Swift .uppercaseString or .lowercaseString property replacement

Xcode 6.0 / Swift 1.0

String is bridged seamlessly to NSString, so it does have uppercaseString and lowercaseString properties as long as you import Foundation (or really almost any framework since they'll usually import Foundation internally. From the Strings and Characters section of the Swift Programming Guide:

Swift’s String type is bridged seamlessly to Foundation’s NSString class. If you are working with the Foundation framework in Cocoa or Cocoa Touch, the entire NSString API is available to call on any String value you create, in addition to the String features described in this chapter. You can also use a String value with any API that requires an NSString instance.

Xcode 6.1 / Swift 1.1

As @newacct pointed out, in Xcode 6.1 / Swift 1.1, uppercaseString and lowercaseString are in Swift's String class so you don't need to use the ones defined in NSString. However, it's implemented as an extension to the String class in the Foundation framework so the solution is still the same: import Foundation

In a playground:

import Foundation

var sillyString = "This is a string!" // --> This is a string!
let yellyString = sillyString.uppercaseString // --> THIS IS A STRING!
let silentString = sillyString.lowercaseString // --> this is a string!

Swift 3.0

In a playground:

import Foundation

var sillyString = "This is a string!" // --> This is a string!
let yellyString = sillyString.uppercased() // --> THIS IS A STRING!
let silentString = sillyString.lowercased() // --> this is a string!

Swift 4 & 5

TL;DR

The new names in Swift 3 use the -ed postfix to indicate that uppercased() and lowercased() return a copy rather than a modified original:

import Foundation

let title = "Castration: The Advantages and the Disadvantages" // Don't `var` unless you have to!
title.uppercased() // "CASTRATION: THE ADVANTAGES AND THE DISADVANTAGES"
title.lowercased() // "castration: the advantages and the disadvantages"
title.capitalized  // "Castration: The Advantages And The Disadvantages"

Inconsistencies: .method() vs .property

Note the odd discrepancy where uppercased() and lowercased() are functions, while capitalized is a property. This seems like an oversight, in which case hopefully someone comfortable with the swift evolution process will make a correction before 3.0 leaves beta.

If happen to know you're working with NSString, there is a property available for all three:

NSString(string: "abcd").uppercased
NSString(string: "ABCD").lowercased
NSString(string: "abCd").capitalized

Language is Hard

The methods above hide a whole string of method delegations to NSString and CFString with a default Locale of nil. This works most of the time. At least, it does in English. The fact is, I don't really understand the rest of what I'm about to paste from my playground.

let turkishI = "\u{0130} is not I"                  // "İ is not I"
turkishI.uppercased()                               // "İ IS NOT I"
turkishI.uppercased(with: Locale(identifier: "en")) // "İ IS NOT I"
turkishI.uppercased(with: Locale(identifier: "tr")) // "İ İS NOT I"
turkishI.lowercased()                               // "i̇ is not i"
turkishI.capitalized                                // "İ Is Not I"
turkishI.lowercased(with: Locale(identifier: "en")) // "i̇ is not i"
turkishI.lowercased(with: Locale(identifier: "tr")) // "i is not ı"

Swift 3

The Locale initializer has a slightly longer parameter name in Swift 3…

turkishI.uppercased(with: Locale(localeIdentifier: "en"))

Light Humor

"✈️".uppercased()  // 🚀 (Swift 4)
"✈️".uppercased()  // 🛸 (Swift 5)

The uppercaseString and lowercaseString properties on String are not in the Swift standard library anymore. Instead, Foundation provides them now. So you have to

import Foundation

to use it.