Split String into groups with specific length

Question

How can I split the given String in Swift into groups with given length, reading from right to left?

For example, I have string 123456789 a

Answer 1

Swift 4

I adapted the answer given by cafedeichi to operate either left-to-right or right-to-left depending on a function parameter, so it's more versatile.

extension String {
    /// Splits a string into groups of `every` n characters, grouping from left-to-right by default. If `backwards` is true, right-to-left.
    public func split(every: Int, backwards: Bool = false) -> [String] {
        var result = [String]()

        for i in stride(from: 0, to: self.count, by: every) {
            switch backwards {
            case true:
                let endIndex = self.index(self.endIndex, offsetBy: -i)
                let startIndex = self.index(endIndex, offsetBy: -every, limitedBy: self.startIndex) ?? self.startIndex
                result.insert(String(self[startIndex..

Example:

"abcde".split(every: 2)                     // ["ab", "cd", "e"]
"abcde".split(every: 2, backwards: true)    // ["a", "bc", "de"]

"abcde".split(every: 4)                     // ["abcd", "e"]
"abcde".split(every: 4, backwards: true)    // ["a", "bcde"]