String Indexing in

About String Indexing

Individual characters in a String can be accessed using the subscripting method used with arrays. However, the indices used for strings are not integers and can not be worked with directly. Instead they must be computed based off of some other known index, such as startIndex, which points to the position of the first character in a nonempty string, using the methods available in the String and NSString libraries (when the Foundation module is imported, strings in Swift have access to all of the NSString properties and methods).

For example, given the following string:

let csv = "apple,pear,peach,orange,cherry,lime,goosberry"

One cannot write csv[21] to get the "g" in "orange", they must instead compute a value of type String.Index and supply that index instead. Note that these indices are not meant to be human-consumable on their own. They are what is referred to as opaque indices ,as humans need not know what is inside them.

let index = csv.index(csv.startIndex, offsetBy: 21)
csv[index]
// "g"
print(index)
// prints Index(_rawBits: 1376513)

Note, however, that if the offset is not a valid index, i.e. if it would return the index before startIndex or after endIndex the operation will raise an error, crashing the program. To prevent this problem, one can specify a limiting index. This returns an optional index and it will return nil for otherwise invalid indices.

let tooFar = csv.index(csv.startIndex, offsetBy: 200)
// error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).
let tooFarButSafe = csv.index(csv.startIndex, offsetBy: 200, limitedBy: csv.endIndex)
// nil

Additionally, indices cannot be shared between strings. For example, using the index of the "g" in "orange" computed above to index into the string fam = "This is my family: 👨‍👩‍👦‍👦, this is our dog: 🐶, this is our cat: 🐱!", i.e. fam[index] will crash your program.

There are many reasons for all of this, but they all basically boil down to "Unicode is tricky".

Generally speaking if you need random access to individual characters, you likely want to be using some other data structure, like Array<Character>.

Some useful methods

Swift has the methods first and last to get the first and last character of a string. These methods return an optional character, so if the string is empty, they will return nil.

let str = "Hello, world!"
print(str.first) // Prints Optional("H")
print(str.last)  // Prints Optional("!")

You can also use the prefix and suffix methods to get a substring of a string. These methods take an integer as an argument and return a substring containing the first or last n characters of the string.

let str = "Hello, world!"
print(str.prefix(5)) // Prints "Hello"
print(str.suffix(6)) // Prints "world!"

Deep Dive: Unicode indexing