I've been updating some of my old code and answers with Swift 3 but when I got to Swift Strings and Indexing it has been a pain to understand things.
Specifically I was trying the following:
let str = "Hello, playground"
let prefixRange = str.startIndex..<str.startIndex.advancedBy(5) // error
where the second line was giving me the following error
'advancedBy' is unavailable: To advance an index by n steps call 'index(_:offsetBy:)' on the CharacterView instance that produced the index.
I see that String
has the following methods.
str.index(after: String.Index)
str.index(before: String.Index)
str.index(String.Index, offsetBy: String.IndexDistance)
str.index(String.Index, offsetBy: String.IndexDistance, limitedBy: String.Index)
These were really confusing me at first so I started playing around with them until I understood them. I am adding an answer below to show how they are used.
I appreciate this question and all the info with it. I have something in mind that's kind of a question and an answer when it comes to String.Index.
I'm trying to see if there is an O(1) way to access a Substring (or Character) inside a String because string.index(startIndex, offsetBy: 1) is O(n) speed if you look at the definition of index function. Of course we can do something like:
let characterArray = Array(string)
then access any position in the characterArray however SPACE complexity of this is n
= length of string, O(n) so it's kind of a waste of space.
I was looking at Swift.String documentation in Xcode and there is a frozen public struct called Index
. We can initialize is as:
let index = String.Index(encodedOffset: 0)
Then simply access or print any index in our String object as such:
print(string[index])
Note: be careful not to go out of bounds`
This works and that's great but what is the run-time and space complexity of doing it this way? Is it any better?
func change(string: inout String) {
var character: Character = .normal
enum Character {
case space
case newLine
case normal
}
for i in stride(from: string.count - 1, through: 0, by: -1) {
// first get index
let index: String.Index?
if i != 0 {
index = string.index(after: string.index(string.startIndex, offsetBy: i - 1))
} else {
index = string.startIndex
}
if string[index!] == "\n" {
if character != .normal {
if character == .newLine {
string.remove(at: index!)
} else if character == .space {
let number = string.index(after: string.index(string.startIndex, offsetBy: i))
if string[number] == " " {
string.remove(at: number)
}
character = .newLine
}
} else {
character = .newLine
}
} else if string[index!] == " " {
if character != .normal {
string.remove(at: index!)
} else {
character = .space
}
} else {
character = .normal
}
}
// startIndex
guard string.count > 0 else { return }
if string[string.startIndex] == "\n" || string[string.startIndex] == " " {
string.remove(at: string.startIndex)
}
// endIndex - here is a little more complicated!
guard string.count > 0 else { return }
let index = string.index(before: string.endIndex)
if string[index] == "\n" || string[index] == " " {
string.remove(at: index)
}
}
Create a UITextView inside of a tableViewController. I used function: textViewDidChange and then checked for return-key-input. then if it detected return-key-input, delete the input of return key and dismiss keyboard.
func textViewDidChange(_ textView: UITextView) {
tableView.beginUpdates()
if textView.text.contains("\n"){
textView.text.remove(at: textView.text.index(before: textView.text.endIndex))
textView.resignFirstResponder()
}
tableView.endUpdates()
}
Source: Stackoverflow.com