This is bit of fast paced intro into Swift Strings.

ASCII, Unicode and the challenges they introduce.

ASCII

Ages ago, only characters that existed were just a to z, A to Z, and bunch of other English characters. This was problematic. You were limited to only 7 bits i.e. only 2 ^ 7 - 1 = 127 characters. Also non-English characters where not part of ASCII. Literally the name says ‘American Standard Code for Information Interchange’. So characters like å or ä were not part of it. Let alone ب or ج or characters of other languages.

Character mapping in ascii

let uppdercaseA: Character = "A"
print(String(describing: uppdercaseA.asciiValue)) // 65

let lowercaseA: Character = "a"
print(String(describing: lowercaseA.asciiValue)) // 97

let variantA: Character = "á"
print(String(describing: lowercaseA.asciiValue)) // nil

Unicode

Then came Unicode. It was roomy. Currently Unicode has 144,697 characters.

  • Unlike ASCII where it only supported English, Unicode supports characters in most languages.
  • It has the same concept of mapping in ASCI.
  • Anything is asci will have the same value it did in Unicode. This helps backwards compatibility.
  • Has combining logic. Examples:

Unicode is a giant table of code-points (Swift calls scalars) matching to some full characters, some combining accents, and some invisibles.

(Swift) Character

It’s a heavily overloaded term. Anything perceivable by us as humans as a single character. a, â, õ, !, ~, , 1 etc. A character is more or less an array (one or more) code points without a (grapheme) break. Examples:

let x = "a"
let y = "á"
let z = "a\u{301}"

func visualize(_ string: String) {
    print("string:",string)
    print("used codepoints:")
    for s in string.unicodeScalars {
        print(s,":",s.value)
    }
    print("----")
}

visualize(x)
visualize(y)
visualize(z)

Output:

string: a
used codepoints:
a : 97 // See https://unicode.scarfboy.com/?s=U%2b0061
----
string: á
used codepoints:
á : 225 // See https://unicode.scarfboy.com/?s=U%2b00E1
----
string: á
used codepoints:
a : 97 // See https://unicode.scarfboy.com/?s=U%2b0061
́ : 769 // See https://unicode.scarfboy.com/?s=U%2b0301
----

What’s interesting is that a\u{301} produces á. Unicode has logic to combine codepoints.

👆 is one of the main reasons why Swift Strings are complicated.

Like if you’re reading

a then its palindrome is just a.
However if you’re reading á then it’s reverse could be either:

  • á (the reverse of y)
  • ́a (the reverse of z)

i.e. one could argue that the palindrome of á is ́a i.e. two codepoints can end up being palindrome with one codepoint.

Depending on how the string/character is constructed the character count could be different — i.e. a is 1 character, but á can be either just á or a ́.

Other notes from https://unicode.org/glossary

  • A grapheme is a logical entity, not visual: it can be composed of one or multiple codepoints.

  • A “glyph”, also in the glossary (https://unicode.org/glossary/#glyph) which is typically used to mean “the drawn representation of graphemes”.

  • Italic and bold don’t create a different code point. If they did, then Unicode would have exploded. Basically:

    • grapheme + (italic) font -> (italic) glyph
    • grapheme + (bold) font -> (bold) glyph

What should you do in interviews?

If all you want is looping then do:

Just do:

let str = "hello world"

for s in str {
    print(type(of:s))
}

If you need indexing then without giving away too much time for explanation just convert the string to an array of characters. 99.99% of questions you get asked aren’t considerate of previous Unicode complexities.

let str = "hello world"
let characters = Array(str)

print([characters[2]]) // "l"

However if you’re confident and you’re not losing time or you think it works to your advantage then discuss the complexities. It could help you stand out vs. the rest of the pack.

Converting a string to an array has a time complexity of O(n). Since most algorithmic problems can’t be solved better than O(n) then it’s totally fine to do so.

What if I don’t want/can’t use an array of Characters?

Try mastering the String - Manipulating indices section.

A helper method I use is this:

extension String { 
    func at(_ i: Int) -> Character {
        return self[self.index(self.startIndex, offsetBy: i)]
    }
    
    func safeAt(_ i: Int) -> Character? {
        guard i < count  else { return nil }
        return at(i)
    }
}

Also it’s good to understand that startIndex is the position of the first character in an nonempty string.

The endIndex is: A string’s “past the end” position – that is, the position one greater than the last valid subscript argument.

With an empty string, accessing the startIndex and endIndex will both cause crashes. In an empty string, startIndex is equal to endIndex.

Why is endIndex “past the end” position?!

If endIndex were the last valid subscript, then a question would arise as: in an empty collection what should the value of endIndex be? You can’t change the logic for calculating the endIndex — only for empty arrays. You need consistency.

  • first and last return optional values because the thing might be empty, so there is no “first” nor “last” element or index

  • On the other hand, startIndex is where the collection begins, and whether it’s empty or non-empty, an array always begins at 0. Because of this, accessing an array using startIndex or endIndex can cause out of bounds crashes. Example:

extension String { 
    func at(_ i: Int) -> Character {
        return self[self.index(self.startIndex, offsetBy: i)] // ❌ CRASH: String index is out of bounds ❌
    }
}

let ss: String = ""
ss.at(0)

This is also partly for ease of comparisons/calculations:

  • An array’s count is endIndex - startIndex
  • isEmpty is startIndex == endIndex

If endIndex was the last index, you’d need to do extra math in there for those calculations.

The docs have a nice example of putting the above together to:

let name = "Marie Curie"
let firstSpace = name.firstIndex(of: " ") ?? name.endIndex
let firstName = name[..<firstSpace]
print(firstName)
// Prints "Marie"

Try not to memorize the above. If you understand the ‘why’ then you’ll memorize it naturally.

Also see this discussion in the dev forums and the original post by Dijkstra himself

if empty check

To check whether a string is empty, use its isEmpty property instead of comparing the length of one of the views to 0. Unlike with isEmpty, calculating a view’s count property requires iterating through the elements of the string.

Summary

  • a has a codepoint of 97. It’s a grapheme by itself.
  • ́ has a codepoint of 769. It’s a grapheme by itself.
  • Together they form a new grapheme cluster: á.
  • á, á, á are all the same grapheme, but because of the font they’re different glyphs.
  • For interviewing most people that are comfortable with Swift, find it easier to convert the string to an array of Characters. Because the focus of the interview isn’t on your String skills it’s about your interview, algo and DS abilities. Or at least that’s what it should be.

References