {

    

    /// Replaces the character at the specified index with the given character. Returns true if the string was changed, false if not.

    

    mutating func replaceCharacterAtIndex(index: String.Index, with char: Character) -> Bool {

        if index == endIndex { return false }

        if self[index] == char { return false }

        let range = Range(start: index, end: index.successor())

        self.replaceRange(range, with: String(char))

        return true

    }

}

Example usage:

var str = "123456789"

str.replaceCharacterAtIndex(str.startIndex.successor(), with: "F")

print(str)

Prints:

"1F3456789"

changeCaseOfCharacterAtIndex

This extension needs the three preceding extensions.

extension String {

        

    /// Changes the case of the character at the specified index. Returns true if the string was changed, false if not.

    

    mutating func changeCaseOfCharacterAtIndex(index: String.Index) -> Bool {

        if isLowercaseCharacterAtIndex(index) {

            return self.replaceCharacterAtIndex(index, with: String(self[index]).uppercaseString[startIndex])

        } else if isUppercaseCharacterAtIndex(index) {

            return self.replaceCharacterAtIndex(index, with: String(self[index]).lowercaseString[startIndex])

        } else {

            return false

        }

    }

}

Happy coding...

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