I'm learning about default arguments and I ran aground of something weird:
import UIKit func greet(name: String = "world") { println("hello \(name)") } greet("jiaaro") this throws an error:
Playground execution failed: error::9:7: error: missing argument label 'name:' in call greet("jiaaro") ^ name:
I understand that it wants greet(name: "jiaaro") but I don't understand why that should be necessary.
Swift functions can specify local and external argument names:
func greet(who name: String = "world") { println("hello \(name)") } // prints "hello world" greet() // prints "hello jiaaro" greet(who:"jiaaro") // error greet("jiaaro") // error greet(name: "jiaaro") To opt out of this behavior you can use an underscore for the external name. Note that the first parameter implicitly uses the "no external name" behavior:
func greet(name: String = "world", _ hello: String = "hello") { println("\(hello) \(name)") } // prints "hello world" greet() // prints "hello jiaaro" greet("jiaaro") // prints "hi jiaaro" greet("jiaaro", "hi") // error greet(name: "jiaaro") The following is now disallowed in Swift 2.0, see below for equivalent code.
You can use the # prefix to use the same local and external name for the first parameter:
func greet(#name: String = "world", hello: String = "hello") { println("\(hello) \(name)") } // prints "hi jiaaro" greet(name: "jiaaro", hello: "hi") Swift 2.0 code:
func greet(name name: String = "world", hello: String = "hello") { println("\(hello) \(name)") } // prints "hi jiaaro" greet(name: "jiaaro", hello: "hi")
Swift requires argument labels by default, because it supports classes with multiple initializers. The benefit of argument labels comes from the ability of Swift to infer which initializer to use; not only by argument type, but argument name as well.
struct Celsius { var temperatureInCelsius: Double = 0.0 init(fromFahrenheit fahrenheit: Double) { temperatureInCelsius = (fahrenheit - 32.0) / 1.8 } init(fromKelvin kelvin: Double) { temperatureInCelsius = kelvin - 273.15 } } let boilingPointOfWater = Celsius(fromFahrenheit: 212.0) // boilingPointOfWater.temperatureInCelsius is 100.0 let freezingPointOfWater = Celsius(fromKelvin: 273.15) // freezingPointOfWater.temperatureInCelsius is 0.0 See this page for more details: https://developer.apple.com/library/prerelease/ios/documentation/swift/conceptual/Swift_Programming_Language/Initialization.html#//apple_ref/doc/uid/TP40014097-CH18-XID_272
I just wanted to add that now your code
func greet(name: String = "world") { print("hello \(name)") } greet("jiaaro") works fine in xcode, i just changed "println" with "print"