Variadics are one of the simplest and most overlooked features in Swift. They are used to specifcy a parameter that can accept a variable number of values.
Variadic parameters are specified by suffixing their type with .... Their values are then made available to the function as an array.
func sum(_ numbers: Int...) -> Int {
numbers.reduce(0, +)
}
Variadics can accept zero or more values:
sum() // 0
sum(1, 2) // 3
sum(1, 2, 3, 4, 5) // 15
When mixing multiple parameters, including variadics, the compiler must be able to distinguish which values belong to the different parameters. For this to be possible, the first parameter following a variadic (which may or may not be another variadic parameter) must be labeled.
// This won't compile since prefix must be labeled
func sum(_ numbers: Int..., _ prefix: String) -> String {
let total = numbers.reduce(0, +)
return "\(prefix): \(total)"
}
sum(1, 2, 3, "Sum")
// This will compile
func sum(_ numbers: Int..., prefix: String) -> String {
let total = numbers.reduce(0, +)
return "\(prefix): \(total)"
}
sum(1, 2, 3, prefix: "Sum")
Why not just use arrays?
One could argue that any API using variadics could equally be replaced with arrays directly. While that is not wrong, variadics can sometimes simplify signatures and provide a more elegant and user-friendly API.
Array version:
func log(_ text: String, _ numbers: [Int] = []) {
print(text, numbers.map(String.init).joined(separator: ";"))
}
log("Data", [1, 2, 3, 4])
log("No data")
Variadic version:
func log(_ text: String, _ numbers: Int...) {
print(text, numbers.map(String.init).joined(separator: ";"))
}
log("Data", 1, 2, 3, 4)
log("No data")