[swift-dev] Make offset index available for String

Cao, Jiannan frogcjn at 163.com
Thu Dec 14 00:30:02 CST 2017


I would like to discuss the String.Index problem within Swift. I know the current situation of String.Index is based on the nature of the underlaying data structure of the string.

But could we just make String.Index contain offset information? Or make offset index subscript available for accessing character in String?

for example:
let a = "01234"
print(a[0]) // 0
print(a[0...4]) // 01234
print(a[...]) // 01234
print(a[..<2]) // 01
print(a[...2]) // 012
print(a[2...]) // 234
print(a[2...3]) // 23
print(a[2...2]) // 2
if let number = a.index(of: "1") {
    print(number) // 1
    print(a[number...]) // 1234

0 equals to Collection.Index of collection.index(startIndex, offsetBy: 0)
1 equals to Collection.Index of collection.index(startIndex, offsetBy: 1)
we keep the String.Index, but allow another kind of index, which is called "offsetIndex" to access the String.Index and the character in the string.
Any Collection could use the offset index to access their element, regarding the real index of it.

I have make the Collection OffsetIndexable protocol available here, and make it more accessible for StringProtocol considering all API related to the index.

https://github.com/frogcjn/OffsetIndexableCollection-String-Int-Indexable- <https://github.com/frogcjn/OffsetIndexableCollection-String-Int-Indexable->

If someone want to make the offset index/range available for any collection, you just need to extend the collection:
extension String : OffsetIndexableCollection {

extension Substring : OffsetIndexableCollection {

I hope the Swift core team could consider bring the offset index to string, or make it available to other collection, thus let developer to decide whether their collection could use offset indices as an assistant for the real index of the collection.


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