How to Get and Set a string's character at a specific index in Swift

Question

I would like to use something like arrayString[firstString][index] = "1"

But that results in: Cannot assign through subscript: subscript is get-only

And an explanation why the following code var arrayString: [String] = struct.strings

Does not permit the comparison first[index] == "0" so i need to use var arrayString = struct.strings

Answer 1

Depending on how many times you need to do this, it's probably best to just create a character array from the string, and index into that.

You can either do it for just the string you care about:

let strings = ["abcdefg", "123", "qwerty"]
let characters = Array(strings[1].characters) // => ["1", "2", "3"]
print(characters[0]) // => "1"

or if you'll be doing lots of accesses to all of the strings, you can preemptively turn all strings into [Character] in advance, like so:

let strings = ["abcdefg", "123", "qwerty"]
let stringCharacters = strings.map { Array(string.characters) }
/* [
       ["a", "b", "c", "d", "e", "f", "g"],
       ["1", "2", "3"],
       ["q", "w", "e", "r", "t", "y"]
   ] */
print(characters[1][0]) // => "1"

If you want to make a change, then simply convert the character array back into a String, and assign it wherever you want it:

var strings = ["abcdefg", "123", "qwerty"]
var characters = Array(strings[1].characters) // => ["1", "2", "3"]
characters[0] = "0"
strings[1] = String(characters)
print(strings) // => ["abcdefg", "023", "qwerty"]

Also, here's a handy extension I use for mutating many characters of a string:

extension String {
    mutating func modifyCharacters(_ closure: (inout [Character]) -> Void) {
        var characterArray = Array(self.characters)
        closure(&characterArray)
        self = String(characterArray)
    }
}


var string = "abcdef"

string.modifyCharacters {
    $0[0] = "1"
    $0[1] = "2"
    $0[2] = "3"
}

print(string)