Swift Map Sorted Closure

Question

I am trying to understand closures in swift and am missing something fundamental.
Given the following example:

var numbers = [20, 19, 7, 12]
numbers.map({(number: Int) -> Int in return 3 * number})
numbers.map {(number: Int) -> Int in return 3 * number}
numbers.sorted {(n1:Int, n2:Int) -> Bool in return n1 < n2}

Why do the extra braces work for map but not sorted? i.e.

numbers.sorted({(n1:Int, n2:Int) -> Bool in return n1 < n2})

does not compile...

"error: argument passed to call that takes no arguments"

Please could someone could explain the difference? Thanks.

Answer 1

Because sort method is func sorted(by:). So you have to add by param name:

numbers.sorted(by: {(n1:Int, n2:Int) -> Bool in return n1 < n2})

Answer 2

sorted():

A sorted array of the collection’s elements.

Means it returns a new -sorted- array, which it should assigned to a new instance. The simple way to implement it is:

let numbers = [20, 19, 7, 12]
let sortedNumbers = numbers.sorted { $0 < $1 } // [7, 12, 19, 20]

If you need to sort the array itself, use sort() instead:

var numbers = [20, 19, 7, 12]
numbers.sort { $0 < $1 }

print(numbers) // [7, 12, 19, 20]

Note: when using sort(), make sure that numbers array is var (mutable).

The same behavior is also appiled to map(_:):

Returns an array containing the results of mapping the given closure over the sequence’s elements.

You can also implment the map() in a simpler way:

let numbers = [20, 19, 7, 12]

let mappedNumbers = numbers.map { $0 * 3 }

print(mappedNumbers) // [60, 57, 21, 36]

If you want to map the array itself, you should implement:

var numbers = [20, 19, 7, 12]
numbers = numbers.map { $0 * 3 }

print(numbers) // [60, 57, 21, 36]

Or in a single line:

let numbers = [20, 19, 7, 12].map { $0 * 3 }

print(numbers) // [60, 57, 21, 36]