Swift: How to get substring from start to last index of character

Question

I want to learn the best/simplest way to turn a string into another string but with only a subset, starting at the beginning and going to the last index of a character.

Answer 1

String has builtin substring feature:

extension String : Sliceable {
    subscript (subRange: Range) -> String { get }
}

If what you want is "going to the first index of a character", you can get the substring using builtin find() function:

var str = "www.stackexchange.com"
str[str.startIndex ..< find(str, ".")!] // -> "www"

To find last index, we can implement findLast().

/// Returns the last index where `value` appears in `domain` or `nil` if
/// `value` is not found.
///
/// Complexity: O(\ `countElements(domain)`\ )
func findLast(domain: C, value: C.Generator.Element) -> C.Index? {
    var last:C.Index? = nil
    for i in domain.startIndex..

---

ADDED:

Maybe, BidirectionalIndexType specialized version of findLast is faster:

func findLast(domain: C, value: C.Generator.Element) -> C.Index? {
    for i in lazy(domain.startIndex ..< domain.endIndex).reverse() {
        if domain[i] == value {
            return i
        }
    }
    return nil
}