EducaSwift

CompactMap

compactMap() works the same as map() but returns only the non-nil results.

var strings: [String] = ["1", "2", "three", "4"]

let numbers: [Int] = strings.compactMap { Int($0) } // [1, 2, 4]

In this example, we use the Int initializer that tries to convert a String to an integer, if it fails, it returns nil. So with compactMap, we'll only get the numbers that were successfully converted.

FlatMap

flatMap converts a multi-dimensional array into a single-dimensional array. A multi-dimensional array is an array of arrays, also known as a matrix.

let matrix: [[Int]] = [
    [2, 2],
    [3, 3, 3],
    [4, 4, 4, 4],
]

let array = matrix.flatMap { $0 } // [2, 2, 3, 3, 3, 4, 4, 4, 4]

Map on Optionals

We can use the map() function on an Optional variable to apply a transformation to its wrapped value if it's not nil. Otherwise, the function returns nil.

var nameNil: String? = nil
var nameNotNil: String? = "Pablo"

let nameNilResult = nameNil.map { $0.uppercased() } // nil
let nameNotNilResult = nameNotNil.map { $0.uppercased() } // PABLO

Map on Optionals vs Map on Arrays

It's common to confuse the map() applied to an Optional with the one applied to the wrapped value of an Optional. Let's see both examples.

let optionalArray: [Int]? = [1, 2, 3]

let resultSquares = optionalArray?.map { $0 * $0 } // [1, 4, 9]

let resultCount = optionalArray.map { $0.count } // 3

In the first example, using the ? symbol, we access the wrapped value of optionalArray, which, being an array, allows us to apply map(). In the second example, we apply map() to the Optional itself, so $0 will be the entire array.

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