index(of: Int) method of Range in swift return incorrect value

Question

When using index(of: Int) method on a half open range object, it always returns an incorrect value if the range does not start at 0. See the code below.

let range = (3 ..< 10)
let indexOfRange = range.index(of: 5) // return 5


let array = Array(5 ..< 10)
let indexOfArray = array.index(of: 5)  // returns 0

I don't understand why such result is produced. Can anyone please explain?

Answer 1

Indices are opaque objects. If it's not an array, you shouldn't assume they are zero-based or even that they are integers (for an example see String.Index). To get a zero-based integer index, you need to get the distance from the startIndex:

let range = (3 ..< 10)
let opaqueIndex = range.index(of: 5)
let integerIndex = range.distance(from: range.startIndex, to: opaqueIndex!)
print(integerIndex) // 2

However, for Int ranges that's basically the same as:

let integerIndex = 5 - range.lowerOffset

The interesting part is that it seems ranges cannot be subscripted (ambiguous definition), therefore there is probably no point to get the index in the first place.