I've written an extension for String that provides getter and setter subscripts for indices, open ranges, closed ranges, PartialRangeFrom, PartialRangeThrough, and PartialRangeUpTo. The extension even supports passing in negative numbers to access characters from the end backwards!
My package, which includes many other helpful extensions, is available here. Here are all the String extensions:
public extension String {
/**
Enables passing in negative indices to access characters
starting from the end and going backwards.
if num is negative, then it is added to the
length of the string to retrieve the true index.
*/
func negativeIndex(_ num: Int) -> Int {
return num < 0 ? num + self.count : num
}
func strOpenRange(index i: Int) -> Range<String.Index> {
let j = negativeIndex(i)
return strOpenRange(j..<(j + 1), checkNegative: false)
}
func strOpenRange(
_ range: Range<Int>, checkNegative: Bool = true
) -> Range<String.Index> {
var lower = range.lowerBound
var upper = range.upperBound
if checkNegative {
lower = negativeIndex(lower)
upper = negativeIndex(upper)
}
let idx1 = index(self.startIndex, offsetBy: lower)
let idx2 = index(self.startIndex, offsetBy: upper)
return idx1..<idx2
}
func strClosedRange(
_ range: CountableClosedRange<Int>, checkNegative: Bool = true
) -> ClosedRange<String.Index> {
var lower = range.lowerBound
var upper = range.upperBound
if checkNegative {
lower = negativeIndex(lower)
upper = negativeIndex(upper)
}
let start = self.index(self.startIndex, offsetBy: lower)
let end = self.index(start, offsetBy: upper - lower)
return start...end
}
// MARK: - Subscripts
/**
Gets and sets a character at a given index.
Negative indices are added to the length so that
characters can be accessed from the end backwards
Usage: `string[n]`
*/
subscript(_ i: Int) -> String {
get {
return String(self[strOpenRange(index: i)])
}
set {
let range = strOpenRange(index: i)
replaceSubrange(range, with: newValue)
}
}
/**
Gets and sets characters in an open range.
Supports negative indexing.
Usage: `string[n..<n]`
*/
subscript(_ r: Range<Int>) -> String {
get {
return String(self[strOpenRange(r)])
}
set {
replaceSubrange(strOpenRange(r), with: newValue)
}
}
/**
Gets and sets characters in a closed range.
Supports negative indexing
Usage: `string[n...n]`
*/
subscript(_ r: CountableClosedRange<Int>) -> String {
get {
return String(self[strClosedRange(r)])
}
set {
replaceSubrange(strClosedRange(r), with: newValue)
}
}
/// `string[n...]`. See PartialRangeFrom
subscript(r: PartialRangeFrom<Int>) -> String {
get {
return String(self[strOpenRange(r.lowerBound..<self.count)])
}
set {
replaceSubrange(strOpenRange(r.lowerBound..<self.count), with: newValue)
}
}
/// `string[...n]`. See PartialRangeThrough
subscript(r: PartialRangeThrough<Int>) -> String {
get {
let upper = negativeIndex(r.upperBound)
return String(self[strClosedRange(0...upper, checkNegative: false)])
}
set {
let upper = negativeIndex(r.upperBound)
replaceSubrange(
strClosedRange(0...upper, checkNegative: false), with: newValue
)
}
}
/// `string[...<n]`. See PartialRangeUpTo
subscript(r: PartialRangeUpTo<Int>) -> String {
get {
let upper = negativeIndex(r.upperBound)
return String(self[strOpenRange(0..<upper, checkNegative: false)])
}
set {
let upper = negativeIndex(r.upperBound)
replaceSubrange(
strOpenRange(0..<upper, checkNegative: false), with: newValue
)
}
}
}
Usage:
let text = "012345"
print(text[2]) // "2"
print(text[-1] // "5"
print(text[1...3]) // "123"
print(text[2..<3]) // "2"
print(text[3...]) // "345"
print(text[...3]) // "0123"
print(text[..<3]) // "012"
print(text[(-3)...] // "345"
print(text[...(-2)] // "01234"
All of the above works with assignment as well. All subscripts have getters and setters.