Useful String extensions

Substrings.swift

extension String {
    
    // string length
    var len: Int {
        return self.characters.count
    }
    
    // retrieve a single character
    subscript(n: Int) -> Character {
        if n < 0 || n > self.len {
            return " "
        } else {
            let index = startIndex.advancedBy(n)
            return self[index]
        }
    }
    
    // retrieve a substring using an integer range
    subscript(r: Range<Int>) -> String {
        let start = startIndex.advancedBy(r.startIndex)
        let end = startIndex.advancedBy(r.endIndex)
        let range = start..<end
        return self[range]
    }
}