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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.
For example, convert "" to "www.stackoverflow".
What code snippet would do that, and being the most swift-like? (I hope this doesn't bring a debate, but I can't find good lesson on how to handle substrings in Swift.
1,26841332
Just accessing backwards
The best way is to use substringToIndex combined to the endIndexproperty and the advance global function.
var string1 = ""
var index1 = advance(string1.endIndex, -4)
var substring1 = string1.substringToIndex(index1)
Looking for a string starting from the back
Use rangeOfString and set options to .BackwardsSearch
var string2 = ""
var index2 = string2.rangeOfString(".", options: .BackwardsSearch)?.startIndex
var substring2 = string2.substringToIndex(index2!)
No extensions, pure idiomatic Swift
Here's how I do it. You could do it the same way, or use this code for ideas.
let s = ""
s.substringWithRange(0..&s.lastIndexOf("."))
Here are the extensions I use:
import Foundation
extension String {
var length: Int {
return countElements(self)
func indexOf(target: String) -& Int {
var range = self.rangeOfString(target)
if let range = range {
return distance(self.startIndex, range.startIndex)
func indexOf(target: String, startIndex: Int) -& Int {
var startRange = advance(self.startIndex, startIndex)
var range = self.rangeOfString(target, options: NSStringCompareOptions.LiteralSearch, range: Range&String.Index&(start: startRange, end: self.endIndex))
if let range = range {
return distance(self.startIndex, range.startIndex)
func lastIndexOf(target: String) -& Int {
var index = -1
var stepIndex = self.indexOf(target)
while stepIndex & -1 {
index = stepIndex
if stepIndex + target.length & self.length {
stepIndex = indexOf(target, startIndex: stepIndex + target.length)
stepIndex = -1
return index
func substringWithRange(range:Range&Int&) -& String {
let start = advance(self.startIndex, range.startIndex)
let end = advance(self.startIndex, range.endIndex)
return self.substringWithRange(start..&end)
Generally I am a strong proponent of extensions, especially for needs like string manipulation, searching, and slicing.
For example, convert "" to "www.stackoverflow". What code snippet would do that, and being the most swift-like?
Can it be more Swift-like?
let myString = "".stringByDeletingPathExtension
// "www.stackoverflow"
20.1k32741
String has builtin substring feature:
extension String : Sliceable {
subscript (subRange: Range&String.Index&) -& 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 = ""
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&C: CollectionType where C.Generator.Element: Equatable&(domain: C, value: C.Generator.Element) -& C.Index? {
var last:C.Index? = nil
for i in domain.startIndex..&domain.endIndex {
if domain[i] == value {
return last
let str = ""
let substring = map(findLast(str, ".")) { str[str.startIndex ..& $0] } // as String?
// if "." is found, substring has some, otherwise `nil`
Maybe, BidirectionalIndexType specialized version of findLast is faster:
func findLast&C: CollectionType where C.Generator.Element: Equatable, C.Index: BidirectionalIndexType&(domain: C, value: C.Generator.Element) -& C.Index? {
for i in lazy(domain.startIndex ..& domain.endIndex).reverse() {
if domain[i] == value {
return nil
18.2k32151
You can use this extensions:
extension String
func substringFromIndex(index: Int) -& String
if (index & 0 || index & self.characters.count)
print("index \(index) out of bounds")
return self.substringFromIndex(self.startIndex.advancedBy(index))
func substringToIndex(index: Int) -& String
if (index & 0 || index & self.characters.count)
print("index \(index) out of bounds")
return self.substringToIndex(self.startIndex.advancedBy(index))
func substringWithRange(start: Int, end: Int) -& String
if (start & 0 || start & self.characters.count)
print("start index \(start) out of bounds")
else if end & 0 || end & self.characters.count
print("end index \(end) out of bounds")
let range = Range(start: self.startIndex.advancedBy(start), end: self.startIndex.advancedBy(end))
return self.substringWithRange(range)
func substringWithRange(start: Int, location: Int) -& String
if (start & 0 || start & self.characters.count)
print("start index \(start) out of bounds")
else if location & 0 || start + location & self.characters.count
print("end index \(start + location) out of bounds")
let range = Range(start: self.startIndex.advancedBy(start), end: self.startIndex.advancedBy(start + location))
return self.substringWithRange(range)
let string = ""
let substring = string.substringToIndex(string.characters.count-4)
For Swift 2.0, it's like this:
var string1 = ""
var index1 = string1.endIndex.advanceBy(-4)
var substring1 = string1.substringToIndex(index1)
1,80011327
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