[swift] How does String substring work in Swift

I am new in Swift 3, but looking the String (index) syntax for analogy I think that index is like a "pointer" constrained to string and Int can help as an independent object. Using the base + offset syntax , then we can get the i-th character from string with the code bellow:

let s = "abcdefghi"
let i = 2
print (s[s.index(s.startIndex, offsetBy:i)])
// print c

For a range of characters ( indexes) from string using String (range) syntax we can get from i-th to f-th characters with the code bellow:

let f = 6
print (s[s.index(s.startIndex, offsetBy:i )..<s.index(s.startIndex, offsetBy:f+1 )])
//print cdefg

For a substring (range) from a string using String.substring (range) we can get the substring using the code bellow:

print (s.substring (with:s.index(s.startIndex, offsetBy:i )..<s.index(s.startIndex, offsetBy:f+1 ) ) )
//print cdefg

Notes:

1. The i-th and f-th begin with 0.

2. To f-th, I use offsetBY: f + 1, because the range of subscription use ..< (half-open operator), not include the f-th position.

3. Of course must include validate errors like invalid index.