Swift 3 Functional Programming by Fatih Nayebi

Author:Fatih Nayebi
Language: eng
Format: epub
Publisher: Packt Publishing

FlatMap and flatten

The flatMap method on arrays can be used to flatten one level of dimension of an array. The following example presents a two-dimensional array, in other words, nested arrays. Calling flatMap on this array reduces one dimension and flattens it so the resulting array becomes [1, 3, 5, 2, 4, 6]:

let twoDimensionalArray = [[1, 3, 5], [2, 4, 6]] let oneDimensionalArray = twoDimensionalArray.flatMap { $0 }

In this example, flatMap returns an Array containing the concatenated results of the mapping transform over itself. We can achieve the same result by calling flatten on our array and then map, as follows:

let oneDimensionalArray = twoDimensionalArray.flatten().map { $0 }

To be able to transform each element into an array, we will need to provide a map method as the closure to the flatMap method as follows:

let transofrmedOneDimensionalArray = twoDimensionalArray.flatMap { $0.map { $0 + 2 } }

The result will be [3, 5, 7, 4, 6, 8].

The same result can be achieved with the following:

let oneDimensionalArray = twoDimensionalArray.flatten().map { $0 + 2 }

Let's examine another example with an Array of three dimensions:

let threeDimensionalArray = [[1, [3, 5]], [2, [4, 6]]] let twoDimensionalArray = threeDimensionalArray.flatMap { $0 }

The resulting array will be [1, [3, 5], 2, [4, 6]].

Therefore, flatMap and flatten only flatten one dimension, and to handle more dimensions and transformations, we need to call the flatMap and map methods multiple times accordingly.

We also know that twoDimensionalArray and threeDimensionalArray are Monads, as we could call map and flatMap on them.

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