I recently answered a Stack Overflow question about transforming an array of Characters into a String in Swift. Looking at the question and the Swift standard lib, it appears that there is a method joinWithSeparator, but the current implementation only supports SequenceType instances where the Element is a String. As an exercise to myself, I wanted to write an extension on SequenceType that could flatten an array of Characters into a String:

extension SequenceType where Generator.Element == Character {

    public func joinWithSeparator(separator: String) -> String {

        var str = ""


            if let arr = self as? [Character], endIndex: Int = arr.endIndex {
                if $0 < endIndex - 1 {

        return str

How can I further optimize this code? Would it be possible to replace the forEach loop with flatMap? Or is a map call inappropriate since I need to append each character to a String, rather than create a new array?


There are two problems with your method:

Problem #1: The method takes a separator: String parameter, but actually it is expected that a single character string is passed, and it crashes at Character(separator) if a multi-character string is passed:

let myChars: [Character] = ["a", "b", "c"]
let joined = myChars.joinWithSeparator("==")
// fatal error: Can't form a Character from a String containing more than one extended grapheme cluster

The solution is easy: change the parameter to separator: Character. Btw, this makes the method faster.

Problem #2: The method is an extension of SequenceType, but it actually works only for Arrays. For arbitrary sequences of characters, the optional cast as? [Character] fails. This is silently ignored and the separator not inserted:

let charSeq = Repeat(count: 4, repeatedValue: Character("x"))
let joined = charSeq.joinWithSeparator(",")
print(joined) // Output: xxxx

Two more remarks before I suggest possible solutions:

  1. The name of the local variable var str = "" is quite non-descriptive. I would call it result or joinedString (but that may be opinion based).

  2. Your loop

    self.enumerate().forEach {
        /* do something with `$0` (index) and `$1` (char)

    is correct, but I would write it as

    for (index, char) in self.enumerate() {
        /* do something with `index` and `char`

So how can problem #2 be solved? You cast the sequence to an array in order to get the index of the last element, so that you can insert the separator after all elements but the last.

It becomes simpler if you think the other way around: Insert the separator before all elements but the first. No need to determine the endIndex anymore:

public func joinWithSeparator(separator: Character) -> String {

    var result = ""
    for (idx, char) in self.enumerate() {
        if idx > 0 {
    return result

This has about the same speed as your method when applied to an array, but works for arbitrary character sequence.

You asked:

Would it be possible to replace the forEach loop with flatMap?

and the answer is "yes":

public func joinWithSeparator(separator: Character) -> String {

    let joinedChars = self.enumerate().flatMap {
        (idx, char) in 
        idx == 0 ? [ char ] : [ separator, char ]
    return String(joinedChars)

The shorted implementation (but not the fastest) would be to convert all characters to strings, and use the existing method to join strings:

public func joinWithSeparator(separator: Character) -> String {

    return self.map { String($0) }.joinWithSeparator(String(separator))

Performance comparison

Here is my test code (compiled in Release mode on a 3.5Ghz iMac):

let myChars = Array(Repeat(count: 1_000_000, repeatedValue: Character("x")))
let d1 = NSDate()
let j = myChars.joinWithSeparator(",")
let d2 = NSDate()


  • Your original code: 0.163 sec.
  • Your code, with the separator parameter changed to Character: 0.104 sec.
  • My first suggestion: 0.095 sec.
  • Using flatMap: 0.276 sec.
  • Convert all characters to strings: 0.305 sec.
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